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An artist's impression of what Mars' surface and atmosphere might look like, if Mars were terraformed.

For centuries people have speculated about the possibility of life on Mars due to the planet's proximity and similarity to Earth. Serious searches for evidence of life began in the 19th century, and they continue today via telescopic investigations and landed missions. While early work focused on phenomenology and bordered on fantasy, modern scientific inquiry has emphasized the search for water, chemical biosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.^[1]

Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks plate tectonics or continental drift, so it has remained almost unchanged since the end of the Hesperian period. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to abiogenesis, even if life does not or has never existed there.^[2]^[3] It remains an open question whether life currently exists on Mars or has existed there in the past, and fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries.

On January 24, 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic, and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable.^[4]^[5]^[6]^[7] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.^[4]

Early speculation

Mars' polar ice caps were observed as early as the mid-17th century, and they were first proven to grow and shrink alternately, in the summer and winter of each hemisphere, by William Herschel in the latter part of the 18th century. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does — but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.

In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization.^[8] This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet’s desiccation.

Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere.^[9] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.

Habitability

Chemical, physical, geological and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.^[10] The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from Solar ultraviolet and galactic cosmic radiation.^[11]^[12]

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below.^[10] Similarly, for each group of parameters, the habitability threshold for each is to be determined.^[10] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.^[13] There are no full-Mars simulations published yet that include all of the biocidal factors combined.^[13]

Some habitability factors^[12]
Water	· liquid water activity (a_w) · Past/future liquid (ice) inventories · Salinity, pH, and Eh of available water
Chemical environment	Nutrients: · C, H, N, O, P, S, essential metals, essential micronutrients · Fixed nitrogen · Availability/mineralogy Toxin abundances and lethality: · Heavy metals (e.g., Zn, Ni, Cu, Cr, As, Cd, etc., some essential, but toxic at high levels) · Globally distributed oxidizing soils
Energy for metabolism	Solar (surface and near-surface only) Geochemical (subsurface) · Oxidants · Reductants · Redox gradients
Conducive physical conditions	· Temperature · Extreme diurnal temperature fluctuations · Low pressure (Is there a low-pressure threshold for terrestrial anaerobes?) · Strong ultraviolet germicidal irradiation · Galactic cosmic radiation and solar particle events (long-term accumulated effects) · Solar UV-induced volatile oxidants, e.g., O ₂^–, O^–, H₂O₂, O₃ · Climate/variability (geography, seasons, diurnal, and eventually, obliquity variations) · Substrate (soil processes, rock microenvironments, dust composition, shielding) · High CO₂ concentrations in the global atmosphere · Transport (aeolian, ground water flow, surface water, glacial)

Past

The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.^[14] When there was a magnetic field, the atmosphere would have been protected from erosion by solar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.^[15]

Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors.^[16] The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment — perhaps a lake or an ancient streambed — that was neutral and not too salty.^[16] On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life.^[17]^[18] The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and Solar radiation,^[19]^[20] together strongly suggest that Mars could have had the environmental factors to support life.^[21] However, the assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively.^[14]

Present

No definitive evidence for biosignatures or organics of Martian origin has been identified, and assessment will continue not only through the Martian seasons, but also back in time as the Curiosity rover studies what is recorded in the depositional history of the rocks in Gale Crater.^[10] While scientists have not identified the minimum number of parameters for determination of habitability potential, some teams have proposed hypotheses based on simulations.

Subsurface

Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms,^[10] life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen.^[22]^[23]^[24]^[25]^[26]^[27] Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet.^[14]^[27]^[28]^[29]^[30]^[31] The extensive volcanism in the past possibly created subsurface cracks and caves within different strata where liquid water could have been stored, forming large aquifers with deposits of saline liquid water, minerals, organic molecules, and geothermal heat – potentially providing a habitable environment away from the harsh surface conditions.^[27]^[32]^[33]^[34]

Surface brines

Although liquid water does not appear at the surface of Mars,^[35]^[36] several modeling studies suggest that potential locations on Mars could include regions where thin films of salty liquid brine or perchlorate may form near the surface^[36]^[36]^[37] that may provide a potential location for terrestrial salt and cold-loving microorganisms (halophile psychrophilic).^[38] Various salts present in the Martian soil may act as an antifreeze and could keep water liquid well below its normal freezing point, if water was present at certain favorable locations.^[36]^[39]^[40] Astrobiologists are keen to find out more, as not much is known about these brines at the moment. The briny water may or may not be habitable to microbes from Earth or Mars.^[41] Another researcher argues that although chemically important, thin films of transient liquid water are not likely to provide suitable sites for life.^[39] In addition, an astrobiology team asserted that the activity of water on salty films, the temperature, or both are less than the biological thresholds across the entire Martian surface and shallow subsurface.^[12]

The damaging effect of ionizing radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats.^[25]^[26]^[42] Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.^[26] Also, solar ultraviolet (UV) radiation proved particularly devastating for the survival of cold-resistant microbes under simulated surface conditions on Mars, as UV radiation was readily and easily able to penetrate the salt-organic matrix that the bacterial cells were embedded in.^[43] In addition, NASA's Mars Exploration Program states that life on the surface of Mars is unlikely, given the presence of superoxides that break down organic (carbon-based) molecules on which life is based.^[44]

Cosmic radiation

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.^[45] Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years.^[46] As a result, the planet has been vulnerable to radiation from space for about 4 billion years.^[47] Currently, ionizing radiation on Mars is typically two orders of magnitude (or 100 times) higher than on Earth.^[48] Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars for that long.^[22]^[49] After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.^[22]^[23]^[24] The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 metres below the planet's surface.^[23]

Even the most radiation-tolerant Earthly bacteria would survive in dormant spore state only 18,000 years at the surface; at 2 meters —the greatest depth at which the ExoMars rover will be capable of reaching— survival time would be 90,000 to half million years, depending on the type of rock.^[24]

The Radiation assessment detector (RAD) on board the Curiosity rover is currently quantifying the flux of biologically hazardous radiation at the surface of Mars today, and will help determine how these fluxes vary on diurnal, seasonal, solar cycle and episodic (flare, storm) timescales. These measurements will allow calculations of the depth in rock or soil to which this flux, when integrated over long timescales, provides a lethal dose for known terrestrial organisms.^[50]

Research published in January 2014 of data collected by the RAD instrument, revealed that the actual absorbed dose measured is 76 mGy/year at the surface,^[51] and that "ionizing radiation strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic matter."^[51] Regardless of the source of Martian organic matter (meteoritic, geological, or biological), its carbon bonds are susceptible to breaking and reconfigurating with surrounding elements by ionizing charged particle radiation.^[51] These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic biosignatures as a function of depth as well as survival times of possible microbial or bacterial life forms left dormant beneath the surface.^[51] The report concludes that the in situ "surface measurements —and subsurface estimates— constrain the preservation window for Martian organic matter following exhumation and exposure to ionizing radiation in the top few meters of the Martian surface."^[51]

Nitrogen fixation

After carbon, nitrogen is arguably the most important element needed for life. Thus, measurements of nitrate over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N₂) in the atmosphere at low levels, but this is not adequate to support nitrogen fixation for biological incorporation.^[52] Nitrogen in the form of nitrate, if present, could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed in shock and electrical processes. Currently there is no data on its availability.^[52]

Low pressure

Further complicating estimates of the habitability of the Martian surface is the fact that very little is known on the growth of microorganisms at pressures close to the conditions found on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar).^[53] In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only Serratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0°C, and CO₂-enriched anoxic atmospheres.^[53]

Liquid water

Liquid water, necessary for life as we know it, cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.^[54]^[55] Liquid water does not appear at the surface itself,^[56] but it could form in minuscule amounts around dust particles in snow heated by the Sun.^[57]^[57]^[58]^[58] Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.^[59]^[60]^[61]^[62]

Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.^[63]^[64] A small amount of water vapor is present in the atmosphere.^[65] There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago,^[66] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,^[67]^[68]^[69]^[70] including large oceans.^[71]^[72]^[73]^[74]^[75] It has been estimated that the primordial oceans on Mars would have covered between 36%^[76] and 75% of the planet.^[77]

Analysis of Martian sandstones, using data obtained from orbital spectrometry, suggests that the waters that previously existed on the surface of Mars would have had too high a salinity to support most Earth-like life. Tosca et al. found that the Martian water in the locations they studied all had water activity, a_w ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.^[78] Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.^[79]

In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.^[80]^[81] Additional similar images were published in 2006, taken by the Mars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in steep crater walls and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago.

There is disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.^[82]^[83]^[84]^[85] Others suggest it may be liquid brine near the surface,^[86]^[87]^[88] but the exact source of the water and the mechanism behind its motion are not understood.^[89]

Silica

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in silica (90%).^[90] The feature is reminiscent of the effect of hot spring water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life, and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.^[91]

Based on Earth analogs, hydrothermal systems on Mars would be highly attractive for their potential for preserving organic and inorganic biosignatures.^[92]^[93]^[94] For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.^[95]^[96]^[97]

Possible biosignatures

Methane

Trace amounts of methane in the atmosphere of Mars were discovered in 2003 and verified in 2004.^[98]^[99]^[100]^[101]^[102]^[103] As methane is an unstable gas, its presence indicates that there must be an active source on the planet in order to keep such levels in the atmosphere. It is estimated that Mars must produce 270 ton/year of methane,^[104]^[105] but asteroid impacts account for only 0.8% of the total methane production. Although geologic sources of methane such as serpentinization are possible, the lack of current volcanism, hydrothermal activity or hotspots^[106] are not favorable for geologic methane. It has been suggested that the methane was produced by chemical reactions in meteorites, driven by the intense heat during entry through the atmosphere. Although research published in December 2009 ruled out this possibility,^[107] research published in 2012 suggest that a source may be organic compounds on meteorites that are converted to methane by ultraviolet radiation.^[108]

The existence of life in the form of microorganisms such as methanogens is among possible, but as yet unproven sources. If microscopic Martian life is producing the methane, it probably resides far below the surface, where it is still warm enough for liquid water to exist.^[28]

Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and in vitro experiments testing growth of methanogenic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0wt% perchlorate salt.^[109] The results reported indicate that the perchlorates discovered by the Phoenix Lander would not rule out the possible presence of methanogens on Mars.^[109]^[110]

A team led by Levin suggested that both phenomena—methane production and degradation—could be accounted for by an ecology of methane-producing and methane-consuming microorganisms.^[110]^[111]

In June 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.^[112]^[113] According to the scientists, "...low H₂/CH₄ ratios (less than approximately 40) indicate that life is likely present and active."^[112] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.^[114]^[115]

In contrast to the findings described above, studies by Kevin Zahnle, a planetary scientist at NASA's Ames Research Center, and two colleagues, conclude that "there is as yet no compelling evidence for methane on Mars". They argue that the strongest reported observations of the gas to date have been taken at frequencies where interference from methane in Earth's atmosphere is particularly difficult to remove, and are thus unreliable. Additionally, they claim that the published observations most favorable to interpretation as indicative of Martian methane are also consistent with no methane being present on Mars.^[116]^[117]^[118]

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane;^[119] but even if the mission is to determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside of the rover's reach.^[120] The first measurements with the Tunable Laser Spectrometer (TLS) in the Curiosity rover indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.^[121]^[122]^[123]^[124] On July 19, 2013, NASA scientists published the results of a new analysis of the atmosphere of Mars, reporting a lack of methane around the landing site of the Curiosity rover.^[125]^[126]^[127] On September 19, 2013, NASA again reported no detection of atmospheric methane with a measured value of 0.18±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, conclude that the probability of current methanogenic microbial activity on Mars is reduced.^[128]^[129]^[130]

India's Mars Orbiter Mission, launched on November 5, 2013, will search for methane in the atmosphere of Mars using its Methane Sensor for Mars (MSM). The orbiter is scheduled to arrive at Mars on September 24, 2014. The Mars Trace Gas Mission orbiter planned to launch in 2016 would further study the methane, if present,^[131]^[132] as well as its decomposition products such as formaldehyde and methanol.

Formaldehyde

In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter had detected traces of formaldehyde in the atmosphere of Mars. Vittorio Formisano, the director of the PFS, has speculated that the formaldehyde could be the byproduct of the oxidation of methane and, according to him, would provide evidence that Mars is either extremely geologically active or harbouring colonies of microbial life.^[133]^[134] NASA scientists consider the preliminary findings well worth a follow-up, but have also rejected the claims of life.^[135]^[136]

Meteorites

NASA maintains a catalog of 34 Mars meteorites.^[137] These assets are highly valuable since they are the only physical samples available of Mars. Studies conducted by NASA's Johnson Space Center show that at least three of the meteorites contain potential evidence of past life on Mars, in the form of microscopic structures resembling fossilized bacteria (so-called biomorphs). Although the scientific evidence collected is reliable, its interpretation varies. To date, none of the original lines of scientific evidence for the hypothesis that the biomorphs are of exobiological origin (the so-called biogenic hypothesis) have been either discredited or positively ascribed to non-biological explanations.^[138]

Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:^[138]

Is the geologic context of the sample compatible with past life?
Is the age of the sample and its stratigraphic location compatible with possible life?
Does the sample contain evidence of cellular morphology and colonies?
Is there any evidence of biominerals showing chemical or mineral disequilibria?
Is there any evidence of stable isotope patterns unique to biology?
Are there any organic biomarkers present?
Are the features indigenous to the sample?

For general acceptance of past life in a geologic sample, essentially most or all of these criteria must be met. All seven criteria have not yet been met for any of the Martian samples, but continued investigations are in progress.^[138]

As of 2010, reexaminations of the biomorphs found in the three Martian meteorites are underway with more advanced analytical instruments than previously available.

ALH84001

The ALH84001 meteorite was found in December 1984 in Antarctica, by members of the ANSMET project; the meteorite weighs 1.93 kilograms (4.3 lb).^[139] The sample was ejected from Mars about 17 million years ago and spent 11,000 years in or on the Antarctic ice sheets. Composition analysis by NASA revealed a kind of magnetite that on Earth, is only found in association with certain microorganisms.^[138] Then, in August 2002, another NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH 84001 occurs as small, uniform-sized crystals that, on Earth, is associated only with biologic activity, and that the remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biological magnetite was organized into chains as would be expected. The meteorite displays indication of relatively low temperature secondary mineralization by water and shows evidence of preterrestrial aqueous alteration.^{[clarification needed]} Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface.

Some structures resembling the mineralized casts of terrestrial bacteria and their appendages (fibrils) or by-products (extracellular polymeric substances) occur in the rims of carbonate globules and preterrestrial aqueous alteration regions.^[140]^[141] The size and shape of the objects is consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial.

In November 2009, NASA scientists reported after more detailed analyses, that a biogenic explanation is a more viable hypothesis for the origin of the magnetites in the meteorite.^[142]^[143]

Nakhla

The Nakhla meteorite fell on Earth on June 28, 1911 on the locality of Nakhla, Alexandria, Egypt.^[144]^[145]

In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects^[146] of the size and shape consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial. Analysis with gas chromatography and mass spectrometry (GC-MS) studied its high molecular weight polycyclic aromatic hydrocarbons in 2000, and NASA scientists concluded that as much as 75% of the organic matter in Nakhla "may not be recent terrestrial contamination".^[138]^[147]

This caused additional interest in this meteorite, so in 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic carbon content was observed. When the results and evidence were published on 2006, some independent researchers claimed that the carbon deposits are of biologic origin. However, it was remarked that since carbon is the fourth most abundant element in the Universe, finding it in curious patterns is not indicative or suggestive of biological origin.^[148]^[149]

Shergotty

The Shergotty meteorite, a 4 kg Martian meteorite, fell on Earth on Shergotty, India on August 25, 1865 and was retrieved by witnesses almost immediately.^[150] This meteorite is relatively young, calculated to have been formed on Mars only 165 million years ago from volcanic origin. It is composed mostly of pyroxene and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest remnants of a biofilm and its associated microbial communities.^[138] Work is in progress on searching for magnetites within alteration phases.

Yamato 000593

Yamato 000593 is the second largest meteorite from Mars found on Earth. Studies suggest the Martian meteorite was formed about 1.3 billion years ago from a lava flow on Mars. An impact occurred on Mars about 12 million years ago and ejected the meteorite from the Martian surface into space. The meteorite landed on Earth in Antarctica about 50,000 years ago. The mass of the meteorite is 13.7 kg (30 lb) and has been found to contain evidence of past water movement.^[151]^[152]^[153] At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by biotic activity according to NASA scientists.^[151]^[152]^[153]

Geysers on Mars

The seasonal frosting and defrosting of the southern ice cap results in the formation of spider-like radial channels carved on 1 meter thick ice by sunlight. Then, sublimed CO₂ – and probably water –increase pressure in their interior producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.^[154]^[155]^[156]^[157] This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars.

A team of Hungarian scientists proposes that the geysers' most visible features, dark dune spots and spider channels, may be colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap, and as the sunlight returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesize and heat their immediate surroundings. A pocket of liquid water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through grey. When the layer has completely melted, the microorganisms rapidly desiccate and turn black, surrounded by a grey aureole.^[158]^[159]^[159]^[160] The Hungarian scientists believe that even a complex sublimation process is insufficient to explain the formation and evolution of the dark dune spots in space and time.^[161]^[162] Since their discovery, fiction writer Arthur C. Clarke promoted these formations as deserving of study from an astrobiological perspective.^[163]

A multinational European team suggests that if liquid water is present in the spiders' channels during their annual defrost cycle, they might provide a niche where certain microscopic life forms could have retreated and adapted while sheltered from solar radiation.^[164] A British team also considers the possibility that organic matter, microbes, or even simple plants might co-exist with these inorganic formations, especially if the mechanism includes liquid water and a geothermal energy source.^[165] However, they also remark that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis.^[165] It has been proposed to develop the Mars Geyser Hopper lander to study the geysers up close.^[166]

Forward contamination

Planetary protection of Mars aims to prevent biological contamination of the planet.^[167] A major goal is to preserve the planetary record of natural processes by preventing human-caused microbial introductions, also called forward contamination. There is abundant evidence as to what can happen when organisms from regions on Earth that have been isolated from one another for significant periods of time are introduced into each other's environment. Species that are constrained in one environment can thrive – often out of control – in another environment much to the detriment of the original species that were present. In some ways this problem could be compounded if life forms from one planet were introduced into the totally alien ecology of another world.^[168]

The prime concern of hardware contaminating Mars, derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (extremophiles) despite best efforts.^[12]^[169] Hardware includes landers, crashed probes, end of mission disposal of hardware, and hard landing of entry, descent, and landing systems. This has prompted research on radiation-resistant microorganisms including Brevundimonas, Rhodococcus, Pseudomonas genera and Deinococcus radiodurans survival rates under simulated Martian conditions.^[170] Results from one of these this experimental irradiation experiments, combined with previous radiation modeling, indicate that Brevundimonas sp. MV.7 emplaced only 30 cm deep in Martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10⁶ population reduction.^[170] Surprisingly, the diurnal Mars-like cycles in temperature and relative humidity affected the viability of Deinococcus radiodurans cells quite severely.^[171] In other simulations, Deinococcus radiodurans also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.^[172]

Life under simulated Martian conditions

On 26 April 2012, scientists reported that an extremophile lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).^[173]^[174]^[175]^[176]^[177]^[178] However, the ability to survive in an environment is not the same as the ability to thrive, reproduce, and evolve in that same environment, necessitating further study.

Missions

Mariner 4

Mariner 4 probe performed the first successful flyby of the planet Mars, returning the first pictures of the Martian surface in 1965. The photographs showed an arid Mars without rivers, oceans, or any signs of life. Further, it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4 billion years. The probe also found that Mars has no global magnetic field that would protect the planet from potentially life-threatening cosmic rays. The probe was able to calculate the atmospheric pressure on the planet to be about 0.6 kPa (compared to Earth's 101.3 kPa), meaning that liquid water could not exist on the planet's surface.^[9] After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.

Viking orbiters

Liquid water is necessary for known life and metabolism, so if water was present on Mars, the chances of it having supported life may have been determinant. The Viking orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams.^[179]^[180]^[181]

File:Sagan Viking.jpg

Carl Sagan poses next to a replica of the Viking landers.

Viking experiments

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.^[182] The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result,^{[dubious – discuss]} showing increased ¹⁴CO₂ production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled ¹⁴CO₂ was evolved in the Labeled Release experiment, and that the GCMS detected no organic molecules. However, there are vastly different interpretations of what those results imply.

A 2011 astrobiology textbook notes that the GCMS was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that the Viking missions failed to detect life in the Martian soil."^[183]

One of the designers of the Labeled Release experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars.^[9] Levin's interpretation is disputed by many scientists.^[184] A 2006 astrobiology textbook noted that "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas."^[185] Other scientists argue that superoxides in the soil could have produced this effect without life being present.^[186] An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph & mass spectrometer, designed to identify natural organic matter, did not detect organic molecules.^[187] The results of the Viking mission concerning life are considered by the general expert community, at best, as inconclusive.^[9]^[186]^[188]

In 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, D.C., USA), Gilbert Levin's investigation was assessed once more.^[187] Levin still maintains that his original data were correct, as the positive and negative control experiments were in order.^[189] Moreover, Levin's team, on 12 April 2012, reported a statistical speculation, based on old data —reinterpreted mathematically through cluster analysis— of the Labeled Release experiments, that may suggest evidence of "extant microbial life on Mars."^[189]^[190] Critics counter that the method has not yet been proven effective for differentiating between biological and non-biological processes on Earth so it is premature to draw any conclusions.^[191]

A research team from the National Autonomous University of Mexico headed by Rafael Navarro-González, concluded that the GCMS equipment (TV-GC-MS) used by the Viking program to search for organic molecules, may not be sensitive enough to detect low levels of organics.^[192] Klaus Biemann, the principal investigator of the GCMS experiment on Viking wrote a rebuttal.^[193] Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, so Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection.

After the discovery of perchlorates on Mars by the Phoenix lander, practically the same team of Navarro-González published a paper arguing that the Viking GCMS results were compromised by presence of perchlorates.^[194] A 2011 astrobiology textbook notes that "while perchlorate is too poor an oxidizer to reproduce the LR results (under the conditions of that experiment perchlorate does not oxidize organics), it does oxidize, and thus destroy, organics at the higher temperatures used in the Viking GCMS experiment."^[183] Biemann has written a commentary critical of this Navarro-González paper as well,^[195] to which the latter have replied;^[196] the exchange was published in December 2011.

Gillevinia straata

The claim for life on Mars, in the form of Gillevinia straata, is based on old data reinterpreted as sufficient evidence of life, mainly by Gilbert Levin.^[187] The evidence supporting the existence of Gillevinia straata microorganisms relies on the data collected by the two Mars Viking landers that searched for biosignatures of life, but the analytical results were, officially, inconclusive.^[9]

In 2006, Mario Crocco, a neurobiologist at the Neuropsychiatric Hospital Borda in Buenos Aires, Argentina, proposed the creation of a new nomenclatural rank that classified the Viking landers' results as 'metabolic' and therefore belonging to a form of life. Crocco proposed to create new biological ranking categories (taxa), in the new kingdom system of life, in order to be able to accommodate the genus of Martian microorganisms. Crocco proposed the following taxonomical entry:^[197]

Organic life system: Solaria
Biosphere: Marciana
Kingdom: Jakobia (named after neurobiologist Christfried Jakob)
Genus and species: Gillevinia straata

As a result, the hypothetical Gillevinia straata would not be a bacterium (which rather is a terrestrial taxon), but a member of the kingdom 'Jakobia' in the biosphere 'Marciana' of the 'Solaria' system. The intended effect of the new nomenclature was to reverse the burden of proof concerning the life issue, but the taxonomy proposed by Crocco has not been accepted by the scientific community and is considered a single nomen nudum. Further, no Mars mission has found traces of biomolecules.^[198]^[199]

An artist's concept of the Phoenix spacecraft

Phoenix lander, 2008

The Phoenix mission landed a robotic spacecraft in the polar region of Mars on May 25, 2008 and it operated until November 10, 2008. One of the mission's two primary objectives was to search for a "habitable zone" in the Martian regolith where microbial life could exist, the other main goal being to study the geological history of water on Mars. The lander has a 2.5 meter robotic arm that was capable of digging shallow trenches in the regolith. There was an electrochemistry experiment which analysed the ions in the regolith and the amount and type of antioxidants on Mars. The Viking program data indicate that oxidants on Mars may vary with latitude, noting that Viking 2 saw fewer oxidants than Viking 1 in its more northerly position. Phoenix landed further north still.^[200] Phoenix's preliminary data revealed that Mars soil contains perchlorate, and thus may not be as life-friendly as thought earlier.^[201]^[202]^[203] The pH and salinity level were viewed as benign from the standpoint of biology. The analysers also indicated the presence of bound water and CO₂.^[204]

Mars Science Laboratory

The Mars Science Laboratory mission is a NASA project that launched on November 26, 2011 the Curiosity rover, a nuclear-powered robotic vehicle, bearing instruments designed to assess past and present habitability conditions on Mars.^[205]^[206] The Curiosity rover landed on Mars on Aeolis Palus in Gale Crater, near Aeolis Mons (a.k.a. Mount Sharp),^[207]^[208]^[209]^[210] on August 6, 2012.^[211]^[212]^[213]

Future missions

ExoMars is a European-led multi-spacecraft programme currently under development by the European Space Agency (ESA) and the Russian Federal Space Agency for launch in 2016 and 2018.^[214] Its primary scientific mission will be to search for possible biosignatures on Mars, past or present. A rover with a 2 metres (6.6 ft)^* core drill will be used to sample various depths beneath the surface where liquid water may be found and where microorganisms might survive cosmic radiation.^[21]
Mars 2020 rover mission – The Mars 2020 rover mission is a Mars planetary rovermission concept under study by NASA with a possible launch in 2020. It is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures within accessible geological materials.^[168]
Mars Sample Return Mission — The best life detection experiment proposed is the examination on Earth of a soil sample from Mars. However, the difficulty of providing and maintaining life support over the months of transit from Mars to Earth remains to be solved. Providing for still unknown environmental and nutritional requirements is daunting. Should dead organisms be found in a sample, it would be difficult to conclude that those organisms were alive when obtained.

References

^ Mumma, Michael J. (January 8, 2012). The Search for Life on Mars. Origin of Life Gordon Research Conference. Galveston, TX.
^ McKay, Christopher P.; Stoker, Carol R. (1989). "The early environment and its evolution on Mars: Implication for life". Reviews of Geophysics. 27 (2): 189–214. Bibcode:1989RvGeo..27..189M. doi:10.1029/RG027i002p00189.
^ Gaidos, Eric; Selsis, Franck (2007). "From Protoplanets to Protolife: The Emergence and Maintenance of Life". Protostars and Planets V: 929–44. arXiv:astro-ph/0602008. Bibcode:2007prpl.conf..929G.
^ ^a ^b Grotzinger, John P. (January 24, 2014). "Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science. 343 (6169): 386–387. doi:10.1126/science.1249944. Retrieved January 24, 2014. {{cite journal}}: More than one of |number= and |issue= specified (help)
^ "Special Issue - Table of Contents - Exploring Martian Habitability". Science. 343 (6169): 345–452. January 24, 2014. Retrieved 24 January 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
^ "Special Collection - Curiosity - Exploring Martian Habitability". Science. January 24, 2014. Retrieved January 24, 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
^ Grotzinger, J. P.; Sumner, D. Y.; Kah, L. C.; Stack, K.; Gupta, S.; Edgar, L.; Rubin, D.; Lewis, K.; Schieber, J.; Mangold, N.; Milliken, R.; Conrad, P. G.; Desmarais, D.; Farmer, J.; Siebach, K.; Calef, F.; Hurowitz, J.; McLennan, S. M.; Ming, D.; Vaniman, D.; Crisp, J.; Vasavada, A.; Edgett, K. S.; Malin, M.; Blake, D.; Gellert, R.; Mahaffy, P.; Wiens, R. C.; Maurice, S.; et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169): 1242777. doi:10.1126/science.1242777. Retrieved January 24, 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help); More than one of |number= and |issue= specified (help)
^ Wallace, Alfred Russel (1907). Is Mars habitable?: A critical examination of Professor Percival Lowell's book 'Mars and its canals,' with an alternative explanation. London: Macmillan. OCLC 263175453.^{[page needed]}
^ ^a ^b ^c ^d ^e Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.^{[page needed]}
^ ^a ^b ^c ^d ^e Conrad, P. G.; Archer, D.; Coll, P.; De La Torre, M.; Edgett, K.; Eigenbrode, J. L.; Fisk, M.; Freissenet, C.; Franz, H.; et al. (2013). "Habitability Assessment at Gale Crater: Implications from Initial Results". 44th Lunar and Planetary Science Conference. 1719: 2185. Bibcode:2013LPICo1719.2185C.
^ Schuerger, Andrew C.; Golden, D. C.; Ming, Doug W. (2012). "Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions". Planetary and Space Science. 72 (1): 91–101. Bibcode:2012P&SS...72...91S. doi:10.1016/j.pss.2012.07.026.
^ ^a ^b ^c ^d MEPAG Special Regions-Science Analysis Group; Beaty, D.; Buxbaum, K.; Meyer, M.; Barlow, N.; Boynton, W.; Clark, B.; Deming, J.; Doran, P. T.; et al. (2006). "Findings of the Mars Special Regions Science Analysis Group". Astrobiology. 6 (5): 677–732. Bibcode:2006AsBio...6..677M. doi:10.1089/ast.2006.6.677. PMID 17067257.
^ ^a ^b "Mars Contamination Dust-Up". Astrobiology Magazine. 17 May 2010. Retrieved 2013-07-04. Whenever multiple biocidal factors are combined, the survival rates plummet quickly, {{cite web}}: |first= missing |last= (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
^ ^a ^b ^c Summons, Roger E.; Amend, Jan P.; Bish, David; Buick, Roger; Cody, George D.; Des Marais, David J.; Dromart, Gilles; Eigenbrode, Jennifer L.; et al. (2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group". Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. PMID 21417945. There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance.
^ Dehant, V.; Lammer, H.; Kulikov, Y. N.; Grießmeier, J. -M.; Breuer, D.; Verhoeven, O.; Karatekin, Ö.; Hoolst, T.; et al. (2007). "Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars". Geology and Habitability of Terrestrial Planets. Space Sciences Series of ISSI. Vol. 24. pp. 279–300. doi:10.1007/978-0-387-74288-5_10. ISBN 978-0-387-74287-8.
^ ^a ^b "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars". NASA. 12 March 2013. Retrieved 2013-06-06.
^ Chang, Kenneth (December 9, 2013). "On Mars, an Ancient Lake and Perhaps Life". New York Times. Retrieved December 9, 2013.
^ "Science - Special Collection - Curiosity Rover on Mars". Science. December 9, 2013. Retrieved December 9, 2013. {{cite journal}}: Cite uses deprecated parameter |authors= (help)
^ Neal-Jones, Nancy (October 12, 2005). "New Map Provides More Evidence Mars Once Like Earth". Goddard Space Flight Center. NASA. Retrieved 5 June 2013. {{cite web}}: Unknown parameter |coauthor= ignored (|author= suggested) (help)
^ "Martian Interior: Paleomagnetism". Mars Express. European Space Agency. 4 January 2007.
^ ^a ^b Wall, Mike (25 March 2011). "Q & A with Mars Life-Seeker Chris Carr". Space.com. Retrieved 2011-03-25.
^ ^a ^b ^c "Study: Surface of Mars Devoid of Life". Space.com. 29 January 2007. Retrieved 28 May 2013. After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation. {{cite web}}: |first= missing |last= (help)
^ ^a ^b ^c Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology". Geophysical Research Letters. 34 (2): L02207. Bibcode:2007GeoRL..34.2207D. doi:10.1029/2006GL027494. Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2 m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m.
^ ^a ^b ^c Richard A. Lovet (February 2, 2007). "Mars Life May Be Too Deep to Find, Experts Conclude". National Geographic News. That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere. Cite error: The named reference "DartnellGeographic" was defined multiple times with different content (see the help page).
^ ^a ^b Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology". Geophysical Research Letters. 34 (2). Bibcode:2007GeoRL..3402207D. doi:10.1029/2006GL027494. The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats.
^ ^a ^b ^c Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Martian sub-surface ionising radiation: biosignatures and geology". Biogeosciences. 4 (4): 545–558. Bibcode:2007BGeo....4..545D. doi:10.5194/bg-4-545-2007. This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. [..] Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.{{cite journal}}: CS1 maint: unflagged free DOI (link)
^ ^a ^b ^c "Scientists find evidence Mars subsurface could hold life". Digital Journal – Science. 21 January. Retrieved 2013-06-05. There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell. {{cite news}}: |first= missing |last= (help); Check date values in: |date= (help)
^ ^a ^b Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center. NASA. Retrieved June 6 24, 2013. If microscopic Martian life is producing the methane, it probably resides far below the surface, where it's still warm enough for liquid water to exist {{cite news}}: Check date values in: |accessdate= (help)
^ NASA – Mars Rovers Sharpen Questions About Livable Conditions
^ "Mars: 'Strongest evidence' planet may have supported life, scientists say". BBC News. 20 January 2013. Retrieved 2013-01-22.
^ Michalski, Joseph R.; Cuadros, Javier; Niles, Paul B.; Parnell, John; Deanne Rogers, A.; Wright, Shawn P. (2013). "Groundwater activity on Mars and implications for a deep biosphere". Nature Geoscience. 6 (2): 133–8. Bibcode:2013NatGe...6..133M. doi:10.1038/ngeo1706.
^ De Morais, A. (2012). "A Possible Biogeochemical Model for Mars". 43rd Lunar and Planetary Science Conference. 43: 2943. Bibcode:2012LPI....43.2943D. The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth.
^ Anderson, Paul S. (15 December 2011). "New Study Says Large Regions of Mars Could Sustain Life". Universe Today. Retrieved 2013-06-05. Most scientists would agree that the best place that any organisms could hope to survive and flourish would be underground.
^ "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 2013-06-06. If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.
^ Hecht, Michael H.; Vasavada, Ashwin R. (2006). "Transient liquid water near an artificial heat source on Mars". International Journal of Mars Science and Exploration. 2: 83–96. Bibcode:2006IJMSE...2...83H. doi:10.1555/mars.2006.0006. In summary, on present-day Mars, liquid water is unlikely except as the result of a quick and dramatic change in environmental conditions such as from a landslide that exposes buried ice to sunlight (Costard et al. 2002), or from the introduction of an artificial heat source.
^ ^a ^b ^c ^d Haberle, Robert M.; McKay, Christopher P.; Schaeffer, James; Cabrol, Nathalie A.; Grin, Edmon A.; Zent, Aaron P.; Quinn, Richard (2001). "On the possibility of liquid water on present-day Mars". Journal of Geophysical Research: Planets. 106 (El0): 23317–26. Bibcode:bibcode=2001JGR...10623317H. doi:10.1029/2000JE001360. Introduction: The mean annual surface pressure and temperature on present-day Mars do not allow for the stability of liquid water on the surface. […] Conclusion: It is possible, even likely, that solar-heated liquid water never forms on present-day Mars. {{cite journal}}: Check |bibcode= length (help); Missing pipe in: |bibcode= (help)
^ "Conference: The Present-Day Habitability of Mars 2013" (PDF). The UCLA Institute for Planets and Exoplanets. February 4–6, 2013. Retrieved 2013-06-17.
^ Jones, E. G.; Lineweaver, C. H. (2012). "Using the phase diagram of liquid water to search for life". Australian Journal of Earth Sciences. 59 (2): 253–62. Bibcode:2012AuJES..59..253J. doi:10.1080/08120099.2011.591430.
^ ^a ^b Lobitz, B.; Wood, BL; Averner, MM; McKay, CP (2001). "Use of spacecraft data to derive regions on Mars where liquid water would be stable". Proc. Natl. Acad. Sci. 98 (5): 2132–2137. Bibcode:2001PNAS...98.2132L. doi:10.1073/pnas.031581098. PMC 30104. PMID 11226204. These results do not indicate that water is present at these locations, only that, if it were present and heat sources were sufficient to bring the water in thermal equilibrium with the surface, the resulting liquid would be stable against freezing or boiling.
^ Parro, Victor Parro (December 2011). "A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars". Astroiology. 11 (10): 969–996. Bibcode:2011AsBio..11..969P. doi:10.1089/ast.2011.0654. {{cite journal}}: |format= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Rod Pyle (25 February 2013). "Mars May Be Habitable Today, Scientists Say". space.com.
^ "42nd Lunar and Planetary Science Conference" (Document). The Woodlands, Texas: Lunar and Planetary Institute. March 7–11, 2011{{inconsistent citations}} {{cite document}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |contribution-url= ignored (help); Unknown parameter |contribution= ignored (help); Unknown parameter |format= ignored (help)CS1 maint: postscript (link)
^ "Scarce Shelter on Mars". Astrobiology Magazine. 1 June 2009. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help)
^ "Goal 1: Determine if Life Ever Arose On Mars". NASA. Retrieved 2013-06-29. {{cite web}}: |first= missing |last= (help)
^ Luhmann, J. G.; Russell, C. T. (1997). "Mars: Magnetic Field and Magnetosphere". In Shirley, J. H.; Fainbridge, R. W. (eds.). Encyclopedia of Planetary Sciences. New York: Chapman and Hall. pp. 454–6. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
^ Phillips, Tony (January 31, 2001). "The Solar Wind at Mars". NASA.
^ "What makes Mars so hostile to life?". BBC News. 7 January 2013. Retrieved 2013-06-15.
^ Keating, A. Keating (November 2012). "The impact of Mars geological evolution in high energy ionizing radiation environment through time". Planetary and Space Science – Eslevier. 72 (1): 70–77. Bibcode:2012P&SS...72...70K. doi:10.1016/j.pss.2012.04.009. Retrieved 2013-06-28. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ "IMPLICATIONS OF COSMIC RADIATION ON THE MARTIAN SURFACE FOR MICROBIAL SURVIVAL AND DETECTION OF FLUORESCENT BIOSIGNATURES" (PDF). Lunar and Planetary Institute. 2011. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Hassler, Donald M.; Zeitlin, Cary; Wimmer-Schweingruber, Robert F.; Ehresmann, Bent; Rafkin, Scot; Martin, Cesar; Boettcher, Stephan; Koehler, Jan; Guo, Jingnan; et al. (2013). "The Radiation Environment on the Martian Surface and during MSL's Cruise to Mars". EGU General Assembly 2013. 15: 12596. Bibcode:2013EGUGA..1512596H.
^ ^a ^b ^c ^d ^e Hassler, Donald M. (24 January 2014). "Mars' Surface Radiation Environment Measured with the Mars ScienceLaboratory's Curiosity Rover" (PDF). Science. 343 (6169): 1244797. doi:10.1126/science.1244797. PMID 24324275. Retrieved 2014-01-27. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ ^a ^b McKay, Christopher P. McKay (April 5, 2013). "The Icebreaker Life Mission to Mars: A Search for Biomolecular Evidence for Life". Astrobiology. 13 (4): 334–353. Bibcode:2013AsBio..13..334M. doi:10.1089/ast.2012.0878. PMID 23560417. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ ^a ^b Schuerger, Andrew C. Schuerger (February 2013). "Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres". Astrobiology. 13 (2): 115–131. Bibcode:2013AsBio..13..115S. doi:10.1089/ast.2011.0811. PMC 3582281. PMID 23289858. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Heldmann, Jennifer L.; Toon, Owen B.; Pollard, Wayne H.; Mellon, Michael T.; Pitlick, John; McKay, Christopher P.; Andersen, Dale T. (2005). "Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions". Journal of Geophysical Research. 110 (E5): E05004. Bibcode:2005JGRE..11005004H. doi:10.1029/2004JE002261.
^ Kostama, V.-P.; Kreslavsky, M. A.; Head, J. W. (2006). "Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement". Geophysical Research Letters. 33 (11): 11201. Bibcode:2006GeoRL..3311201K. doi:10.1029/2006GL025946.
^ Hecht, Michael H.; Vasavada, Ashwin R. (2006). "Transient liquid water near an artificial heat source on Mars". International Journal of Mars Science and Exploration. 2: 83. Bibcode:2006IJMSE...2...83H. doi:10.1555/mars.2006.0006.
^ ^a ^b Shiga, David (December 7, 2009). "Watery niche may foster life on Mars". New Scientist.
^ ^a ^b Tudor Vieru (2009-12-07). "Greenhouse Effect on Mars May Be Allowing for Life". News.softpedia.com. Retrieved 2011-08-20.^{[unreliable source?]}
^ Michael T. Mellon Subsurface Ice at Mars: A review of ice and water in the equatorial regions University of Colorado 10 May 2011 Planetary Protection Subcommittee Meeting
^ Robert Roy Britt Ice Packs and Methane on Mars Suggest Present Life Possible space.com 22 February 2005
^ Mellon, Michael T.; Jakosky, Bruce M.; Postawko, Susan E. (1997). "The persistence of equatorial ground ice on Mars". Journal of Geophysical Research. 102: 19357–69. Bibcode:1997JGR...10219357M. doi:10.1029/97JE01346.
^ Arfstrom, J. D. (2012). "A Conceptual Model of Equatorial Ice Sheets on Mars". Comparative Climatology of Terrestrial Planets. 1675: 8001. Bibcode:2012LPICo1675.8001A.
^ "Mars Odyssey: Newsroom". Mars.jpl.nasa.gov. May 28, 2002. Retrieved December 19, 2010.
^ Feldman, W. C. (2004). "Global distribution of near-surface hydrogen on Mars". Journal of Geophysical Research. 109. Bibcode:2004JGRE..10909006F. doi:10.1029/2003JE002160.
^ "Mars Global Surveyor Measures Water Clouds". Retrieved March 7, 2009.
^ Baker, V. R.; Strom, R. G.; Gulick, V. C.; Kargel, J. S.; Komatsu, G.; Kale, V. S. (1991). "Ancient oceans, ice sheets and the hydrological cycle on Mars". Nature. 352 (6336): 589. Bibcode:1991Natur.352..589B. doi:10.1038/352589a0.
^ "Flashback: Water on Mars Announced 10 Years Ago". SPACE.com. June 22, 2000. Retrieved December 19, 2010.
^ "Science@NASA, The Case of the Missing Mars Water". Retrieved March 7, 2009.
^ "Mars Rover Opportunity Examines Clay Clues in Rock". NASA. Jet Propulsion Laboratory. May 17, 2013. Retrieved 2013-06-14.
^ "NASA Rover Helps Reveal Possible Secrets of Martian Life". NASA. November 29, 2005. Retrieved 2013-06-20.
^ ISBN 0-312-24551-3^{[page needed]}^{[full citation needed]}
^ "PSRD: Ancient Floodwaters and Seas on Mars". Psrd.hawaii.edu. July 16, 2003. Retrieved December 19, 2010.
^ "Gamma-Ray Evidence Suggests Ancient Mars Had Oceans". SpaceRef. November 17, 2008. Retrieved December 19, 2010.
^ Carr, Michael H.; Head, James W. (2003). "Oceans on Mars: An assessment of the observational evidence and possible fate". Journal of Geophysical Research (Planets). 108: 5042. Bibcode:2003JGRE..108.5042C. doi:10.1029/2002JE001963.
^ Harwood, William (January 25, 2013). "Opportunity rover moves into 10th year of Mars operations". Space Flight Now. Retrieved February 18, 2013.
^ Di Achille, Gaetano; Hynek, Brian M. (2010). "Ancient ocean on Mars supported by global distribution of deltas and valleys". Nature Geoscience. 3 (7): 459–63. Bibcode:2010NatGe...3..459D. doi:10.1038/ngeo891. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)
^ Smith, D. E.; Sjogren, W. L.; Tyler, G. L.; Balmino, G.; Lemoine, F. G.; Konopliv, A. S. (1999). "The gravity field of Mars: Results from Mars Global Surveyor". Science. 286 (5437): 94–7. Bibcode:1999Sci...286...94S. doi:10.1126/science.286.5437.94. PMID 10506567.
^ Tosca, Nicholas J.; Knoll, Andrew H.; McLennan, Scott M. (2008). "Water Activity and the Challenge for Life on Early Mars". Science. 320 (5880): 1204–7. Bibcode:2008Sci...320.1204T. doi:10.1126/science.1155432. PMID 18511686.
^ DasSarma, Shiladitya (2006). "Extreme Halophiles Are Models for Astrobiology". Microbe. 1 (3): 120–6.
^ Malin, Michael C.; Edgett, Kenneth S. (2000). "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars". Science. 288 (5475): 2330–5. Bibcode:2000Sci...288.2330M. doi:10.1126/science.288.5475.2330. PMID 10875910.
^ Martínez, G. M.; Renno, N. O.; Elliott, H. M.; Fischer, E. (2013). Present Day Liquid Water On Mars: Theoretical Expectations, Observational Evidence And Preferred Locations (PDF). The Present-day Mars Habitability Conference. Los Angeles.
^ Kolb, K.; Pelletier, Jon D.; McEwen, Alfred S. (2010). "Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water". Icarus. 205: 113–137. Bibcode:2010Icar..205..113K. doi:10.1016/j.icarus.2009.09.009.
^ "Icarus | Vol 218, Iss 1, In Progress, (March, 2012)". ScienceDirect.com. 2004-06-08. Retrieved 2012-01-16.^{[page needed]}
^ "University of Arizona Press Release" March 16, 2006.
^ Kerr, Richard (2006-12-08). "Mars Orbiter's Swan Song: The Red Planet Is A-Changin'". Science. 314 (5805): 1528–1529. doi:10.1126/science.314.5805.1528. PMID 17158298.
^ "NASA Finds Possible Signs of Flowing Water on Mars". voanews.com. Retrieved August 5, 2011.
^ Source: Ames Research Center Posted Saturday, June 6, 2009 (June 6, 2009). "NASA Scientists Find Evidence for Liquid Water on a Frozen Early Mars". SpaceRef. Retrieved December 19, 2010.{{cite web}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
^ "Dead Spacecraft on Mars Lives on in New Study". SPACE.com. June 10, 2008. Retrieved December 19, 2010.
^ McEwen, Alfred S.; Ojha, Lujendra; Dundas, Colin M.; Mattson, Sarah S.; Byrne, Shane; Wray, James J.; Cull, Selby C.; Murchie, Scott L.; et al. (2011). "Seasonal Flows on Warm Martian Slopes". Science. 333 (6043): 740–3. Bibcode:2011Sci...333..740M. doi:10.1126/science.1204816. PMID 21817049.
^ "Mars Rover Spirit Unearths Surprise Evidence of Wetter Past" (Press release). Jet Propulsion Laboratory. May 21, 2007. Retrieved June 27, 2013.
^ "Mars Rover Investigates Signs of Steamy Martian Past" (Press release). Jet Propulsion Laboratory. December 10, 2007. Retrieved June 27, 2013.
^ Leveille, R. J. (2010). "Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars". American Geophysical Union. 12: 07. Bibcode:2010AGUFM.P12A..07L.
^ Walter, M. R.; Des Marais, David J. (1993). "Preservation of Biological Information in Thermal Spring Deposits: Developing a Strategy for the Search for Fossil Life on Mars". Icarus. 101 (1): 129–43. Bibcode:1993Icar..101..129W. doi:10.1006/icar.1993.1011. PMID 11536937.
^ Allen, Carlton C.; Albert, Fred G.; Chafetz, Henry S.; Combie, Joan; Graham, Catherine R.; Kieft, Thomas L.; Kivett, Steven J.; McKay, David S.; et al. (2000). "Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars". Icarus. 147 (1): 49–67. Bibcode:2000Icar..147...49A. doi:10.1006/icar.2000.6435. PMID 11543582.
^ Wade, Manson L.; Agresti, David G.; Wdowiak, Thomas J.; Armendarez, Lawrence P.; Farmer, Jack D. (1999). "A Mössbauer investigation of iron-rich terrestrial hydrothermal vent systems: Lessons for Mars exploration". Journal of Geophysical Research. 104 (E4): 8489–507. Bibcode:1999JGR...104.8489W. doi:10.1029/1998JE900049. PMID 11542933.
^ Agresti, D. G.; Wdowiak, T. J.; Wade, M. L.; Armendarez, L. P.; Farmer, J. D. (1995). "A Mossbauer Investigation of Hot Springs Iron Deposits". Abstracts of the Lunar and Planetary Science Conference. 26: 7. Bibcode:1995LPI....26....7A.
^ Agresti, D. G.; Wdowiak, T. J.; Wade, M. L.; Armendarez, L. P. (1997). "Mössbauer Spectroscopy of Thermal Springs Iron Deposits as Martian Analogs". Early Mars: Geologic and Hydrologic Evolution. 916: 1. Bibcode:1997LPICo.916....1A.
^ Mumma, M. J.; Novak, R. E.; Disanti, M. A.; Bonev, B. P. (2003). "A Sensitive Search for Methane on Mars". American Astronomical Society. 35: 937. Bibcode:2003DPS....35.1418M.
^ Mumma, Michael J.; Villanueva, Geronimo L.; Novak, Robert E.; Hewagama, Tilak; Bonev, Boncho P.; Disanti, Michael A.; Mandell, Avi M.; Smith, Michael D. (2009). "Strong Release of Methane on Mars in Northern Summer 2003". Science. 323 (5917): 1041–5. Bibcode:2009Sci...323.1041M. doi:10.1126/science.1165243. PMID 19150811.
^ Formisano, Vittorio; Atreya, Sushil; Encrenaz, Thérèse; Ignatiev, Nikolai; Giuranna, Marco (2004). "Detection of Methane in the Atmosphere of Mars". Science. 306 (5702): 1758–61. Bibcode:2004Sci...306.1758F. doi:10.1126/science.1101732. PMID 15514118.
^ Krasnopolsky, Vladimir A.; Maillard, Jean Pierre; Owen, Tobias C. (2004). "Detection of methane in the martian atmosphere: Evidence for life?". Icarus. 172 (2): 537–47. Bibcode:2004Icar..172..537K. doi:10.1016/j.icarus.2004.07.004.
^ "Mars Express confirms methane in the Martian atmosphere" (Press release). ESA. March 30, 2004. Retrieved March 17, 2006.
^ Moran, Mark; Miller, Joseph D.; Kral, Tim; Scott, Dave (2005). "Desert methane: Implications for life detection on Mars". Icarus. 178: 277–80. Bibcode:2005Icar..178..277M. doi:10.1016/j.icarus.2005.06.008.
^ Krasnopolsky, Vladimir A. (2006). "Some problems related to the origin of methane on Mars". Icarus. 180 (2): 359–67. Bibcode:2006Icar..180..359K. doi:10.1016/j.icarus.2005.10.015.
^ Planetary Fourier Spectrometer website (ESA, Mars Express)^{[verification needed]}
^ "Hunting for young lava flows". Geophysical Research Letters. Red Planet. June 1, 2011. Retrieved 2013-10-04.
^ Court, Richard W.; Sephton, Mark A. (2009). "Investigating the contribution of methane produced by ablating micrometeorites to the atmosphere of Mars". Earth and Planetary Science Letters. 288 (3–4): 382–5. Bibcode:2009E&PSL.288..382C. doi:10.1016/j.epsl.2009.09.041. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)
^ Keppler, Frank; Vigano, Ivan; McLeod, Andy; Ott, Ulrich; Früchtl, Marion; Röckmann, Thomas (2012). "Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere". Nature. 486 (7401): 93–6. Bibcode:2012Natur.486...93K. doi:10.1038/nature11203. PMID 22678286.
^ ^a ^b Kral, T. A.; Goodhart, T.; Howe, K. L.; Gavin, P. (2009). "Can Methanogens Grow in a Perchlorate Environment on Mars?". 72nd Annual Meeting of the Meteoritical Society. 72: 5136. Bibcode:2009M&PSA..72.5136K.
^ ^a ^b Howe, K. L.; Gavin, P.; Goodhart, T.; Kral, T. A. (2009). "Methane Production by Methanogens in Perchlorate-supplemented Media". 40th Lunar and Planetary Science Conference. 40: 1287. Bibcode:2009LPI....40.1287H.
^ Levin, Gilbert V.; Straat, Patricia Ann (2009). "Methane and life on Mars". In Hoover, Richard B; Levin, Gilbert V; Rozanov, Alexei Y; Retherford, Kurt D (eds.). Instruments and Methods for Astrobiology and Planetary Missions XII. Vol. 7441. pp. 12–27. Bibcode:2009SPIE.7441E..12L. doi:10.1117/12.829183. ISBN 978-0-8194-7731-6.
^ ^a ^b Oze, Christopher; Jones, Camille; Goldsmith, Jonas I.; Rosenbauer, Robert J. (June 7, 2012). "Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces". PNAS. 109 (25): 9750–9754. Bibcode:2012PNAS..109.9750O. doi:10.1073/pnas.1205223109. PMC 3382529. PMID 22679287.
^ Staff (June 25, 2012). "Mars Life Could Leave Traces in Red Planet's Air: Study". Space.com. Retrieved June 27, 2012.
^ Brogi, Matteo; Snellen, Ignas A. G.; de Krok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernest J. W. (June 28, 2012). "The signature of orbital motion from the dayside of the planet τ Boötis b". Nature. 486 (7404): 502–504. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161.
^ Mann, Adam (June 27, 2012). "New View of Exoplanets Will Aid Search for E.T." Wired (magazine). Retrieved June 28, 2012.
^ "Is there Methane on Mars? Part II" (Document). Lunar and Planetary Institute. 2011{{inconsistent citations}} {{cite document}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |contribution-url= ignored (help); Unknown parameter |contribution= ignored (help); Unknown parameter |format= ignored (help)CS1 maint: postscript (link)
^ "Methane on Mars. Now you see it. Now you don't..." The Economist. 29 December 2010. Retrieved 2011-08-06.
^ Kevin Zahnle, Richard S. Freedman and David C. Catling (2011). "Is there methane on Mars?". Icarus. 212 (2): 493–503. Bibcode:2011Icar..212..493Z. doi:10.1016/j.icarus.2010.11.027.
^ Tenenbaum, David (June 9, 2008). "Making Sense of Mars Methane". Astrobiology Magazine. Archived from the original on 23 September 2008. Retrieved October 8, 2008. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
^ Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center. NASA. Archived from the original on 17 January 2009. Retrieved January 24, 2009. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
^ "Mars Curiosity Rover News Telecon -November 2, 2012".
^ Kerr, Richard A. (November 2, 2012). "Curiosity Finds Methane on Mars, or Not". Science (journal). Retrieved November 3, 2012.
^ Wall, Mike (November 2, 2012). "Curiosity Rover Finds No Methane on Mars — Yet". Space.com. Retrieved November 3, 2012.
^ Chang, Kenneth (November 2, 2012). "Hope of Methane on Mars Fades". New York Times. Retrieved November 3, 2012.
^ Mann, Adam (18 July 2013). "Mars Rover Finds Good News for Past Life, Bad News for Current Life on Mars". Wired (magazine). Retrieved 19 July 2013.
^ Webster Chris R.; et al. (19 July 2013). "Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere". Science. 341 (6143): 260–263. doi:10.1126/science.1237961. Retrieved 19 July 2013. {{cite journal}}: Explicit use of et al. in: |author= (help)
^ Mahaffy, Paul R.; et al. (19 July 2013). "Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover". Science. 341 (6143): 263–266. doi:10.1126/science.1237966. Retrieved 19 July 2013. {{cite journal}}: Explicit use of et al. in: |author= (help)
^ Webster, Christopher R.; Mahaffy, Paul R.; Atreya, Sushil K.; Flesch, Gregory J.; Farley, Kenneth A. (September 19, 2013). "Low Upper Limit to Methane Abundance on Mars". Science. doi:10.1126/science.1242902. Retrieved September 19, 2013.
^ Cho, Adrian (September 19, 2013). "Mars Rover Finds No Evidence of Burps and Farts". Science (journal). Retrieved September 19, 2013.
^ Chang, Kenneth (September 19, 2013). "Mars Rover Comes Up Empty in Search for Methane". New York Times. Retrieved September 19, 2013.
^ Rincon, Paul (July 9, 2009). "Agencies outline Mars initiative". BBC News. Retrieved July 26, 2009.
^ "NASA orbiter to hunt for source of Martian methane in 2016". Thaindian News. Asian News International. March 6, 2009. Retrieved July 26, 2009.
^ Peplow, Mark (February 25, 2005). "Formaldehyde claim inflames martian debate". Nature. doi:10.1038/news050221-15.
^ Hogan, Jenny (February 16, 2005). "A whiff of life on the Red Planet". New Scientist magazine. Retrieved August 18, 2008.
^ Peplow, Mark (September 7, 2005). "Martian methane probe in trouble". Nature. doi:10.1038/news050905-10.
^ "NASA Statement on False Claim of Evidence of Life on Mars". NASA News. NASA. February 18, 2005. Retrieved August 18, 2008.
^ "Mars Meteorites". NASA. Retrieved February 16, 2010.
^ ^a ^b ^c ^d ^e ^f Evidence for ancient Martian life. E. K. Gibson Jr., F. Westall, D. S. McKay, K. Thomas-Keprta, S. Wentworth, and C. S. Romanek, Mail Code SN2, NASA Johnson Space Center, Houston TX 77058, USA.
^ "Allan Hills 84001". The Meteorolitical Society. April 2008. Retrieved August 21, 2008.
^ Crenson, Matt (August 6, 2006). "After 10 years, few believe life on Mars". Associated Press (on space.com). Retrieved August 6, 2006. {{cite web}}: External link in |publisher= (help)^{[dead link]}
^ McKay, David S.; Gibson, Everett K.; Thomas-Keprta, Kathie L.; Vali, Hojatollah; Romanek, Christopher S.; Clemett, Simon J.; Chillier, Xavier D. F.; Maechling, Claude R.; Zare, Richard N. (1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–30. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069.
^ "New Study Adds to Finding of Ancient Life Signs in Mars Meteorite". NASA. NASA News. 30 November 2009. Retrieved 2013-06-04. {{cite news}}: |first= missing |last= (help)
^ Thomas-Keprta, K. L.; Clemett, S. J.; McKay, D. S.; Gibson, E. K.; Wentworth, S. J. (2009). "Origins of magnetite nanocrystals in Martian meteorite ALH84001". Geochimica et Cosmochimica Acta. 73 (21): 6631–77. Bibcode:2009GeCoA..73.6631T. doi:10.1016/j.gca.2009.05.064.
^ Baalke, Ron (1995). "The Nakhla Meteorite". Jet Propulsion Lab. NASA. Retrieved August 17, 2008.
^ "Rotating image of a Nakhla meteorite fragment". London Natural History Museum. 2008. Retrieved August 17, 2008.
^ Rincon, Paul (February 8, 2006). "Space rock re-opens Mars debate". BBC News. Retrieved August 17, 2008.
^ Meyer, C. (2004). "Mars Meteorite Compendium" (PDF). NASA. Retrieved August 21, 2008.
^ Whitehouse, David (August 27, 1999). "Life on Mars – new claims". BBC News. Retrieved August 20, 2008.
^ Compilation of scientific research references on the Nakhla meteorite: http://curator.jsc.nasa.gov/antmet/marsmets/nakhla/references.cfm
^ Shergoti Meteorite – JPL, NASA
^ ^a ^b Webster, Guy (February 27, 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved February 27, 2014.
^ ^a ^b White, Lauren M.; Gibson, Everett K.; Thomnas-Keprta, Kathie L.; Clemett, Simon J.; McKay, David (February 19, 2014). "Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593". Astrobiology. 14 (2): 170–181. doi:10.1089/ast.2011.0733. Retrieved February 27, 2014.
^ ^a ^b Gannon, Megan (February 28, 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com. Retrieved February 28, 2014.
^ "NASA Findings Suggest Jets Bursting From Martian Ice Cap". Jet Propulsion Laboratory. NASA. August 16, 2006. Retrieved August 11, 2009.
^ Kieffer, H. H. (2000). "Annual Punctuated CO2 Slab-Ice and Jets on Mars". International Conference on Mars Polar Science and Exploration: 93. Bibcode:2000mpse.conf...93K.
^ Portyankina, G.; Markiewicz, W. J.; Garcia-Comas, M.; Keller, H. U.; Bibring, J.-P.; Neukum, G. (2006). "Simulations of Geyser-type Eruptions in Cryptic Region of Martian South Polar Cap". Fourth International Conference on Mars Polar Science and Exploration. 1323: 8040. Bibcode:2006LPICo1323.8040P.
^ Kieffer, Hugh H.; Christensen, Philip R.; Titus, Timothy N. (2006). "CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap". Nature. 442 (7104): 793–6. Bibcode:2006Natur.442..793K. doi:10.1038/nature04945. PMID 16915284.
^ Horváth, A.; Gánti, T.; Gesztesi, A.; Bérczi, Sz.; Szathmáry, E. (2001). "Probable Evidences of Recent Biological Activity on Mars: Appearance and Growing of Dark Dune Spots in the South Polar Region". 32nd Annual Lunar and Planetary Science Conference. 32: 1543. Bibcode:2001LPI....32.1543H.
^ ^a ^b Pócs, T.; Horváth, A.; Gánti, T.; Bérczi, Sz.; Szathemáry, E. (2004). "Possible crypto-biotic-crust on Mars?". Proceedings of the Third European Workshop on Exo-Astrobiology. 545: 265–6. Bibcode:2004eab..conf..265P.
^ Gánti, Tibor; Horváth, András; Bérczi, Szaniszló; Gesztesi, Albert; Szathmáry, Eörs (2003). "Dark Dune Spots: Possible Biomarkers on Mars?". Origins of Life and Evolution of the Biosphere. 33 (4/5): 515–57. doi:10.1023/A:1025705828948.
^ Horváth, A.; Gánti, T.; Bérczi, Sz.; Gesztesi, A.; Szathmáry, E. (2002). "Morphological Analysis of the Dark Dune Spots on Mars: New Aspects in Biological Interpretation". 33rd Annual Lunar and Planetary Science Conference. 33: 1108. Bibcode:2002LPI....33.1108H.
^ András Sik, Ákos Kereszturi. "Dark Dune Spots – Could it be that it's alive?". Monochrom. Retrieved September 4, 2009. (Audio interview, MP3 6 min.)
^ Orme, Greg M.; Ness, Peter K. (June 9, 2003). "Martian Spiders" (PDF). Marsbugs. 10 (23): 5–7. Archived from the original (PDF) on September 27, 2007. Retrieved September 6, 2009.
^ Manrubia, S. C.; Prieto Ballesteros, O.; González Kessler, C.; Fernández Remolar, D.; Córdoba-Jabonero, C.; Selsis, F.; Bérczi, S.; Gánti, T.; Horváth, A. (2004). "Comparative analysis of geological features and seasonal processes in 'Inca City' and 'Pityusa Patera' regions on Mars". Proceedings of the Third European Workshop on Exo-Astrobiology. 545: 77–80. Bibcode:2004eab..conf...77M. ISBN 92-9092-856-5.
^ ^a ^b Ness, Peter K.; Orme, Greg M. (2002). "Spider-Ravine Models and Plant-Like Features on Mars – Possible Geophysical and Biogeophysical Modes of Origin". Journal of the British Interplanetary Society. 55 (3/4): 85–108. Bibcode:2002JBIS...55...85N.
^ Landis, Geoffrey; Oleson, Steven; McGuire, Melissa (2012). Design Study for a Mars Geyser Hopper. 50th AIAA Aerospace Sciences Meeting. Nashville. doi:10.2514/6.2012-631. {{cite conference}}: Unknown parameter |doi_brokendate= ignored (|doi-broken-date= suggested) (help)
^ Committee on an Astrobiology Strategy for the Exploration of Mars; National Research Council (2007). "Planetary Protection for Mars Missions". An Astrobiology Strategy for the Exploration of Mars. The National Academies Press. pp. 95–8. ISBN 978-0-309-10851-5. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
^ ^a ^b Cowing, Keith (April 11, 2013). "Planetary Protection: A Work in Progress". Astrobiology. Retrieved 2013-06-02. Cite error: The named reference "Cowing" was defined multiple times with different content (see the help page).
^ Debus, A. (2005). "Estimation and assessment of Mars contamination". Advances in Space Research. 35 (9): 1648–53. Bibcode:2005AdSpR..35.1648D. doi:10.1016/j.asr.2005.04.084. PMID 16175730.
^ ^a ^b Dartnell, Lewis R.; Hunter, Stephanie J.; Lovell, Keith V.; Coates, Andrew J.; Ward, John M. (2010). "Low-Temperature Ionizing Radiation Resistance of Deinococcus radiodurans and Antarctic Dry Valley Bacteria". Astrobiology. 10 (7): 717–32. Bibcode:2010AsBio..10..717D. doi:10.1089/ast.2009.0439. PMID 20950171.
^ de la Vega, U. Pogoda; Rettberg, P.; Reitz, G. (2007). "Simulation of the environmental climate conditions on martian surface and its effect on Deinococcus radiodurans". Advances in Space Research. 40 (11): 1672–7. Bibcode:2007AdSpR..40.1672D. doi:10.1016/j.asr.2007.05.022.
^ "Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres". Astrobiology. 13 (2): 115–131. February 2013. Bibcode:2013AsBio..13..115S. doi:10.1089/ast.2011.0811. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ de Vera, Jean-Pierre; Möhlmann, Diedrich; Butina, Frederike; Lorek, Andreas; Wernecke, Roland; Ott, Sieglinde (2010). "Survival Potential and Photosynthetic Activity of Lichens Under Mars-Like Conditions: A Laboratory Study". Astrobiology. 10 (2): 215–27. Bibcode:2010AsBio..10..215D. doi:10.1089/ast.2009.0362. PMID 20402583.
^ de Vera, J.-P. P.; Schulze-Makuch, D.; Khan, A.; Lorek, A.; Koncz, A.; Möhlmann, D.; Spohn, T. (2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars". EGU General Assembly 2012. 14: 2113. Bibcode:2012EGUGA..14.2113D.
^ Surviving the conditions on Mars DLR, 26 April 2012
^ de Vera, Jean-Pierre (2012). "Lichens as survivors in space and on Mars". Fungal Ecology. 5 (4): 472–9. doi:10.1016/j.funeco.2012.01.008.
^ R. de la Torre Noetzel, F.J. Sanchez Inigo, E. Rabbow, G. Horneck, J. P. de Vera, L.G. Sancho Survival of lichens to simulated Mars conditions^{[self-published source?]}
^ Sánchez, F. J.; Mateo-Martí, E.; Raggio, J.; Meeßen, J.; Martínez-Frías, J.; Sancho, L. G.; Ott, S.; de la Torre, R. (2012). "The resistance of the lichen Circinaria gyrosa (nom. Provis.) towards simulated Mars conditions—a model test for the survival capacity of an eukaryotic extremophile". Planetary and Space Science. 72 (1): 102–10. Bibcode:2012P&SS...72..102S. doi:10.1016/j.pss.2012.08.005.
^ Strom, R.G., Steven K. Croft, and Nadine G. Barlow, "The Martian Impact Cratering Record," Mars, University of Arizona Press, ISBN 0-8165-1257-4, 1992.^{[page needed]}
^ Raeburn, P. 1998. Uncovering the Secrets of the Red Planet Mars. National Geographic Society. Washington D.C.^{[page needed]}
^ Moore, P. et al. 1990. The Atlas of the Solar System. Mitchell Beazley Publishers NY, NY.^{[page needed]}
^ "Astrobiology". Biology Cabinet. September 26, 2006. Retrieved 2011-01-17.
^ ^a ^b Kevin W. Plaxco; Michael Gross (2011). Astrobiology: A Brief Introduction. JHU Press. pp. 282–283. ISBN 978-1-4214-0194-2. Cite error: The named reference "PlaxcoGross2011" was defined multiple times with different content (see the help page).
^ Richard Stenger Mars sample return plan carries microbial risk, group warns, CNN, November 7, 2000
^ Kevin W. Plaxco and Michael Gross (2006). Astrobiology: A Brief Introduction. JHU Press. p. 223. ISBN 978-0-8018-8366-8.
^ ^a ^b Kevin W. Plaxco; Michael Gross (2011). Astrobiology: A Brief Introduction (2nd ed.). JHU Press. pp. 285–286. ISBN 978-1-4214-0194-2.
^ ^a ^b ^c Levin, Gilbert V. (2007). "Analysis of evidence of Mars life". Electroneurobiología. 15 (2): 39–47. arXiv:0705.3176. Bibcode:2007arXiv0705.3176L.
^ Klein, Harold P.; Horowitz, Norman H.; Levin, Gilbert V.; Oyama, Vance I.; Lederberg, Joshua; Rich, Alexander; Hubbard, Jerry S.; Hobby, George L.; Straat, Patricia A. (1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090.
^ ^a ^b Bianciardi, Giorgio; Miller, Joseph D.; Straat, Patricia Ann; Levin, Gilbert V. (2012). "Complexity Analysis of the Viking Labeled Release Experiments". International Journal of Aeronautical and Space Sciences. 13 (1): 14–26. Bibcode:2012IJASS..13...14B. doi:10.5139/IJASS.2012.13.1.14.
^ Life on Mars Found by NASA's Viking Mission?
^ Klotz, Irene (12 April 2012). "Mars Viking Robots 'Found Life'". DiscoveryNews. Retrieved 16 April 2012.
^ Navarro-González, Rafael; Navarro, Karina F.; de la Rosa, José; Iñiguez, Enrique; Molina, Paola; Miranda, Luis D.; Morales, Pedro; Cienfuegos, Edith; Coll, Patrice (2006). "The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results". Proceedings of the National Academy of Sciences. 103 (44): 16089–94. Bibcode:2006PNAS..10316089N. doi:10.1073/pnas.0604210103. JSTOR 30052117. PMC 1621051. PMID 17060639.
^ Biemann, Klaus (2007). "On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter". Proceedings of the National Academy of Sciences of the United States of America. 104 (25): 10310–10313. Bibcode:2007PNAS..10410310B. doi:10.1073/pnas.0703732104. PMC 1965509. PMID 17548829.
^ Webster, Guy; Hoover, Rachel; Marlaire, Ruth; Frias, Gabriela (2010-09-03). "Missing Piece Inspires New Look at Mars Puzzle". NASA Jet Propulsion Laboratory. Retrieved 2010-10-24.
^ doi:10.1029/2011JE003869
^ doi:10.1029/2011JE003880
^ Crocco, Mario (April 14, 2007). "Los taxones mayores de la vida orgánica y la nomenclatura de la vida en Marte: primera clasificación biológica de un organismo marciano". Electroneurobiología (in Spanish). 15 (2): 1–34. Retrieved August 14, 2008. {{cite journal}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)
^ What Life Leaves Behind by Veronique Greenwood.
^ Curiosity tasked with hunting for elusive Mars organics. Astronomy Now. 22 May 2013
^ Piecing Together Life's Potential
^ "NASA Spacecraft Confirms Perchlorate on Mars". NASA. NASA. August 5, 2008. Retrieved January 28, 2009.
^ Johnson, John (August 6, 2008). "Perchlorate found in Martian soil". Los Angeles Times.
^ "Martian Life Or Not? NASA's Phoenix Team Analyzes Results". Science Daily. August 6, 2008.
^ Lakdawalla, Emily (June 26, 2008). "Phoenix sol 30 update: Alkaline soil, not very salty, "nothing extreme" about it!". The Planetary Society weblog. Planetary Society. Retrieved June 26, 2008. {{cite web}}: External link in |work= (help)
^ "Mars Science Laboratory Launch". 26 November 2011. Retrieved 2011-11-26.
^ Associated Press (26 November 2011). "NASA Launches Super-Size Rover to Mars: 'Go, Go!'". New York Times. Retrieved 2011-11-26.
^ USGS (16 May 2012). "Three New Names Approved for Features on Mars". USGS. Retrieved 28 May 2012.
^ NASA Staff (27 March 2012). "'Mount Sharp' on Mars Compared to Three Big Mountains on Earth". NASA. Retrieved 31 March 2012.
^ Agle, D. C. (28 March 2012). "'Mount Sharp' On Mars Links Geology's Past and Future". NASA. Retrieved 31 March 2012.
^ Staff (29 March 2012). "NASA's New Mars Rover Will Explore Towering 'Mount Sharp'". Space.com. Retrieved 30 March 2012.
^ Webster, Guy; Brown, Dwayne (22 July 2011). "NASA's Next Mars Rover To Land At Gale Crater". NASA JPL. Retrieved 2011-07-22.
^ Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 2011-07-22.
^ Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Retrieved 2011-07-22.
^ "ExoMars: ESA and Roscosmos set for Mars missions". European Space Agency (ESA). 14 March 2013. Retrieved 2013-03-14.

External links

Template:Link GA Template:Link FA Template:Link GA Template:Link FA

[1] Mumma, Michael J. (January 8, 2012). The Search for Life on Mars. Origin of Life Gordon Research Conference. Galveston, TX.

[2] McKay, Christopher P.; Stoker, Carol R. (1989). "The early environment and its evolution on Mars: Implication for life". Reviews of Geophysics. 27 (2): 189–214. Bibcode:1989RvGeo..27..189M. doi:10.1029/RG027i002p00189.

[Fromproto-3] Gaidos, Eric; Selsis, Franck (2007). "From Protoplanets to Protolife: The Emergence and Maintenance of Life". Protostars and Planets V: 929–44. arXiv:astro-ph/0602008. Bibcode:2007prpl.conf..929G.

[SCI-20140124a-4] Grotzinger, John P. (January 24, 2014). "Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science. 343 (6169): 386–387. doi:10.1126/science.1249944. Retrieved January 24, 2014. {{cite journal}}: More than one of |number= and |issue= specified (help)

[SCI-20140124special-5] "Special Issue - Table of Contents - Exploring Martian Habitability". Science. 343 (6169): 345–452. January 24, 2014. Retrieved 24 January 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help)

[SCI-20140124-6] "Special Collection - Curiosity - Exploring Martian Habitability". Science. January 24, 2014. Retrieved January 24, 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help)

[SCI-20140124c-7] Grotzinger, J. P.; Sumner, D. Y.; Kah, L. C.; Stack, K.; Gupta, S.; Edgar, L.; Rubin, D.; Lewis, K.; Schieber, J.; Mangold, N.; Milliken, R.; Conrad, P. G.; Desmarais, D.; Farmer, J.; Siebach, K.; Calef, F.; Hurowitz, J.; McLennan, S. M.; Ming, D.; Vaniman, D.; Crisp, J.; Vasavada, A.; Edgett, K. S.; Malin, M.; Blake, D.; Gellert, R.; Mahaffy, P.; Wiens, R. C.; Maurice, S.; et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169): 1242777. doi:10.1126/science.1242777. Retrieved January 24, 2014. {{cite journal}}: Cite uses deprecated parameter |authors= (help); More than one of |number= and |issue= specified (help)

[8] Wallace, Alfred Russel (1907). Is Mars habitable?: A critical examination of Professor Percival Lowell's book 'Mars and its canals,' with an alternative explanation. London: Macmillan. OCLC 263175453.^{[page needed]}

[chambers-9] Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.^{[page needed]}

[2013_LPS-10] Conrad, P. G.; Archer, D.; Coll, P.; De La Torre, M.; Edgett, K.; Eigenbrode, J. L.; Fisk, M.; Freissenet, C.; Franz, H.; et al. (2013). "Habitability Assessment at Gale Crater: Implications from Initial Results". 44th Lunar and Planetary Science Conference. 1719: 2185. Bibcode:2013LPICo1719.2185C.

[D.C.Golden-11] Schuerger, Andrew C.; Golden, D. C.; Ming, Doug W. (2012). "Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions". Planetary and Space Science. 72 (1): 91–101. Bibcode:2012P&SS...72...91S. doi:10.1016/j.pss.2012.07.026.

[Beaty-12] MEPAG Special Regions-Science Analysis Group; Beaty, D.; Buxbaum, K.; Meyer, M.; Barlow, N.; Boynton, W.; Clark, B.; Deming, J.; Doran, P. T.; et al. (2006). "Findings of the Mars Special Regions Science Analysis Group". Astrobiology. 6 (5): 677–732. Bibcode:2006AsBio...6..677M. doi:10.1089/ast.2006.6.677. PMID 17067257.

[dust-up-13] "Mars Contamination Dust-Up". Astrobiology Magazine. 17 May 2010. Retrieved 2013-07-04. Whenever multiple biocidal factors are combined, the survival rates plummet quickly, {{cite web}}: |first= missing |last= (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)

[Biosignatures_2011-14] Summons, Roger E.; Amend, Jan P.; Bish, David; Buick, Roger; Cody, George D.; Des Marais, David J.; Dromart, Gilles; Eigenbrode, Jennifer L.; et al. (2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group". Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. PMID 21417945. There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance.

[Dehant-15] Dehant, V.; Lammer, H.; Kulikov, Y. N.; Grießmeier, J. -M.; Breuer, D.; Verhoeven, O.; Karatekin, Ö.; Hoolst, T.; et al. (2007). "Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars". Geology and Habitability of Terrestrial Planets. Space Sciences Series of ISSI. Vol. 24. pp. 279–300. doi:10.1007/978-0-387-74288-5_10. ISBN 978-0-387-74287-8.

[ancient_life-16] "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars". NASA. 12 March 2013. Retrieved 2013-06-06.

[NYT-20131209-17] Chang, Kenneth (December 9, 2013). "On Mars, an Ancient Lake and Perhaps Life". New York Times. Retrieved December 9, 2013.

[SCI-20131209-18] "Science - Special Collection - Curiosity Rover on Mars". Science. December 9, 2013. Retrieved December 9, 2013. {{cite journal}}: Cite uses deprecated parameter |authors= (help)

[19] Neal-Jones, Nancy (October 12, 2005). "New Map Provides More Evidence Mars Once Like Earth". Goddard Space Flight Center. NASA. Retrieved 5 June 2013. {{cite web}}: Unknown parameter |coauthor= ignored (|author= suggested) (help)

[20] "Martian Interior: Paleomagnetism". Mars Express. European Space Agency. 4 January 2007.

[Wall-21] Wall, Mike (25 March 2011). "Q & A with Mars Life-Seeker Chris Carr". Space.com. Retrieved 2011-03-25.

[cosmic_radiation-22] "Study: Surface of Mars Devoid of Life". Space.com. 29 January 2007. Retrieved 28 May 2013. After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation. {{cite web}}: |first= missing |last= (help)

[Dartnell-23] Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology". Geophysical Research Letters. 34 (2): L02207. Bibcode:2007GeoRL..34.2207D. doi:10.1029/2006GL027494. Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2 m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m.

[DartnellGeographic-24] Richard A. Lovet (February 2, 2007). "Mars Life May Be Too Deep to Find, Experts Conclude". National Geographic News. That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere. Cite error: The named reference "DartnellGeographic" was defined multiple times with different content (see the help page).

[Dartnell-1-25] Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology". Geophysical Research Letters. 34 (2). Bibcode:2007GeoRL..3402207D. doi:10.1029/2006GL027494. The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats.

[ionising_radiation-26] Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Martian sub-surface ionising radiation: biosignatures and geology". Biogeosciences. 4 (4): 545–558. Bibcode:2007BGeo....4..545D. doi:10.5194/bg-4-545-2007. This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. [..] Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.{{cite journal}}: CS1 maint: unflagged free DOI (link)

[Parnell-27] "Scientists find evidence Mars subsurface could hold life". Digital Journal – Science. 21 January. Retrieved 2013-06-05. There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell. {{cite news}}: |first= missing |last= (help); Check date values in: |date= (help)

[Steigerwald-28] Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center. NASA. Retrieved June 6 24, 2013. If microscopic Martian life is producing the methane, it probably resides far below the surface, where it's still warm enough for liquid water to exist {{cite news}}: Check date values in: |accessdate= (help)

[29] NASA – Mars Rovers Sharpen Questions About Livable Conditions

[30] "Mars: 'Strongest evidence' planet may have supported life, scientists say". BBC News. 20 January 2013. Retrieved 2013-01-22.

[31] Michalski, Joseph R.; Cuadros, Javier; Niles, Paul B.; Parnell, John; Deanne Rogers, A.; Wright, Shawn P. (2013). "Groundwater activity on Mars and implications for a deep biosphere". Nature Geoscience. 6 (2): 133–8. Bibcode:2013NatGe...6..133M. doi:10.1038/ngeo1706.

[subsurface_habitability_model-32] De Morais, A. (2012). "A Possible Biogeochemical Model for Mars". 43rd Lunar and Planetary Science Conference. 43: 2943. Bibcode:2012LPI....43.2943D. The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth.

[33] Anderson, Paul S. (15 December 2011). "New Study Says Large Regions of Mars Could Sustain Life". Universe Today. Retrieved 2013-06-05. Most scientists would agree that the best place that any organisms could hope to survive and flourish would be underground.

[34] "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 2013-06-06. If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.

[35] Hecht, Michael H.; Vasavada, Ashwin R. (2006). "Transient liquid water near an artificial heat source on Mars". International Journal of Mars Science and Exploration. 2: 83–96. Bibcode:2006IJMSE...2...83H. doi:10.1555/mars.2006.0006. In summary, on present-day Mars, liquid water is unlikely except as the result of a quick and dramatic change in environmental conditions such as from a landslide that exposes buried ice to sunlight (Costard et al. 2002), or from the introduction of an artificial heat source.

[Haberle-36] Haberle, Robert M.; McKay, Christopher P.; Schaeffer, James; Cabrol, Nathalie A.; Grin, Edmon A.; Zent, Aaron P.; Quinn, Richard (2001). "On the possibility of liquid water on present-day Mars". Journal of Geophysical Research: Planets. 106 (El0): 23317–26. Bibcode:bibcode=2001JGR...10623317H. doi:10.1029/2000JE001360. Introduction: The mean annual surface pressure and temperature on present-day Mars do not allow for the stability of liquid water on the surface. […] Conclusion: It is possible, even likely, that solar-heated liquid water never forms on present-day Mars. {{cite journal}}: Check |bibcode= length (help); Missing pipe in: |bibcode= (help)

[2013_conference-37] "Conference: The Present-Day Habitability of Mars 2013" (PDF). The UCLA Institute for Planets and Exoplanets. February 4–6, 2013. Retrieved 2013-06-17.

[38] Jones, E. G.; Lineweaver, C. H. (2012). "Using the phase diagram of liquid water to search for life". Australian Journal of Earth Sciences. 59 (2): 253–62. Bibcode:2012AuJES..59..253J. doi:10.1080/08120099.2011.591430.

[Lobitz-39] Lobitz, B.; Wood, BL; Averner, MM; McKay, CP (2001). "Use of spacecraft data to derive regions on Mars where liquid water would be stable". Proc. Natl. Acad. Sci. 98 (5): 2132–2137. Bibcode:2001PNAS...98.2132L. doi:10.1073/pnas.031581098. PMC 30104. PMID 11226204. These results do not indicate that water is present at these locations, only that, if it were present and heat sources were sufficient to bring the water in thermal equilibrium with the surface, the resulting liquid would be stable against freezing or boiling.

[Parro_2011-40] Parro, Victor Parro (December 2011). "A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars". Astroiology. 11 (10): 969–996. Bibcode:2011AsBio..11..969P. doi:10.1089/ast.2011.0654. {{cite journal}}: |format= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[41] Rod Pyle (25 February 2013). "Mars May Be Habitable Today, Scientists Say". space.com.

[42] "42nd Lunar and Planetary Science Conference" (Document). The Woodlands, Texas: Lunar and Planetary Institute. March 7–11, 2011{{inconsistent citations}} {{cite document}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |contribution-url= ignored (help); Unknown parameter |contribution= ignored (help); Unknown parameter |format= ignored (help)CS1 maint: postscript (link)

[Scarce_shelter-43] "Scarce Shelter on Mars". Astrobiology Magazine. 1 June 2009. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help)

[44] "Goal 1: Determine if Life Ever Arose On Mars". NASA. Retrieved 2013-06-29. {{cite web}}: |first= missing |last= (help)

[45] Luhmann, J. G.; Russell, C. T. (1997). "Mars: Magnetic Field and Magnetosphere". In Shirley, J. H.; Fainbridge, R. W. (eds.). Encyclopedia of Planetary Sciences. New York: Chapman and Hall. pp. 454–6. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)

[46] Phillips, Tony (January 31, 2001). "The Solar Wind at Mars". NASA.

[hostile_to_life-47] "What makes Mars so hostile to life?". BBC News. 7 January 2013. Retrieved 2013-06-15.

[48] Keating, A. Keating (November 2012). "The impact of Mars geological evolution in high energy ionizing radiation environment through time". Planetary and Space Science – Eslevier. 72 (1): 70–77. Bibcode:2012P&SS...72...70K. doi:10.1016/j.pss.2012.04.009. Retrieved 2013-06-28. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[49] "IMPLICATIONS OF COSMIC RADIATION ON THE MARTIAN SURFACE FOR MICROBIAL SURVIVAL AND DETECTION OF FLUORESCENT BIOSIGNATURES" (PDF). Lunar and Planetary Institute. 2011. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[50] Hassler, Donald M.; Zeitlin, Cary; Wimmer-Schweingruber, Robert F.; Ehresmann, Bent; Rafkin, Scot; Martin, Cesar; Boettcher, Stephan; Koehler, Jan; Guo, Jingnan; et al. (2013). "The Radiation Environment on the Martian Surface and during MSL's Cruise to Mars". EGU General Assembly 2013. 15: 12596. Bibcode:2013EGUGA..1512596H.

[RAD_January_2014-51] Hassler, Donald M. (24 January 2014). "Mars' Surface Radiation Environment Measured with the Mars ScienceLaboratory's Curiosity Rover" (PDF). Science. 343 (6169): 1244797. doi:10.1126/science.1244797. PMID 24324275. Retrieved 2014-01-27. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[Icebreaker2018-52] McKay, Christopher P. McKay (April 5, 2013). "The Icebreaker Life Mission to Mars: A Search for Biomolecular Evidence for Life". Astrobiology. 13 (4): 334–353. Bibcode:2013AsBio..13..334M. doi:10.1089/ast.2012.0878. PMID 23560417. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[Serratia-53] Schuerger, Andrew C. Schuerger (February 2013). "Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres". Astrobiology. 13 (2): 115–131. Bibcode:2013AsBio..13..115S. doi:10.1089/ast.2011.0811. PMC 3582281. PMID 23289858. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[54] Heldmann, Jennifer L.; Toon, Owen B.; Pollard, Wayne H.; Mellon, Michael T.; Pitlick, John; McKay, Christopher P.; Andersen, Dale T. (2005). "Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions". Journal of Geophysical Research. 110 (E5): E05004. Bibcode:2005JGRE..11005004H. doi:10.1029/2004JE002261.

[55] Kostama, V.-P.; Kreslavsky, M. A.; Head, J. W. (2006). "Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement". Geophysical Research Letters. 33 (11): 11201. Bibcode:2006GeoRL..3311201K. doi:10.1029/2006GL025946.

[56] Hecht, Michael H.; Vasavada, Ashwin R. (2006). "Transient liquid water near an artificial heat source on Mars". International Journal of Mars Science and Exploration. 2: 83. Bibcode:2006IJMSE...2...83H. doi:10.1555/mars.2006.0006.

[Shiga2009-57] Shiga, David (December 7, 2009). "Watery niche may foster life on Mars". New Scientist.

[news.softpedia-58] Tudor Vieru (2009-12-07). "Greenhouse Effect on Mars May Be Allowing for Life". News.softpedia.com. Retrieved 2011-08-20.^{[unreliable source?]}

[59] Michael T. Mellon Subsurface Ice at Mars: A review of ice and water in the equatorial regions University of Colorado 10 May 2011 Planetary Protection Subcommittee Meeting

[60] Robert Roy Britt Ice Packs and Methane on Mars Suggest Present Life Possible space.com 22 February 2005

[61] Mellon, Michael T.; Jakosky, Bruce M.; Postawko, Susan E. (1997). "The persistence of equatorial ground ice on Mars". Journal of Geophysical Research. 102: 19357–69. Bibcode:1997JGR...10219357M. doi:10.1029/97JE01346.

[62] Arfstrom, J. D. (2012). "A Conceptual Model of Equatorial Ice Sheets on Mars". Comparative Climatology of Terrestrial Planets. 1675: 8001. Bibcode:2012LPICo1675.8001A.

[mars.jpl.nasa.gov-63] "Mars Odyssey: Newsroom". Mars.jpl.nasa.gov. May 28, 2002. Retrieved December 19, 2010.

[Feildman,_T._2004-64] Feldman, W. C. (2004). "Global distribution of near-surface hydrogen on Mars". Journal of Geophysical Research. 109. Bibcode:2004JGRE..10909006F. doi:10.1029/2003JE002160.

[ucar-65] "Mars Global Surveyor Measures Water Clouds". Retrieved March 7, 2009.

[Baker-66] Baker, V. R.; Strom, R. G.; Gulick, V. C.; Kargel, J. S.; Komatsu, G.; Kale, V. S. (1991). "Ancient oceans, ice sheets and the hydrological cycle on Mars". Nature. 352 (6336): 589. Bibcode:1991Natur.352..589B. doi:10.1038/352589a0.

[flashback-67] "Flashback: Water on Mars Announced 10 Years Ago". SPACE.com. June 22, 2000. Retrieved December 19, 2010.

[68] "Science@NASA, The Case of the Missing Mars Water". Retrieved March 7, 2009.

[Clay_clues-69] "Mars Rover Opportunity Examines Clay Clues in Rock". NASA. Jet Propulsion Laboratory. May 17, 2013. Retrieved 2013-06-14.

[70] "NASA Rover Helps Reveal Possible Secrets of Martian Life". NASA. November 29, 2005. Retrieved 2013-06-20.

[71] ISBN 0-312-24551-3^{[page needed]}^{[full citation needed]}

[Sea-72] "PSRD: Ancient Floodwaters and Seas on Mars". Psrd.hawaii.edu. July 16, 2003. Retrieved December 19, 2010.

[73] "Gamma-Ray Evidence Suggests Ancient Mars Had Oceans". SpaceRef. November 17, 2008. Retrieved December 19, 2010.

[2003JGRE..108.5042C-74] Carr, Michael H.; Head, James W. (2003). "Oceans on Mars: An assessment of the observational evidence and possible fate". Journal of Geophysical Research (Planets). 108: 5042. Bibcode:2003JGRE..108.5042C. doi:10.1029/2002JE001963.

[SFN-20130125-75] Harwood, William (January 25, 2013). "Opportunity rover moves into 10th year of Mars operations". Space Flight Now. Retrieved February 18, 2013.

[ReferenceA-76] Di Achille, Gaetano; Hynek, Brian M. (2010). "Ancient ocean on Mars supported by global distribution of deltas and valleys". Nature Geoscience. 3 (7): 459–63. Bibcode:2010NatGe...3..459D. doi:10.1038/ngeo891. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)

[Smith-77] Smith, D. E.; Sjogren, W. L.; Tyler, G. L.; Balmino, G.; Lemoine, F. G.; Konopliv, A. S. (1999). "The gravity field of Mars: Results from Mars Global Surveyor". Science. 286 (5437): 94–7. Bibcode:1999Sci...286...94S. doi:10.1126/science.286.5437.94. PMID 10506567.

[78] Tosca, Nicholas J.; Knoll, Andrew H.; McLennan, Scott M. (2008). "Water Activity and the Challenge for Life on Early Mars". Science. 320 (5880): 1204–7. Bibcode:2008Sci...320.1204T. doi:10.1126/science.1155432. PMID 18511686.

[79] DasSarma, Shiladitya (2006). "Extreme Halophiles Are Models for Astrobiology". Microbe. 1 (3): 120–6.

[underground-80] Malin, Michael C.; Edgett, Kenneth S. (2000). "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars". Science. 288 (5475): 2330–5. Bibcode:2000Sci...288.2330M. doi:10.1126/science.288.5475.2330. PMID 10875910.

[81] Martínez, G. M.; Renno, N. O.; Elliott, H. M.; Fischer, E. (2013). Present Day Liquid Water On Mars: Theoretical Expectations, Observational Evidence And Preferred Locations (PDF). The Present-day Mars Habitability Conference. Los Angeles.

[82] Kolb, K.; Pelletier, Jon D.; McEwen, Alfred S. (2010). "Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water". Icarus. 205: 113–137. Bibcode:2010Icar..205..113K. doi:10.1016/j.icarus.2009.09.009.

[ReferenceB-83] "Icarus | Vol 218, Iss 1, In Progress, (March, 2012)". ScienceDirect.com. 2004-06-08. Retrieved 2012-01-16.^{[page needed]}

[moon-84] "University of Arizona Press Release" March 16, 2006.

[85] Kerr, Richard (2006-12-08). "Mars Orbiter's Swan Song: The Red Planet Is A-Changin'". Science. 314 (5805): 1528–1529. doi:10.1126/science.314.5805.1528. PMID 17158298.

[voanews-86] "NASA Finds Possible Signs of Flowing Water on Mars". voanews.com. Retrieved August 5, 2011.

[Ames-87] Source: Ames Research Center Posted Saturday, June 6, 2009 (June 6, 2009). "NASA Scientists Find Evidence for Liquid Water on a Frozen Early Mars". SpaceRef. Retrieved December 19, 2010.{{cite web}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)

[88] "Dead Spacecraft on Mars Lives on in New Study". SPACE.com. June 10, 2008. Retrieved December 19, 2010.

[hirise-89] McEwen, Alfred S.; Ojha, Lujendra; Dundas, Colin M.; Mattson, Sarah S.; Byrne, Shane; Wray, James J.; Cull, Selby C.; Murchie, Scott L.; et al. (2011). "Seasonal Flows on Warm Martian Slopes". Science. 333 (6043): 740–3. Bibcode:2011Sci...333..740M. doi:10.1126/science.1204816. PMID 21817049.

[90] "Mars Rover Spirit Unearths Surprise Evidence of Wetter Past" (Press release). Jet Propulsion Laboratory. May 21, 2007. Retrieved June 27, 2013.

[20071210a-91] "Mars Rover Investigates Signs of Steamy Martian Past" (Press release). Jet Propulsion Laboratory. December 10, 2007. Retrieved June 27, 2013.

[Leveille-92] Leveille, R. J. (2010). "Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars". American Geophysical Union. 12: 07. Bibcode:2010AGUFM.P12A..07L.

[93] Walter, M. R.; Des Marais, David J. (1993). "Preservation of Biological Information in Thermal Spring Deposits: Developing a Strategy for the Search for Fossil Life on Mars". Icarus. 101 (1): 129–43. Bibcode:1993Icar..101..129W. doi:10.1006/icar.1993.1011. PMID 11536937.

[94] Allen, Carlton C.; Albert, Fred G.; Chafetz, Henry S.; Combie, Joan; Graham, Catherine R.; Kieft, Thomas L.; Kivett, Steven J.; McKay, David S.; et al. (2000). "Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars". Icarus. 147 (1): 49–67. Bibcode:2000Icar..147...49A. doi:10.1006/icar.2000.6435. PMID 11543582.

[95] Wade, Manson L.; Agresti, David G.; Wdowiak, Thomas J.; Armendarez, Lawrence P.; Farmer, Jack D. (1999). "A Mössbauer investigation of iron-rich terrestrial hydrothermal vent systems: Lessons for Mars exploration". Journal of Geophysical Research. 104 (E4): 8489–507. Bibcode:1999JGR...104.8489W. doi:10.1029/1998JE900049. PMID 11542933.

[96] Agresti, D. G.; Wdowiak, T. J.; Wade, M. L.; Armendarez, L. P.; Farmer, J. D. (1995). "A Mossbauer Investigation of Hot Springs Iron Deposits". Abstracts of the Lunar and Planetary Science Conference. 26: 7. Bibcode:1995LPI....26....7A.

[97] Agresti, D. G.; Wdowiak, T. J.; Wade, M. L.; Armendarez, L. P. (1997). "Mössbauer Spectroscopy of Thermal Springs Iron Deposits as Martian Analogs". Early Mars: Geologic and Hydrologic Evolution. 916: 1. Bibcode:1997LPICo.916....1A.

[98] Mumma, M. J.; Novak, R. E.; Disanti, M. A.; Bonev, B. P. (2003). "A Sensitive Search for Methane on Mars". American Astronomical Society. 35: 937. Bibcode:2003DPS....35.1418M.

[99] Mumma, Michael J.; Villanueva, Geronimo L.; Novak, Robert E.; Hewagama, Tilak; Bonev, Boncho P.; Disanti, Michael A.; Mandell, Avi M.; Smith, Michael D. (2009). "Strong Release of Methane on Mars in Northern Summer 2003". Science. 323 (5917): 1041–5. Bibcode:2009Sci...323.1041M. doi:10.1126/science.1165243. PMID 19150811.

[100] Formisano, Vittorio; Atreya, Sushil; Encrenaz, Thérèse; Ignatiev, Nikolai; Giuranna, Marco (2004). "Detection of Methane in the Atmosphere of Mars". Science. 306 (5702): 1758–61. Bibcode:2004Sci...306.1758F. doi:10.1126/science.1101732. PMID 15514118.

[101] Krasnopolsky, Vladimir A.; Maillard, Jean Pierre; Owen, Tobias C. (2004). "Detection of methane in the martian atmosphere: Evidence for life?". Icarus. 172 (2): 537–47. Bibcode:2004Icar..172..537K. doi:10.1016/j.icarus.2004.07.004.

[102] "Mars Express confirms methane in the Martian atmosphere" (Press release). ESA. March 30, 2004. Retrieved March 17, 2006.

[103] Moran, Mark; Miller, Joseph D.; Kral, Tim; Scott, Dave (2005). "Desert methane: Implications for life detection on Mars". Icarus. 178: 277–80. Bibcode:2005Icar..178..277M. doi:10.1016/j.icarus.2005.06.008.

[104] Krasnopolsky, Vladimir A. (2006). "Some problems related to the origin of methane on Mars". Icarus. 180 (2): 359–67. Bibcode:2006Icar..180..359K. doi:10.1016/j.icarus.2005.10.015.

[105] Planetary Fourier Spectrometer website (ESA, Mars Express)^{[verification needed]}

[thermal_signature-106] "Hunting for young lava flows". Geophysical Research Letters. Red Planet. June 1, 2011. Retrieved 2013-10-04.

[107] Court, Richard W.; Sephton, Mark A. (2009). "Investigating the contribution of methane produced by ablating micrometeorites to the atmosphere of Mars". Earth and Planetary Science Letters. 288 (3–4): 382–5. Bibcode:2009E&PSL.288..382C. doi:10.1016/j.epsl.2009.09.041. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)

[108] Keppler, Frank; Vigano, Ivan; McLeod, Andy; Ott, Ulrich; Früchtl, Marion; Röckmann, Thomas (2012). "Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere". Nature. 486 (7401): 93–6. Bibcode:2012Natur.486...93K. doi:10.1038/nature11203. PMID 22678286.

[Kral-109] Kral, T. A.; Goodhart, T.; Howe, K. L.; Gavin, P. (2009). "Can Methanogens Grow in a Perchlorate Environment on Mars?". 72nd Annual Meeting of the Meteoritical Society. 72: 5136. Bibcode:2009M&PSA..72.5136K.

[Howe-110] Howe, K. L.; Gavin, P.; Goodhart, T.; Kral, T. A. (2009). "Methane Production by Methanogens in Perchlorate-supplemented Media". 40th Lunar and Planetary Science Conference. 40: 1287. Bibcode:2009LPI....40.1287H.

[Levin_2009-111] Levin, Gilbert V.; Straat, Patricia Ann (2009). "Methane and life on Mars". In Hoover, Richard B; Levin, Gilbert V; Rozanov, Alexei Y; Retherford, Kurt D (eds.). Instruments and Methods for Astrobiology and Planetary Missions XII. Vol. 7441. pp. 12–27. Bibcode:2009SPIE.7441E..12L. doi:10.1117/12.829183. ISBN 978-0-8194-7731-6.

[PNAS-20120607-112] Oze, Christopher; Jones, Camille; Goldsmith, Jonas I.; Rosenbauer, Robert J. (June 7, 2012). "Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces". PNAS. 109 (25): 9750–9754. Bibcode:2012PNAS..109.9750O. doi:10.1073/pnas.1205223109. PMC 3382529. PMID 22679287.

[Space-20120625-113] Staff (June 25, 2012). "Mars Life Could Leave Traces in Red Planet's Air: Study". Space.com. Retrieved June 27, 2012.

[Nature-20120627-114] Brogi, Matteo; Snellen, Ignas A. G.; de Krok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernest J. W. (June 28, 2012). "The signature of orbital motion from the dayside of the planet τ Boötis b". Nature. 486 (7404): 502–504. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161.

[Wired-20120627-115] Mann, Adam (June 27, 2012). "New View of Exoplanets Will Aid Search for E.T." Wired (magazine). Retrieved June 28, 2012.

[116] "Is there Methane on Mars? Part II" (Document). Lunar and Planetary Institute. 2011{{inconsistent citations}} {{cite document}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |contribution-url= ignored (help); Unknown parameter |contribution= ignored (help); Unknown parameter |format= ignored (help)CS1 maint: postscript (link)

[117] "Methane on Mars. Now you see it. Now you don't..." The Economist. 29 December 2010. Retrieved 2011-08-06.

[118] Kevin Zahnle, Richard S. Freedman and David C. Catling (2011). "Is there methane on Mars?". Icarus. 212 (2): 493–503. Bibcode:2011Icar..212..493Z. doi:10.1016/j.icarus.2010.11.027.

[119] Tenenbaum, David (June 9, 2008). "Making Sense of Mars Methane". Astrobiology Magazine. Archived from the original on 23 September 2008. Retrieved October 8, 2008. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)

[120] Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center. NASA. Archived from the original on 17 January 2009. Retrieved January 24, 2009. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)

[121] "Mars Curiosity Rover News Telecon -November 2, 2012".

[Science-20121102-122] Kerr, Richard A. (November 2, 2012). "Curiosity Finds Methane on Mars, or Not". Science (journal). Retrieved November 3, 2012.

[Space-20121102-123] Wall, Mike (November 2, 2012). "Curiosity Rover Finds No Methane on Mars — Yet". Space.com. Retrieved November 3, 2012.

[NYT-20121102-124] Chang, Kenneth (November 2, 2012). "Hope of Methane on Mars Fades". New York Times. Retrieved November 3, 2012.

[Wired-20130718-125] Mann, Adam (18 July 2013). "Mars Rover Finds Good News for Past Life, Bad News for Current Life on Mars". Wired (magazine). Retrieved 19 July 2013.

[SCI-20130719a-126] Webster Chris R.; et al. (19 July 2013). "Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere". Science. 341 (6143): 260–263. doi:10.1126/science.1237961. Retrieved 19 July 2013. {{cite journal}}: Explicit use of et al. in: |author= (help)

[SCI-20130719b-127] Mahaffy, Paul R.; et al. (19 July 2013). "Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover". Science. 341 (6143): 263–266. doi:10.1126/science.1237966. Retrieved 19 July 2013. {{cite journal}}: Explicit use of et al. in: |author= (help)

[SJ-20130919-128] Webster, Christopher R.; Mahaffy, Paul R.; Atreya, Sushil K.; Flesch, Gregory J.; Farley, Kenneth A. (September 19, 2013). "Low Upper Limit to Methane Abundance on Mars". Science. doi:10.1126/science.1242902. Retrieved September 19, 2013.

[SCI-20130919-129] Cho, Adrian (September 19, 2013). "Mars Rover Finds No Evidence of Burps and Farts". Science (journal). Retrieved September 19, 2013.

[NYT-20130919-130] Chang, Kenneth (September 19, 2013). "Mars Rover Comes Up Empty in Search for Methane". New York Times. Retrieved September 19, 2013.

[131] Rincon, Paul (July 9, 2009). "Agencies outline Mars initiative". BBC News. Retrieved July 26, 2009.

[132] "NASA orbiter to hunt for source of Martian methane in 2016". Thaindian News. Asian News International. March 6, 2009. Retrieved July 26, 2009.

[133] Peplow, Mark (February 25, 2005). "Formaldehyde claim inflames martian debate". Nature. doi:10.1038/news050221-15.

[134] Hogan, Jenny (February 16, 2005). "A whiff of life on the Red Planet". New Scientist magazine. Retrieved August 18, 2008.

[PFS-135] Peplow, Mark (September 7, 2005). "Martian methane probe in trouble". Nature. doi:10.1038/news050905-10.

[NASA_Releasease_:_05-052-136] "NASA Statement on False Claim of Evidence of Life on Mars". NASA News. NASA. February 18, 2005. Retrieved August 18, 2008.

[137] "Mars Meteorites". NASA. Retrieved February 16, 2010.

[meteoritos-Bio-138] ^ ^a ^b ^c ^d ^e ^f Evidence for ancient Martian life. E. K. Gibson Jr., F. Westall, D. S. McKay, K. Thomas-Keprta, S. Wentworth, and C. S. Romanek, Mail Code SN2, NASA Johnson Space Center, Houston TX 77058, USA.

[139] "Allan Hills 84001". The Meteorolitical Society. April 2008. Retrieved August 21, 2008.

[disbelief-140] Crenson, Matt (August 6, 2006). "After 10 years, few believe life on Mars". Associated Press (on space.com). Retrieved August 6, 2006. {{cite web}}: External link in |publisher= (help)^{[dead link]}

[141] McKay, David S.; Gibson, Everett K.; Thomas-Keprta, Kathie L.; Vali, Hojatollah; Romanek, Christopher S.; Clemett, Simon J.; Chillier, Xavier D. F.; Maechling, Claude R.; Zare, Richard N. (1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–30. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069.

[142] "New Study Adds to Finding of Ancient Life Signs in Mars Meteorite". NASA. NASA News. 30 November 2009. Retrieved 2013-06-04. {{cite news}}: |first= missing |last= (help)

[143] Thomas-Keprta, K. L.; Clemett, S. J.; McKay, D. S.; Gibson, E. K.; Wentworth, S. J. (2009). "Origins of magnetite nanocrystals in Martian meteorite ALH84001". Geochimica et Cosmochimica Acta. 73 (21): 6631–77. Bibcode:2009GeCoA..73.6631T. doi:10.1016/j.gca.2009.05.064.

[NASA-144] Baalke, Ron (1995). "The Nakhla Meteorite". Jet Propulsion Lab. NASA. Retrieved August 17, 2008.

[145] "Rotating image of a Nakhla meteorite fragment". London Natural History Museum. 2008. Retrieved August 17, 2008.

[146] Rincon, Paul (February 8, 2006). "Space rock re-opens Mars debate". BBC News. Retrieved August 17, 2008.

[147] Meyer, C. (2004). "Mars Meteorite Compendium" (PDF). NASA. Retrieved August 21, 2008.

[148] Whitehouse, David (August 27, 1999). "Life on Mars – new claims". BBC News. Retrieved August 20, 2008.

[149] Compilation of scientific research references on the Nakhla meteorite: http://curator.jsc.nasa.gov/antmet/marsmets/nakhla/references.cfm

[150] Shergoti Meteorite – JPL, NASA

[NASA-20140227-151] Webster, Guy (February 27, 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved February 27, 2014.

[AJ-20140219-152] White, Lauren M.; Gibson, Everett K.; Thomnas-Keprta, Kathie L.; Clemett, Simon J.; McKay, David (February 19, 2014). "Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593". Astrobiology. 14 (2): 170–181. doi:10.1089/ast.2011.0733. Retrieved February 27, 2014.

[SP-20140228-153] Gannon, Megan (February 28, 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com. Retrieved February 28, 2014.

[2006-100-154] "NASA Findings Suggest Jets Bursting From Martian Ice Cap". Jet Propulsion Laboratory. NASA. August 16, 2006. Retrieved August 11, 2009.

[Kieffer2000-155] Kieffer, H. H. (2000). "Annual Punctuated CO2 Slab-Ice and Jets on Mars". International Conference on Mars Polar Science and Exploration: 93. Bibcode:2000mpse.conf...93K.

[Portyankina-156] Portyankina, G.; Markiewicz, W. J.; Garcia-Comas, M.; Keller, H. U.; Bibring, J.-P.; Neukum, G. (2006). "Simulations of Geyser-type Eruptions in Cryptic Region of Martian South Polar Cap". Fourth International Conference on Mars Polar Science and Exploration. 1323: 8040. Bibcode:2006LPICo1323.8040P.

[Hugh2006-157] Kieffer, Hugh H.; Christensen, Philip R.; Titus, Timothy N. (2006). "CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap". Nature. 442 (7104): 793–6. Bibcode:2006Natur.442..793K. doi:10.1038/nature04945. PMID 16915284.

[Andras-158] Horváth, A.; Gánti, T.; Gesztesi, A.; Bérczi, Sz.; Szathmáry, E. (2001). "Probable Evidences of Recent Biological Activity on Mars: Appearance and Growing of Dark Dune Spots in the South Polar Region". 32nd Annual Lunar and Planetary Science Conference. 32: 1543. Bibcode:2001LPI....32.1543H.

[Pócs_et_al_2004-159] Pócs, T.; Horváth, A.; Gánti, T.; Bérczi, Sz.; Szathemáry, E. (2004). "Possible crypto-biotic-crust on Mars?". Proceedings of the Third European Workshop on Exo-Astrobiology. 545: 265–6. Bibcode:2004eab..conf..265P.

[160] Gánti, Tibor; Horváth, András; Bérczi, Szaniszló; Gesztesi, Albert; Szathmáry, Eörs (2003). "Dark Dune Spots: Possible Biomarkers on Mars?". Origins of Life and Evolution of the Biosphere. 33 (4/5): 515–57. doi:10.1023/A:1025705828948.

[Planetary-161] Horváth, A.; Gánti, T.; Bérczi, Sz.; Gesztesi, A.; Szathmáry, E. (2002). "Morphological Analysis of the Dark Dune Spots on Mars: New Aspects in Biological Interpretation". 33rd Annual Lunar and Planetary Science Conference. 33: 1108. Bibcode:2002LPI....33.1108H.

[162] András Sik, Ákos Kereszturi. "Dark Dune Spots – Could it be that it's alive?". Monochrom. Retrieved September 4, 2009. (Audio interview, MP3 6 min.)

[Orme-163] Orme, Greg M.; Ness, Peter K. (June 9, 2003). "Martian Spiders" (PDF). Marsbugs. 10 (23): 5–7. Archived from the original (PDF) on September 27, 2007. Retrieved September 6, 2009.

[Manrubia-164] Manrubia, S. C.; Prieto Ballesteros, O.; González Kessler, C.; Fernández Remolar, D.; Córdoba-Jabonero, C.; Selsis, F.; Bérczi, S.; Gánti, T.; Horváth, A. (2004). "Comparative analysis of geological features and seasonal processes in 'Inca City' and 'Pityusa Patera' regions on Mars". Proceedings of the Third European Workshop on Exo-Astrobiology. 545: 77–80. Bibcode:2004eab..conf...77M. ISBN 92-9092-856-5.

[Ness-165] Ness, Peter K.; Orme, Greg M. (2002). "Spider-Ravine Models and Plant-Like Features on Mars – Possible Geophysical and Biogeophysical Modes of Origin". Journal of the British Interplanetary Society. 55 (3/4): 85–108. Bibcode:2002JBIS...55...85N.

[166] Landis, Geoffrey; Oleson, Steven; McGuire, Melissa (2012). Design Study for a Mars Geyser Hopper. 50th AIAA Aerospace Sciences Meeting. Nashville. doi:10.2514/6.2012-631. {{cite conference}}: Unknown parameter |doi_brokendate= ignored (|doi-broken-date= suggested) (help)

[167] Committee on an Astrobiology Strategy for the Exploration of Mars; National Research Council (2007). "Planetary Protection for Mars Missions". An Astrobiology Strategy for the Exploration of Mars. The National Academies Press. pp. 95–8. ISBN 978-0-309-10851-5. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)

[Cowing-168] Cowing, Keith (April 11, 2013). "Planetary Protection: A Work in Progress". Astrobiology. Retrieved 2013-06-02. Cite error: The named reference "Cowing" was defined multiple times with different content (see the help page).

[Debus-169] Debus, A. (2005). "Estimation and assessment of Mars contamination". Advances in Space Research. 35 (9): 1648–53. Bibcode:2005AdSpR..35.1648D. doi:10.1016/j.asr.2005.04.084. PMID 16175730.

[Planetary_protection_–_radiodurans-170] Dartnell, Lewis R.; Hunter, Stephanie J.; Lovell, Keith V.; Coates, Andrew J.; Ward, John M. (2010). "Low-Temperature Ionizing Radiation Resistance of Deinococcus radiodurans and Antarctic Dry Valley Bacteria". Astrobiology. 10 (7): 717–32. Bibcode:2010AsBio..10..717D. doi:10.1089/ast.2009.0439. PMID 20950171.

[171] Vega, U. Pogoda; Rettberg, P.; Reitz, G. (2007). "Simulation of the environmental climate conditions on martian surface and its effect on Deinococcus radiodurans". Advances in Space Research. 40 (11): 1672–7. Bibcode:2007AdSpR..40.1672D. doi:10.1016/j.asr.2007.05.022.

[172] "Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres". Astrobiology. 13 (2): 115–131. February 2013. Bibcode:2013AsBio..13..115S. doi:10.1089/ast.2011.0811. Retrieved 2013-07-04. {{cite journal}}: |first= missing |last= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[173] Vera, Jean-Pierre; Möhlmann, Diedrich; Butina, Frederike; Lorek, Andreas; Wernecke, Roland; Ott, Sieglinde (2010). "Survival Potential and Photosynthetic Activity of Lichens Under Mars-Like Conditions: A Laboratory Study". Astrobiology. 10 (2): 215–27. Bibcode:2010AsBio..10..215D. doi:10.1089/ast.2009.0362. PMID 20402583.

[EGU-20120426-174] Vera, J.-P. P.; Schulze-Makuch, D.; Khan, A.; Lorek, A.; Koncz, A.; Möhlmann, D.; Spohn, T. (2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars". EGU General Assembly 2012. 14: 2113. Bibcode:2012EGUGA..14.2113D.

[dlrMarsStudy-175] Surviving the conditions on Mars DLR, 26 April 2012

[176] Vera, Jean-Pierre (2012). "Lichens as survivors in space and on Mars". Fungal Ecology. 5 (4): 472–9. doi:10.1016/j.funeco.2012.01.008.

[177] R. de la Torre Noetzel, F.J. Sanchez Inigo, E. Rabbow, G. Horneck, J. P. de Vera, L.G. Sancho Survival of lichens to simulated Mars conditions^{[self-published source?]}

[178] Sánchez, F. J.; Mateo-Martí, E.; Raggio, J.; Meeßen, J.; Martínez-Frías, J.; Sancho, L. G.; Ott, S.; de la Torre, R. (2012). "The resistance of the lichen Circinaria gyrosa (nom. Provis.) towards simulated Mars conditions—a model test for the survival capacity of an eukaryotic extremophile". Planetary and Space Science. 72 (1): 102–10. Bibcode:2012P&SS...72..102S. doi:10.1016/j.pss.2012.08.005.

[179] Strom, R.G., Steven K. Croft, and Nadine G. Barlow, "The Martian Impact Cratering Record," Mars, University of Arizona Press, ISBN 0-8165-1257-4, 1992.^{[page needed]}

[180] Raeburn, P. 1998. Uncovering the Secrets of the Red Planet Mars. National Geographic Society. Washington D.C.^{[page needed]}

[181] Moore, P. et al. 1990. The Atlas of the Solar System. Mitchell Beazley Publishers NY, NY.^{[page needed]}

[BC-182] "Astrobiology". Biology Cabinet. September 26, 2006. Retrieved 2011-01-17.

[PlaxcoGross2011-183] Kevin W. Plaxco; Michael Gross (2011). Astrobiology: A Brief Introduction. JHU Press. pp. 282–283. ISBN 978-1-4214-0194-2. Cite error: The named reference "PlaxcoGross2011" was defined multiple times with different content (see the help page).

[cnn-184] Richard Stenger Mars sample return plan carries microbial risk, group warns, CNN, November 7, 2000

[Plaxco2006-185] Kevin W. Plaxco and Michael Gross (2006). Astrobiology: A Brief Introduction. JHU Press. p. 223. ISBN 978-0-8018-8366-8.

[PlaxcoGross2011_2-186] Kevin W. Plaxco; Michael Gross (2011). Astrobiology: A Brief Introduction (2nd ed.). JHU Press. pp. 285–286. ISBN 978-1-4214-0194-2.

[Levin-187] Levin, Gilbert V. (2007). "Analysis of evidence of Mars life". Electroneurobiología. 15 (2): 39–47. arXiv:0705.3176. Bibcode:2007arXiv0705.3176L.

[188] Klein, Harold P.; Horowitz, Norman H.; Levin, Gilbert V.; Oyama, Vance I.; Lederberg, Joshua; Rich, Alexander; Hubbard, Jerry S.; Hobby, George L.; Straat, Patricia A. (1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090.

[Bianciardi-2012-189] Bianciardi, Giorgio; Miller, Joseph D.; Straat, Patricia Ann; Levin, Gilbert V. (2012). "Complexity Analysis of the Viking Labeled Release Experiments". International Journal of Aeronautical and Space Sciences. 13 (1): 14–26. Bibcode:2012IJASS..13...14B. doi:10.5139/IJASS.2012.13.1.14.

[190] Life on Mars Found by NASA's Viking Mission?

[Discovery-20120412-191] Klotz, Irene (12 April 2012). "Mars Viking Robots 'Found Life'". DiscoveryNews. Retrieved 16 April 2012.

[Navarro-192] Navarro-González, Rafael; Navarro, Karina F.; de la Rosa, José; Iñiguez, Enrique; Molina, Paola; Miranda, Luis D.; Morales, Pedro; Cienfuegos, Edith; Coll, Patrice (2006). "The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results". Proceedings of the National Academy of Sciences. 103 (44): 16089–94. Bibcode:2006PNAS..10316089N. doi:10.1073/pnas.0604210103. JSTOR 30052117. PMC 1621051. PMID 17060639.

[193] Biemann, Klaus (2007). "On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter". Proceedings of the National Academy of Sciences of the United States of America. 104 (25): 10310–10313. Bibcode:2007PNAS..10410310B. doi:10.1073/pnas.0703732104. PMC 1965509. PMID 17548829.

[Webster-194] Webster, Guy; Hoover, Rachel; Marlaire, Ruth; Frias, Gabriela (2010-09-03). "Missing Piece Inspires New Look at Mars Puzzle". NASA Jet Propulsion Laboratory. Retrieved 2010-10-24.

[195] :10.1029/2011JE003869

[196] :10.1029/2011JE003880

[197] Crocco, Mario (April 14, 2007). "Los taxones mayores de la vida orgánica y la nomenclatura de la vida en Marte: primera clasificación biológica de un organismo marciano". Electroneurobiología (in Spanish). 15 (2): 1–34. Retrieved August 14, 2008. {{cite journal}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)

[198] What Life Leaves Behind by Veronique Greenwood.

[199] Curiosity tasked with hunting for elusive Mars organics. Astronomy Now. 22 May 2013

[MarsDaily-200] Piecing Together Life's Potential

[201] "NASA Spacecraft Confirms Perchlorate on Mars". NASA. NASA. August 5, 2008. Retrieved January 28, 2009.

[202] Johnson, John (August 6, 2008). "Perchlorate found in Martian soil". Los Angeles Times.

[203] "Martian Life Or Not? NASA's Phoenix Team Analyzes Results". Science Daily. August 6, 2008.

[204] Lakdawalla, Emily (June 26, 2008). "Phoenix sol 30 update: Alkaline soil, not very salty, "nothing extreme" about it!". The Planetary Society weblog. Planetary Society. Retrieved June 26, 2008. {{cite web}}: External link in |work= (help)

[205] "Mars Science Laboratory Launch". 26 November 2011. Retrieved 2011-11-26.

[NYT-MSL-206] Associated Press (26 November 2011). "NASA Launches Super-Size Rover to Mars: 'Go, Go!'". New York Times. Retrieved 2011-11-26.

[IAU-20120516-207] USGS (16 May 2012). "Three New Names Approved for Features on Mars". USGS. Retrieved 28 May 2012.

[NASA-20120327-208] NASA Staff (27 March 2012). "'Mount Sharp' on Mars Compared to Three Big Mountains on Earth". NASA. Retrieved 31 March 2012.

[NASA-20120328-209] Agle, D. C. (28 March 2012). "'Mount Sharp' On Mars Links Geology's Past and Future". NASA. Retrieved 31 March 2012.

[Space-20120329-210] Staff (29 March 2012). "NASA's New Mars Rover Will Explore Towering 'Mount Sharp'". Space.com. Retrieved 30 March 2012.

[Gale_Crater-211] Webster, Guy; Brown, Dwayne (22 July 2011). "NASA's Next Mars Rover To Land At Gale Crater". NASA JPL. Retrieved 2011-07-22.

[Gale_Crater2-212] Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 2011-07-22.

[Gale_Crater3-213] Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Retrieved 2011-07-22.

[ESA_signed-214] "ExoMars: ESA and Roscosmos set for Mars missions". European Space Agency (ESA). 14 March 2013. Retrieved 2013-03-14.

[1]

[2]

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 Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks [[plate tectonics]] or [[continental drift]], so it has remained almost unchanged since the end of the [[Hesperian]] period. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to [[abiogenesis]], even if life does not or has never existed there.<ref>{{cite journal |doi=10.1029/RG027i002p00189 |title=The early environment and its evolution on Mars: Implication for life |year=1989 |last1=McKay |first1=Christopher P. |last2=Stoker |first2=Carol R. |journal=Reviews of Geophysics |volume=27 |issue=2 |pages=189–214|bibcode = 1989RvGeo..27..189M }}</ref><ref name='Fromproto'>{{cite journal |bibcode=2007prpl.conf..929G |arxiv=astro-ph/0602008 |title=From Protoplanets to Protolife: The Emergence and Maintenance of Life |last1=Gaidos |first1=Eric |last2=Selsis |first2=Franck |year=2007 |pages=929–44 |journal=Protostars and Planets V}}</ref> It remains an open question whether life currently exists on Mars or has existed there in the past, and fictional [[Martian]]s have been a recurring feature of [[popular entertainment]] of the 20th and 21st centuries.
-On January 24, 2014, NASA reported that [[Timeline of Mars Science Laboratory#Current status|current studies]] on the planet [[Mars]] by the [[Curiosity (rover)|''Curiosity'']] and [[Opportunity (rover)|''Opportunity'']] [[Mars rover|rovers]] will now be searching for evidence of ancient life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic, and/or [[Lithotroph#Chemolithotrophs|chemolithoautotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient [[river]]s or [[lake]]s) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal |last=Grotzinger |first=John P. |title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars |url=http://www.sciencemag.org/content/343/6169/386 |journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |number=6169 |pages=386–387 |doi=10.1126/science.1249944 |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124special">{{cite journal |authors=Various |title=Special Issue - Table of Contents - Exploring Martian Habitability |url=http://www.sciencemag.org/content/343/6169.toc#SpecialIssue |date=January 24, 2014|journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452 |accessdate=24 January 2014 }}</ref><ref name="SCI-20140124">{{cite journal |authors=Various |title=Special Collection - Curiosity - Exploring Martian Habitability |url=http://www.sciencemag.org/site/extra/curiosity/|date=January 24, 2014 |journal=[[Science (journal)|Science]] |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal |authors=Grotzinger, J.P. et al. |title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |url=http://www.sciencemag.org/content/343/6169/1242777 |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |number=6169 |doi=10.1126/science.1242777 |accessdate=January 24, 2014 }}</ref> The search for evidence of [[Planetary habitability|habitability]], [[taphonomy]] (related to [[fossils]]), and [[organic carbon]] on the planet [[Mars]] is now a primary [[NASA]] objective.<ref name="SCI-20140124a" />
+On January 24, 2014, NASA reported that [[Timeline of Mars Science Laboratory#Current status|current studies]] on the planet [[Mars]] by the [[Curiosity (rover)|''Curiosity'']] and [[Opportunity (rover)|''Opportunity'']] [[Mars rover|rovers]] will now be searching for evidence of ancient life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic, and/or [[Lithotroph#Chemolithotrophs|chemolithoautotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient [[river]]s or [[lake]]s) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal |last=Grotzinger |first=John P. |title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars |url=http://www.sciencemag.org/content/343/6169/386 |journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |issue=6169 |number=6169 |pages=386–387 |doi=10.1126/science.1249944 |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124special">{{cite journal |authors=Various |title=Special Issue - Table of Contents - Exploring Martian Habitability |url=http://www.sciencemag.org/content/343/6169.toc#SpecialIssue |date=January 24, 2014|journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452 |accessdate=24 January 2014 }}</ref><ref name="SCI-20140124">{{cite journal |authors=Various |title=Special Collection - Curiosity - Exploring Martian Habitability |url=http://www.sciencemag.org/site/extra/curiosity/|date=January 24, 2014 |journal=[[Science (journal)|Science]] |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal |authors=Grotzinger, J.P. et al. |title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |url=http://www.sciencemag.org/content/343/6169/1242777 |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |issue=6169 |pages=1242777 |number=6169 |doi=10.1126/science.1242777 |accessdate=January 24, 2014 |last1=Grotzinger |first1=J. P. |last2=Sumner |first2=D. Y. |last3=Kah |first3=L. C. |last4=Stack |first4=K. |last5=Gupta |first5=S. |last6=Edgar |first6=L. |last7=Rubin |first7=D. |last8=Lewis |first8=K. |last9=Schieber |first9=J. |last10=Mangold |first10=N. |last11=Milliken |first11=R. |last12=Conrad |first12=P. G. |last13=Desmarais |first13=D. |last14=Farmer |first14=J. |last15=Siebach |first15=K. |last16=Calef |first16=F. |last17=Hurowitz |first17=J. |last18=McLennan |first18=S. M. |last19=Ming |first19=D. |last20=Vaniman |first20=D. |last21=Crisp |first21=J. |last22=Vasavada |first22=A. |last23=Edgett |first23=K. S. |last24=Malin |first24=M. |last25=Blake |first25=D. |last26=Gellert |first26=R. |last27=Mahaffy |first27=P. |last28=Wiens |first28=R. C. |last29=Maurice |first29=S. |last30=Grant |first30=J. A. |display-authors=29 }}</ref> The search for evidence of [[Planetary habitability|habitability]], [[taphonomy]] (related to [[fossils]]), and [[organic carbon]] on the planet [[Mars]] is now a primary [[NASA]] objective.<ref name="SCI-20140124a" />
 == Early speculation ==
@@ Line 18: / Line 18: @@
 == Habitability ==
-Chemical, physical, geological and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.<ref name='2013 LPS'>{{cite journal |bibcode=2013LPICo1719.2185C |title=Habitability Assessment at Gale Crater: Implications from Initial Results |last1=Conrad |first1=P. G. |last2=Archer |first2=D. |last3=Coll |first3=P. |last4=De La Torre |first4=M. |last5=Edgett |first5=K. |last6=Eigenbrode |first6=J. L. |last7=Fisk |first7=M. |last8=Freissenet |first8=C. |last9=Franz |first9=H. |volume=1719 |year=2013 |pages=2185 |journal=44th Lunar and Planetary Science Conference}}</ref>  The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from Solar ultraviolet and [[Cosmic ray|galactic cosmic radiation]].<ref name='D.C.Golden'>{{cite journal |bibcode=2012P&SS...72...91S |title=Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions |last1=Schuerger |first1=Andrew C. |last2=Golden |first2=D. C. |last3=Ming |first3=Doug W. |volume=72 |issue=1 |year=2012 |pages=91–101 |journal=Planetary and Space Science |doi=10.1016/j.pss.2012.07.026}}</ref><ref name=Beaty>{{cite journal |bibcode=2006AsBio...6..677M |title=Findings of the Mars Special Regions Science Analysis Group |author1=MEPAG Special Regions-Science Analysis Group |last2=Beaty |first2=D. |last3=Buxbaum |first3=K. |last4=Meyer |first4=M. |last5=Barlow |first5=N. |last6=Boynton |first6=W. |last7=Clark |first7=B. |last8=Deming |first8=J. |last9=Doran |first9=P. T. |volume=6 |year=2006 |pages=677–732 |journal=Astrobiology |doi=10.1089/ast.2006.6.677 |pmid=17067257 |issue=5}}</ref>
+Chemical, physical, geological and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.<ref name='2013 LPS'>{{cite journal |bibcode=2013LPICo1719.2185C |title=Habitability Assessment at Gale Crater: Implications from Initial Results |last1=Conrad |first1=P. G. |last2=Archer |first2=D. |last3=Coll |first3=P. |last4=De La Torre |first4=M. |last5=Edgett |first5=K. |last6=Eigenbrode |first6=J. L. |last7=Fisk |first7=M. |last8=Freissenet |first8=C. |last9=Franz |first9=H. |last10=Glavin |first10=D. P. |last11=Gómez |first11=F. |last12=Haberle |first12=R. |last13=Hamilton |first13=V. |last14=Jones |first14=J. H. |last15=Kah |first15=L. C. |last16=Leshin |first16=L. A. |last17=Mahaffy |first17=P. M. |last18=McAdam |first18=A. |last19=McKay |first19=C. P. |last20=Navarro-González |first20=R. |last21=Steele |first21=A. |last22=Stern |first22=J. |last23=Sumner |first23=D. |last24=Treiman |first24=A. H. |last25=Wong |first25=M. H. |last26=Wray |first26=J. |last27=Yingst |first27=R. A. |volume=1719 |year=2013 |pages=2185 |journal=44th Lunar and Planetary Science Conference|display-authors=9 |author28=MSL Science Team }}</ref>  The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from Solar ultraviolet and [[Cosmic ray|galactic cosmic radiation]].<ref name='D.C.Golden'>{{cite journal |bibcode=2012P&SS...72...91S |title=Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions |last1=Schuerger |first1=Andrew C. |last2=Golden |first2=D. C. |last3=Ming |first3=Doug W. |volume=72 |issue=1 |year=2012 |pages=91–101 |journal=Planetary and Space Science |doi=10.1016/j.pss.2012.07.026}}</ref><ref name=Beaty>{{cite journal |bibcode=2006AsBio...6..677M |title=Findings of the Mars Special Regions Science Analysis Group |author1=MEPAG Special Regions-Science Analysis Group |last2=Beaty |first2=D. |last3=Buxbaum |first3=K. |last4=Meyer |first4=M. |last5=Barlow |first5=N. |last6=Boynton |first6=W. |last7=Clark |first7=B. |last8=Deming |first8=J. |last9=Doran |first9=P. T. |last10=Edgett |first10=K. |last11=Hancock |first11=S. |last12=Head |first12=J. |last13=Hecht |first13=M. |last14=Hipkin |first14=V. |last15=Kieft |first15=T. |last16=Mancinelli |first16=R. |last17=McDonald |first17=E. |last18=McKay |first18=C. |last19=Mellon |first19=M. |last20=Newsom |first20=H. |last21=Ori |first21=G. |last22=Paige |first22=D. |last23=Schuerger |first23=A. C. |last24=Sogin |first24=M. |last25=Spry |first25=J. A. |last26=Steele |first26=A. |last27=Tanaka |first27=K. |last28=Voytek |first28=M. |volume=6 |year=2006 |pages=677–732 |journal=Astrobiology |doi=10.1089/ast.2006.6.677 |pmid=17067257 |issue=5|display-authors=9 }}</ref>
 Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below.<ref name='2013 LPS' /> Similarly, for each group of parameters, the habitability threshold for each is to be determined.<ref name='2013 LPS' /> Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.<ref name='dust-up'>{{cite web | url = http://www.astrobio.net/exclusive/3495/mars-contamination-dust-up | title = Mars Contamination Dust-Up | accessdate = 2013-07-04 | first = Charles Q. Choi, | date = 17 May 2010 | publisher = Astrobiology Magazine | quote = Whenever multiple biocidal factors are combined, the survival rates plummet quickly,}}</ref> There are no full-Mars simulations published yet that include all of the biocidal factors combined.<ref name='dust-up' />
@@ Line 36: / Line 36: @@
 === Past ===
-The loss of the Martian [[magnetosphere|magnetic field]] strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.<ref name='Biosignatures 2011' /> When there was a magnetic field, the atmosphere would have been protected from erosion by [[solar wind]], which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.<ref name='Dehant'>{{cite book |doi=10.1007/978-0-387-74288-5_10 |chapter=Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars |title=Geology and Habitability of Terrestrial Planets |series=Space Sciences Series of ISSI |year=2007 |last1=Dehant |first1=V. |last2=Lammer |first2=H. |last3=Kulikov |first3=Y. N. |last4=Grießmeier |first4=J. -M. |last5=Breuer |first5=D. |last6=Verhoeven |first6=O. |last7=Karatekin |first7=Ö. |last8=Hoolst |first8=T. |last9=Korablev |first9=O. |isbn=978-0-387-74287-8 |volume=24 |pages=279–300}}</ref>
+The loss of the Martian [[magnetosphere|magnetic field]] strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.<ref name='Biosignatures 2011' /> When there was a magnetic field, the atmosphere would have been protected from erosion by [[solar wind]], which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.<ref name='Dehant'>{{cite book |doi=10.1007/978-0-387-74288-5_10 |chapter=Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars |title=Geology and Habitability of Terrestrial Planets |series=Space Sciences Series of ISSI |year=2007 |last1=Dehant |first1=V. |last2=Lammer |first2=H. |last3=Kulikov |first3=Y. N. |last4=Grießmeier |first4=J. -M. |last5=Breuer |first5=D. |last6=Verhoeven |first6=O. |last7=Karatekin |first7=Ö. |last8=Hoolst |first8=T. |last9=Korablev |first9=O. |last10=Lognonné |first10=P. |isbn=978-0-387-74287-8 |volume=24 |pages=279–300|display-authors=8 }}</ref>
 Soil and rock samples studied in 2013 by NASA's [[Curiosity (rover)|''Curiosity'' rover's]] onboard instruments brought about additional information on several habitability factors.<ref name='ancient life'>{{cite news | title = NASA Rover Finds Conditions Once Suited for Ancient Life on Mars | date = 12 March 2013 | url = http://www.nasa.gov/mission_pages/msl/news/msl20130312.html | work = NASA | accessdate = 2013-06-06}}</ref> The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment — perhaps a lake or an ancient streambed — that was neutral and not too salty.<ref name='ancient life' /> On December 9, 2013, NASA reported that, based on evidence from ''Curiosity'' studying [[Aeolis Palus]], [[Gale (crater)|Gale Crater]] contained an ancient [[freshwater lake]] which could have been a hospitable environment for [[microbial life]].<ref name="NYT-20131209">{{cite news |last=Chang |first=Kenneth |title=On Mars, an Ancient Lake and Perhaps Life |url=http://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |date=December 9, 2013 |work=[[New York Times]]|accessdate=December 9, 2013 }}</ref><ref name="SCI-20131209">{{cite journal |authors=Various |title=Science - Special Collection - Curiosity Rover on Mars |url=http://www.sciencemag.org/site/extra/curiosity/ |date=December 9, 2013 |journal=[[Science (journal)|Science]] |accessdate=December 9, 2013 }}</ref>  The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past [[magnetosphere|magnetic field]] that protected the planet from cosmic and Solar radiation,<ref>{{cite web | url=http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html | title=New Map Provides More Evidence Mars Once Like Earth | publisher=NASA | work=Goddard Space Flight Center | date=October 12, 2005 | accessdate=5 June 2013 | author= Neal-Jones, Nancy | coauthor =O'Carroll, Cynthia}}</ref><ref>{{cite web| title=Martian Interior: Paleomagnetism |publisher=European Space Agency |work=Mars Express |date=4 January 2007 |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31028&fbodylongid=645}}</ref> together strongly suggest that Mars could have had the environmental factors to support life.<ref name='Wall' /> However, the assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably [[microorganism|microbial]], existing communally in fluids or on sediments, either free-living or as [[biofilm]]s, respectively.<ref name='Biosignatures 2011' />
@@ Line 44: / Line 44: @@
 ==== Subsurface ====
-Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms,<ref name='2013 LPS' /> life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen.<ref name='cosmic radiation' /><ref name="Dartnell" /><ref name='DartnellGeographic' /><ref name='Dartnell-1' /><ref name='ionising radiation' /><ref name='Parnell'>{{cite news | first = JohnThomas Didymus | title = Scientists find evidence Mars subsurface could hold life | date = 21 January | url = http://digitaljournal.com/article/341801 | work = Digital Journal – Science | accessdate = 2013-06-05 | quote = There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell.}}</ref> Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet.<ref name='Biosignatures 2011'>{{cite journal |doi=10.1089/ast.2010.0506 |quote=There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance. |title=Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group |year=2011 |last1=Summons |first1=Roger E. |last2=Amend |first2=Jan P. |last3=Bish |first3=David |last4=Buick |first4=Roger |last5=Cody |first5=George D. |last6=Des Marais |first6=David J. |last7=Dromart |first7=Gilles |last8=Eigenbrode |first8=Jennifer L. |last9=Knoll |first9=Andrew H. |journal=Astrobiology |volume=11 |issue=2 |pages=157–81 |pmid=21417945|bibcode = 2011AsBio..11..157S }}</ref><ref name='Parnell' /><ref name=Steigerwald /><ref>[http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20080215a.html NASA – Mars Rovers Sharpen Questions About Livable Conditions]</ref><ref>{{cite news | title = Mars: 'Strongest evidence' planet may have supported life, scientists say | date = 20 January 2013 | url = http://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-21063817 | work = BBC News | accessdate = 2013-01-22}}</ref><ref>{{cite journal |doi=10.1038/ngeo1706 |title=Groundwater activity on Mars and implications for a deep biosphere |year=2013 |last1=Michalski |first1=Joseph R. |last2=Cuadros |first2=Javier |last3=Niles |first3=Paul B. |last4=Parnell |first4=John |last5=Deanne Rogers |first5=A. |last6=Wright |first6=Shawn P. |journal=Nature Geoscience |volume=6 |issue=2 |pages=133–8|bibcode = 2013NatGe...6..133M }}</ref> The extensive [[Volcanology of Mars|volcanism]] in the past possibly created subsurface cracks and caves within different strata where liquid water could have been stored, forming large [[aquifer]]s with deposits of saline liquid water, minerals, organic molecules, and [[Geothermal gradient|geothermal heat]] – potentially providing a habitable environment away from the harsh surface conditions.<ref name='Parnell' /><ref name='subsurface habitability model'>{{cite journal |bibcode=2012LPI....43.2943D |quote=The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth. |title=A Possible Biogeochemical Model for Mars |last1=De Morais |first1=A. |volume=43 |year=2012 |pages=2943 |journal=43rd Lunar and Planetary Science Conference}}</ref><ref>{{cite news | first = Paul S. | last = Anderson | title = New Study Says Large Regions of Mars Could Sustain Life | date = 15 December 2011 | url = http://www.universetoday.com/91848/new-study-says-large-regions-of-mars-could-sustain-life/ | work = Universe Today | accessdate = 2013-06-05 | quote = Most scientists would agree that the best place that any organisms could hope to survive and flourish would be underground. }}</ref><ref>{{cite web | url = http://phoenix.lpl.arizona.edu/mars143.php | title = Habitability and Biology: What are the Properties of Life? | accessdate = 2013-06-06 | work = Phoenix Mars Mission | publisher = The University of Arizona | quote= If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.}}</ref>
+Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms,<ref name='2013 LPS' /> life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen.<ref name='cosmic radiation' /><ref name="Dartnell" /><ref name='DartnellGeographic' /><ref name='Dartnell-1' /><ref name='ionising radiation' /><ref name='Parnell'>{{cite news | first = JohnThomas Didymus | title = Scientists find evidence Mars subsurface could hold life | date = 21 January | url = http://digitaljournal.com/article/341801 | work = Digital Journal – Science | accessdate = 2013-06-05 | quote = There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell.}}</ref> Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet.<ref name='Biosignatures 2011'>{{cite journal |doi=10.1089/ast.2010.0506 |quote=There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance. |title=Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group |year=2011 |last1=Summons |first1=Roger E. |last2=Amend |first2=Jan P. |last3=Bish |first3=David |last4=Buick |first4=Roger |last5=Cody |first5=George D. |last6=Des Marais |first6=David J. |last7=Dromart |first7=Gilles |last8=Eigenbrode |first8=Jennifer L. |last9=Knoll |first9=Andrew H. |last10=Sumner |first10=Dawn Y. |journal=Astrobiology |volume=11 |issue=2 |pages=157–81 |pmid=21417945|bibcode = 2011AsBio..11..157S |display-authors=8 }}</ref><ref name='Parnell' /><ref name=Steigerwald /><ref>[http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20080215a.html NASA – Mars Rovers Sharpen Questions About Livable Conditions]</ref><ref>{{cite news | title = Mars: 'Strongest evidence' planet may have supported life, scientists say | date = 20 January 2013 | url = http://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-21063817 | work = BBC News | accessdate = 2013-01-22}}</ref><ref>{{cite journal |doi=10.1038/ngeo1706 |title=Groundwater activity on Mars and implications for a deep biosphere |year=2013 |last1=Michalski |first1=Joseph R. |last2=Cuadros |first2=Javier |last3=Niles |first3=Paul B. |last4=Parnell |first4=John |last5=Deanne Rogers |first5=A. |last6=Wright |first6=Shawn P. |journal=Nature Geoscience |volume=6 |issue=2 |pages=133–8|bibcode = 2013NatGe...6..133M }}</ref> The extensive [[Volcanology of Mars|volcanism]] in the past possibly created subsurface cracks and caves within different strata where liquid water could have been stored, forming large [[aquifer]]s with deposits of saline liquid water, minerals, organic molecules, and [[Geothermal gradient|geothermal heat]] – potentially providing a habitable environment away from the harsh surface conditions.<ref name='Parnell' /><ref name='subsurface habitability model'>{{cite journal |bibcode=2012LPI....43.2943D |quote=The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth. |title=A Possible Biogeochemical Model for Mars |last1=De Morais |first1=A. |volume=43 |year=2012 |pages=2943 |journal=43rd Lunar and Planetary Science Conference}}</ref><ref>{{cite news | first = Paul S. | last = Anderson | title = New Study Says Large Regions of Mars Could Sustain Life | date = 15 December 2011 | url = http://www.universetoday.com/91848/new-study-says-large-regions-of-mars-could-sustain-life/ | work = Universe Today | accessdate = 2013-06-05 | quote = Most scientists would agree that the best place that any organisms could hope to survive and flourish would be underground. }}</ref><ref>{{cite web | url = http://phoenix.lpl.arizona.edu/mars143.php | title = Habitability and Biology: What are the Properties of Life? | accessdate = 2013-06-06 | work = Phoenix Mars Mission | publisher = The University of Arizona | quote= If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.}}</ref>
 ==== Surface brines ====
-Although liquid water does not appear at the surface of Mars,<ref>{{cite journal |bibcode=2006IJMSE...2...83H |quote=In summary, on present-day Mars, liquid water is unlikely except as the result of a quick and dramatic change in environmental conditions such as from a landslide that exposes buried ice to sunlight (Costard et al. 2002), or from the introduction of an artificial heat source. |title=Transient liquid water near an artificial heat source on Mars |last1=Hecht |first1=Michael H. |last2=Vasavada |first2=Ashwin R. |volume=2 |year=2006 |pages=83–96 |journal=International Journal of Mars Science and Exploration |doi=10.1555/mars.2006.0006}}</ref><ref name=Haberle>{{cite journal |last1=Haberle |first1=Robert M. |last2=McKay |first2=Christopher P. |last3=Schaeffer |first3=James |last4=Cabrol |first4=Nathalie A. |last5=Grin |first5=Edmon A. |last6=Zent |first6=Aaron P. |last7=Quinn |first7=Richard |title=On the possibility of liquid water on present-day Mars |doi=10.1029/2000JE001360 |journal=Journal of Geophysical Research: Planets |volume=106 |issue=El0 |year=2001 |pages=23317–26 |bibcode=bibcode=2001JGR...10623317H |quote=Introduction: The mean annual surface pressure and temperature on present-day Mars do not allow for the stability of liquid water on the surface. […] Conclusion: It is possible, even likely, that solar-heated liquid water never forms on present-day Mars.}}</ref> several modeling studies suggest that potential locations on Mars could include regions where thin films of salty liquid [[brine]] or [[perchlorate]] may form near the surface<ref name=Haberle /><ref name=Haberle /><ref name='2013 conference'>{{cite web | url = http://planets.ucla.edu/meetings/mars-habitability-2013/program/ | title = Conference: The Present-Day Habitability of Mars 2013 | accessdate = 2013-06-17 | date = February 4–6, 2013 | format = PDF | publisher = The UCLA Institute for Planets and Exoplanets}}</ref> that may provide a potential location for terrestrial salt and cold-loving microorganisms ([[halophile]] [[psychrophilic]]).<ref>{{cite journal |doi=10.1080/08120099.2011.591430 |title=Using the phase diagram of liquid water to search for life |year=2012 |last1=Jones |first1=E. G. |last2=Lineweaver |first2=C. H. |journal=Australian Journal of Earth Sciences |volume=59 |issue=2 |pages=253–62|bibcode = 2012AuJES..59..253J }}</ref>  Various salts present in the Martian soil may act as an antifreeze and could keep water liquid well below its normal freezing point, if water was present at certain favorable locations.<ref name=Haberle /><ref name=Lobitz>{{cite journal | doi = 10.1073/pnas.031581098 | last1 = Lobitz | first1 = B. | last2 = Wood | year = 2001 | first2 = BL | last3 = Averner | first3 = MM | last4 = McKay | first4 = CP | title = Use of spacecraft data to derive regions on Mars where liquid water would be stable | journal=Proc. Natl. Acad. Sci. | volume = 98 | issue = 5| pages = 2132–2137 | pmid = 11226204 | pmc = 30104 |bibcode = 2001PNAS...98.2132L |quote= These results do not indicate that water is present at these locations, only that, if it were present and heat sources were sufficient to bring the water in thermal equilibrium with the surface, the resulting liquid would be stable against freezing or boiling.}}</ref><ref name='Parro 2011'>{{cite journal | title = A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars | journal = Astroiology | date = December 2011 | first = Victor Parro | coauthors = Graciela de Diego-Castilla, Mercedes Moreno-Paz, Yolanda Blanco, Patricia Cruz-Gil, José A. Rodríguez-Manfredi, David Fernández-Remolar, Felipe Gómez, Manuel J. Gómez, Luis A. Rivas, Cecilia Demergasso, Alex Echeverría, Viviana N. Urtuvia, Marta Ruiz-Bermejo, Miriam García-Villadangos, Marina Postigo, Mónica Sánchez-Román, Guillermo Chong-Díaz, and Javier Gómez-Elvira. | volume = 11 | issue = 10 | pages = 969–996 | doi = 10.1089/ast.2011.0654 | url = http://online.liebertpub.com/doi/abs/10.1089/ast.2011.0654 | format = PDF | accessdate = 2013-06-30|bibcode = 2011AsBio..11..969P }}</ref>  Astrobiologists are keen to find out more, as not much is known about these brines at the moment. The briny water may or may not be habitable to microbes from Earth or Mars.<ref>{{cite web |url=http://www.space.com/19928-mars-habitable-life-possible.html  |title= Mars May Be Habitable Today, Scientists Say|author=Rod Pyle |date= 25 February 2013|work= space.com}}</ref> Another researcher argues that although chemically important, thin films of transient liquid water are not likely to provide suitable sites for life.<ref name=Lobitz />  In addition, an astrobiology team asserted that the [[Water activity|activity of water]] on salty films, the temperature, or both are less than the biological thresholds across the entire Martian surface and shallow subsurface.<ref name=Beaty />
+Although liquid water does not appear at the surface of Mars,<ref>{{cite journal |bibcode=2006IJMSE...2...83H |quote=In summary, on present-day Mars, liquid water is unlikely except as the result of a quick and dramatic change in environmental conditions such as from a landslide that exposes buried ice to sunlight (Costard et al. 2002), or from the introduction of an artificial heat source. |title=Transient liquid water near an artificial heat source on Mars |last1=Hecht |first1=Michael H. |last2=Vasavada |first2=Ashwin R. |volume=2 |year=2006 |pages=83–96 |journal=International Journal of Mars Science and Exploration |doi=10.1555/mars.2006.0006}}</ref><ref name=Haberle>{{cite journal |last1=Haberle |first1=Robert M. |last2=McKay |first2=Christopher P. |last3=Schaeffer |first3=James |last4=Cabrol |first4=Nathalie A. |last5=Grin |first5=Edmon A. |last6=Zent |first6=Aaron P. |last7=Quinn |first7=Richard |title=On the possibility of liquid water on present-day Mars |doi=10.1029/2000JE001360 |journal=Journal of Geophysical Research: Planets |volume=106 |issue=El0 |year=2001 |pages=23317–26 |bibcode=bibcode=2001JGR...10623317H |quote=Introduction: The mean annual surface pressure and temperature on present-day Mars do not allow for the stability of liquid water on the surface. […] Conclusion: It is possible, even likely, that solar-heated liquid water never forms on present-day Mars.}}</ref> several modeling studies suggest that potential locations on Mars could include regions where thin films of salty liquid [[brine]] or [[perchlorate]] may form near the surface<ref name=Haberle /><ref name=Haberle /><ref name='2013 conference'>{{cite web | url = http://planets.ucla.edu/meetings/mars-habitability-2013/program/ | title = Conference: The Present-Day Habitability of Mars 2013 | accessdate = 2013-06-17 | date = February 4–6, 2013 | format = PDF | publisher = The UCLA Institute for Planets and Exoplanets}}</ref> that may provide a potential location for terrestrial salt and cold-loving microorganisms ([[halophile]] [[psychrophilic]]).<ref>{{cite journal |doi=10.1080/08120099.2011.591430 |title=Using the phase diagram of liquid water to search for life |year=2012 |last1=Jones |first1=E. G. |last2=Lineweaver |first2=C. H. |journal=Australian Journal of Earth Sciences |volume=59 |issue=2 |pages=253–62|bibcode = 2012AuJES..59..253J }}</ref>  Various salts present in the Martian soil may act as an antifreeze and could keep water liquid well below its normal freezing point, if water was present at certain favorable locations.<ref name=Haberle /><ref name=Lobitz>{{cite journal | doi = 10.1073/pnas.031581098 | last1 = Lobitz | first1 = B. | last2 = Wood | year = 2001 | first2 = BL | last3 = Averner | first3 = MM | last4 = McKay | first4 = CP | title = Use of spacecraft data to derive regions on Mars where liquid water would be stable | journal=Proc. Natl. Acad. Sci. | volume = 98 | issue = 5| pages = 2132–2137 | pmid = 11226204 | pmc = 30104 |bibcode = 2001PNAS...98.2132L |quote= These results do not indicate that water is present at these locations, only that, if it were present and heat sources were sufficient to bring the water in thermal equilibrium with the surface, the resulting liquid would be stable against freezing or boiling.}}</ref><ref name='Parro 2011'>{{cite journal | title = A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars | journal = Astroiology | date = December 2011 | first = Victor Parro | coauthors = Graciela de Diego-Castilla, Mercedes Moreno-Paz, Yolanda Blanco, Patricia Cruz-Gil, José A. Rodríguez-Manfredi, David Fernández-Remolar, Felipe Gómez, Manuel J. Gómez, Luis A. Rivas, Cecilia Demergasso, Alex Echeverría, Viviana N. Urtuvia, Marta Ruiz-Bermejo, Miriam García-Villadangos, Marina Postigo, Mónica Sánchez-Román, Guillermo Chong-Díaz, and Javier Gómez-Elvira. | volume = 11 | issue = 10 | pages = 969–996 | doi = 10.1089/ast.2011.0654 | format = PDF |bibcode = 2011AsBio..11..969P | last1 = Parro }}</ref>  Astrobiologists are keen to find out more, as not much is known about these brines at the moment. The briny water may or may not be habitable to microbes from Earth or Mars.<ref>{{cite web |url=http://www.space.com/19928-mars-habitable-life-possible.html  |title= Mars May Be Habitable Today, Scientists Say|author=Rod Pyle |date= 25 February 2013|work= space.com}}</ref> Another researcher argues that although chemically important, thin films of transient liquid water are not likely to provide suitable sites for life.<ref name=Lobitz />  In addition, an astrobiology team asserted that the [[Water activity|activity of water]] on salty films, the temperature, or both are less than the biological thresholds across the entire Martian surface and shallow subsurface.<ref name=Beaty />
-The damaging effect of [[ionizing radiation]] on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats.<ref name='Dartnell-1'>{{cite journal |bibcode=2007GeoRL..3402207D |doi=10.1029/2006GL027494 |quote=The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |year=2007 |last1=Dartnell |first1=L. R. |last2=Desorgher |first2=L. |last3=Ward |first3=J. M. |last4=Coates |first4=A. J. |journal=Geophysical Research Letters |volume=34 |issue=2}}</ref><ref name='ionising radiation'>{{cite journal | title = Martian sub-surface ionising radiation: biosignatures and geology | journal = Biogeosciences | year = 2007 | first1 = L. R. |last1=Dartnell |first2=L. |last2=Desorgher |first3=J. M. |last3=Ward |first4=A. J. |last4=Coates | volume = 4 | pages = 545–558 | doi = 10.5194/bg-4-545-2007 | bibcode = 2007BGeo....4..545D | quote = This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. [..] Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs. | issue = 4}}</ref><ref>{{citation | first = Lewis R. Dartnell | coauthors = Michael C. Storrie-Lombardi, Jan-Peter. Muller, Andrew. D. Griffiths, Andrew J. Coates, John M. Ward | contribution = Implications of cosmic radiation on the Martian surface for microbial survival and detection of fluorescent biosignatures | title = 42nd Lunar and Planetary Science Conference | publisher = Lunar and Planetary Institute | place = The Woodlands, Texas | date = March 7–11, 2011 | contribution-url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf | format = PDF | accessdate = 2013-06-30}}</ref>  Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.<ref name='ionising radiation' /> Also, solar ultraviolet (UV) radiation proved particularly devastating for the survival of cold-resistant microbes under simulated surface conditions on Mars, as UV radiation was readily and easily able to penetrate the salt-organic matrix that the bacterial cells were embedded in.<ref name='Scarce shelter'>{{cite journal | title = Scarce Shelter on Mars | journal = Astrobiology Magazine | date = 1 June 2009 | first = Jeremy Hsu | url = http://www.astrobio.net/exclusive/3144/scarce-shelter-on-mars | accessdate = 2013-07-04}}</ref> In addition, NASA's [[Mars Exploration Program]] states that life on the surface of Mars is unlikely, given the presence of [[superoxide]]s that break down organic (carbon-based) molecules on which life is based.<ref>{{cite web | url = http://mars.jpl.nasa.gov/programmissions/science/goal1/ | title = Goal 1: Determine if Life Ever Arose On Mars | accessdate = 2013-06-29 | first = The Mars Exploration Program | work = NASA}}</ref>
+The damaging effect of [[ionizing radiation]] on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats.<ref name='Dartnell-1'>{{cite journal |bibcode=2007GeoRL..3402207D |doi=10.1029/2006GL027494 |quote=The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |year=2007 |last1=Dartnell |first1=L. R. |last2=Desorgher |first2=L. |last3=Ward |first3=J. M. |last4=Coates |first4=A. J. |journal=Geophysical Research Letters |volume=34 |issue=2}}</ref><ref name='ionising radiation'>{{cite journal | title = Martian sub-surface ionising radiation: biosignatures and geology | journal = Biogeosciences | year = 2007 | first1 = L. R. |last1=Dartnell |first2=L. |last2=Desorgher |first3=J. M. |last3=Ward |first4=A. J. |last4=Coates | volume = 4 | pages = 545–558 | doi = 10.5194/bg-4-545-2007 | bibcode = 2007BGeo....4..545D | quote = This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. [..] Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs. | issue = 4}}</ref><ref>{{Cite document| first = Lewis R. Dartnell | coauthors = Michael C. Storrie-Lombardi, Jan-Peter. Muller, Andrew. D. Griffiths, Andrew J. Coates, John M. Ward | contribution = Implications of cosmic radiation on the Martian surface for microbial survival and detection of fluorescent biosignatures | title = 42nd Lunar and Planetary Science Conference | publisher = Lunar and Planetary Institute | place = The Woodlands, Texas | date = March 7–11, 2011 | contribution-url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf | format = PDF | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->&#123;&#123;inconsistent citations&#125;&#125; }}</ref>  Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.<ref name='ionising radiation' /> Also, solar ultraviolet (UV) radiation proved particularly devastating for the survival of cold-resistant microbes under simulated surface conditions on Mars, as UV radiation was readily and easily able to penetrate the salt-organic matrix that the bacterial cells were embedded in.<ref name='Scarce shelter'>{{cite journal | title = Scarce Shelter on Mars | journal = Astrobiology Magazine | date = 1 June 2009 | first = Jeremy Hsu | url = http://www.astrobio.net/exclusive/3144/scarce-shelter-on-mars | accessdate = 2013-07-04}}</ref> In addition, NASA's [[Mars Exploration Program]] states that life on the surface of Mars is unlikely, given the presence of [[superoxide]]s that break down organic (carbon-based) molecules on which life is based.<ref>{{cite web | url = http://mars.jpl.nasa.gov/programmissions/science/goal1/ | title = Goal 1: Determine if Life Ever Arose On Mars | accessdate = 2013-06-29 | first = The Mars Exploration Program | work = NASA}}</ref>
 === Cosmic radiation ===
-In 1965, the [[Mariner 4]] probe discovered that Mars had no [[Magnetosphere|global magnetic field]] that would protect the planet from potentially life-threatening [[cosmic radiation]] and [[solar radiation]]; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.<ref>{{cite book |chapterurl=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/ |chapter=Mars: Magnetic Field and Magnetosphere |first1=J. G. |last1=Luhmann |first2=C. T. |last2=Russell |title=Encyclopedia of Planetary Sciences |editor1-first=J. H. |editor1-last=Shirley |editor2-first=R. W. |editor2-last=Fainbridge |pages=454–6 |publisher=Chapman and Hall |location=New York |year=1997}}</ref> Scientists speculate that the lack of magnetic shielding helped the [[solar wind]] blow away much of [[Atmosphere of Mars|Mars's atmosphere]] over the course of several billion years.<ref>{{cite web |url=http://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |title=The Solar Wind at Mars |date=January 31, 2001 |first=Tony |last=Phillips |publisher=NASA}}</ref> As a result, the planet has been vulnerable to radiation from space for about 4 billion years.<ref name='hostile to life'>{{cite news | title = What makes Mars so hostile to life? | date = 7 January 2013 | url = http://www.bbc.co.uk/science/0/20915340 | work = BBC News | accessdate = 2013-06-15}}</ref> Currently, ionizing radiation on Mars is typically two orders of magnitude (or 100 times) higher than on Earth.<ref>{{cite journal | title = The impact of Mars geological evolution in high energy ionizing radiation environment through time | journal = Planetary and Space Science – Eslevier | date = November 2012 | first = A. Keating | coauthors = P. Goncalves | volume = 72 | issue = 1 | pages = 70–77 | url = http://www.sciencedirect.com/science/article/pii/S0032063312001225 | accessdate = 2013-06-28 | doi=10.1016/j.pss.2012.04.009 | bibcode=2012P&SS...72...70K}}</ref> Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars for that long.<ref name='cosmic radiation' /><ref>{{cite journal | title = IMPLICATIONS OF COSMIC RADIATION ON THE MARTIAN SURFACE FOR MICROBIAL SURVIVAL AND DETECTION OF FLUORESCENT BIOSIGNATURES. | journal = Lunar and Planetary Institute | year = 2011 | first = Lewis R. Dartnell | coauthors = Michael C. Storrie-Storrie-Lombardi, Jan-Peter. Muller, Andrew. D. Griffiths, Andrew J. Coates, John M. Ward | url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf | format = PDF | accessdate = 2013-07-04}}</ref> After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.<ref name='cosmic radiation'>{{cite web | url = http://www.space.com/3396-study-surface-mars-devoid-life.html | title = Study: Surface of Mars Devoid of Life | accessdate = 28 May 2013 | first = Ker Than | date = 29 January 2007 | work = Space.com| quote = After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation.}}</ref><ref name='Dartnell'>{{cite journal |doi=10.1029/2006GL027494 |quote=Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2&nbsp;m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m. |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |year=2007 |last1=Dartnell |first1=L. R. |last2=Desorgher |first2=L. |last3=Ward |first3=J. M. |last4=Coates |first4=A. J. |journal=Geophysical Research Letters |volume=34 |issue=2 |pages=L02207|bibcode = 2007GeoRL..34.2207D }}</ref><ref name='DartnellGeographic'>{{cite web | author = Richard A. Lovet | title = Mars Life May Be Too Deep to Find, Experts Conclude| url = http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html | work = National Geographic News |date= February 2, 2007 |quote=That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere.}}</ref> The team calculated that the cumulative damage to [[DNA]] and [[RNA]] by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 metres below the planet's surface.<ref name="Dartnell" />
+In 1965, the [[Mariner 4]] probe discovered that Mars had no [[Magnetosphere|global magnetic field]] that would protect the planet from potentially life-threatening [[cosmic radiation]] and [[solar radiation]]; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.<ref>{{cite book |chapterurl=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/ |chapter=Mars: Magnetic Field and Magnetosphere |first1=J. G. |last1=Luhmann |first2=C. T. |last2=Russell |title=Encyclopedia of Planetary Sciences |editor1-first=J. H. |editor1-last=Shirley |editor2-first=R. W. |editor2-last=Fainbridge |pages=454–6 |publisher=Chapman and Hall |location=New York |year=1997}}</ref> Scientists speculate that the lack of magnetic shielding helped the [[solar wind]] blow away much of [[Atmosphere of Mars|Mars's atmosphere]] over the course of several billion years.<ref>{{cite web |url=http://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |title=The Solar Wind at Mars |date=January 31, 2001 |first=Tony |last=Phillips |publisher=NASA}}</ref> As a result, the planet has been vulnerable to radiation from space for about 4 billion years.<ref name='hostile to life'>{{cite news | title = What makes Mars so hostile to life? | date = 7 January 2013 | url = http://www.bbc.co.uk/science/0/20915340 | work = BBC News | accessdate = 2013-06-15}}</ref> Currently, ionizing radiation on Mars is typically two orders of magnitude (or 100 times) higher than on Earth.<ref>{{cite journal | title = The impact of Mars geological evolution in high energy ionizing radiation environment through time | journal = Planetary and Space Science – Eslevier | date = November 2012 | first = A. Keating | coauthors = P. Goncalves | volume = 72 | issue = 1 | pages = 70–77 | url = http://www.sciencedirect.com/science/article/pii/S0032063312001225 | accessdate = 2013-06-28 | doi=10.1016/j.pss.2012.04.009 | bibcode=2012P&SS...72...70K| last1 = Keating }}</ref> Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars for that long.<ref name='cosmic radiation' /><ref>{{cite journal | title = IMPLICATIONS OF COSMIC RADIATION ON THE MARTIAN SURFACE FOR MICROBIAL SURVIVAL AND DETECTION OF FLUORESCENT BIOSIGNATURES. | journal = Lunar and Planetary Institute | year = 2011 | first = Lewis R. Dartnell | coauthors = Michael C. Storrie-Storrie-Lombardi, Jan-Peter. Muller, Andrew. D. Griffiths, Andrew J. Coates, John M. Ward | url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf | format = PDF | accessdate = 2013-07-04}}</ref> After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.<ref name='cosmic radiation'>{{cite web | url = http://www.space.com/3396-study-surface-mars-devoid-life.html | title = Study: Surface of Mars Devoid of Life | accessdate = 28 May 2013 | first = Ker Than | date = 29 January 2007 | work = Space.com| quote = After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation.}}</ref><ref name='Dartnell'>{{cite journal |doi=10.1029/2006GL027494 |quote=Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2&nbsp;m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m. |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |year=2007 |last1=Dartnell |first1=L. R. |last2=Desorgher |first2=L. |last3=Ward |first3=J. M. |last4=Coates |first4=A. J. |journal=Geophysical Research Letters |volume=34 |issue=2 |pages=L02207|bibcode = 2007GeoRL..34.2207D }}</ref><ref name='DartnellGeographic'>{{cite web | author = Richard A. Lovet | title = Mars Life May Be Too Deep to Find, Experts Conclude| url = http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html | work = National Geographic News |date= February 2, 2007 |quote=That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere.}}</ref> The team calculated that the cumulative damage to [[DNA]] and [[RNA]] by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 metres below the planet's surface.<ref name="Dartnell" />
 Even the most radiation-tolerant Earthly bacteria would survive in dormant [[spore]] state only 18,000 years at the surface; at 2 meters  —the greatest depth at which the [[ExoMars]] rover will be capable of reaching— survival time would be 90,000 to half million years, depending on the type of rock.<ref name='DartnellGeographic'>{{cite web | author = Richard A. Lovet | title = Mars Life May Be Too Deep to Find, Experts Conclude| url = http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html | work = National Geographic News |date= February 2, 2007 }}</ref>
-The [[Radiation assessment detector]] (RAD) on board the [[Curiosity (rover)|''Curiosity'' rover]] is currently quantifying the flux of biologically hazardous radiation at the surface of Mars today, and will help determine how these fluxes vary on diurnal, seasonal, solar cycle and episodic (flare, storm) timescales. These measurements will allow calculations of the depth in rock or soil to which this flux, when integrated over long timescales, provides a lethal dose for known terrestrial organisms.<ref>{{cite journal |bibcode=2013EGUGA..1512596H |title=The Radiation Environment on the Martian Surface and during MSL's Cruise to Mars |last1=Hassler |first1=Donald M. |last2=Zeitlin |first2=Cary |last3=Wimmer-Schweingruber |first3=Robert F. |last4=Ehresmann |first4=Bent |last5=Rafkin |first5=Scot |last6=Martin |first6=Cesar |last7=Boettcher |first7=Stephan |last8=Koehler |first8=Jan |last9=Guo |first9=Jingnan |volume=15 |year=2013 |pages=12596 |journal=EGU General Assembly 2013}}</ref>
+The [[Radiation assessment detector]] (RAD) on board the [[Curiosity (rover)|''Curiosity'' rover]] is currently quantifying the flux of biologically hazardous radiation at the surface of Mars today, and will help determine how these fluxes vary on diurnal, seasonal, solar cycle and episodic (flare, storm) timescales. These measurements will allow calculations of the depth in rock or soil to which this flux, when integrated over long timescales, provides a lethal dose for known terrestrial organisms.<ref>{{cite journal |bibcode=2013EGUGA..1512596H |title=The Radiation Environment on the Martian Surface and during MSL's Cruise to Mars |last1=Hassler |first1=Donald M. |last2=Zeitlin |first2=Cary |last3=Wimmer-Schweingruber |first3=Robert F. |last4=Ehresmann |first4=Bent |last5=Rafkin |first5=Scot |last6=Martin |first6=Cesar |last7=Boettcher |first7=Stephan |last8=Koehler |first8=Jan |last9=Guo |first9=Jingnan |last10=Brinza |first10=David E. |last11=Reitz |first11=Guenther |last12=Posner |first12=Arik |volume=15 |year=2013 |pages=12596 |journal=EGU General Assembly 2013|display-authors=9 |author13=the MSL Science Team }}</ref>
-Research published in January 2014 of data collected by the RAD instrument, revealed that the actual absorbed dose measured is 76 [[Gray (unit)|mGy]]/year at the surface,<ref name='RAD January 2014'/> and that "[[ionizing radiation]] strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic matter."<ref name='RAD January 2014'>{{cite journal | title = Mars’ Surface Radiation Environment Measured with the Mars ScienceLaboratory’s Curiosity Rover | journal = Science | date = 24 January 2014 | first = Donald M. | last = Hassler | coauthors = ''et al'' | volume = 343 | issue = 6169| id =  | url = http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf | format = PDF | accessdate = 2014-01-27 | doi=10.1126/science.1244797}}</ref> Regardless of the source of Martian [[organic matter]] (meteoritic, geological, or biological), its carbon bonds are susceptible to breaking and reconfigurating with surrounding elements by ionizing charged particle radiation.<ref name='RAD January 2014'/>  These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic [[biosignature]]s as a function of depth as well as survival
+Research published in January 2014 of data collected by the RAD instrument, revealed that the actual absorbed dose measured is 76 [[Gray (unit)|mGy]]/year at the surface,<ref name='RAD January 2014'/> and that "[[ionizing radiation]] strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic matter."<ref name='RAD January 2014'>{{cite journal | title = Mars' Surface Radiation Environment Measured with the Mars ScienceLaboratory's Curiosity Rover | journal = Science | date = 24 January 2014 | first = Donald M. | last = Hassler | coauthors = ''et al'' | volume = 343 | issue = 6169| pages = 1244797 | id =  | url = http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf | format = PDF | accessdate = 2014-01-27 | doi=10.1126/science.1244797| pmid = 24324275 }}</ref> Regardless of the source of Martian [[organic matter]] (meteoritic, geological, or biological), its carbon bonds are susceptible to breaking and reconfigurating with surrounding elements by ionizing charged particle radiation.<ref name='RAD January 2014'/>  These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic [[biosignature]]s as a function of depth as well as survival
 times of possible microbial or bacterial life forms left dormant beneath the surface.<ref name='RAD January 2014'/> The report concludes that the ''in situ'' "surface measurements —and subsurface estimates— constrain the preservation window for Martian organic matter following exhumation and exposure to ionizing radiation in the top few meters of the Martian surface."<ref name='RAD January 2014'/>
 === Nitrogen fixation ===
-After carbon, [[nitrogen]] is arguably the most important element needed for life. Thus, measurements of [[nitrate]] over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N<sub>2</sub>) in the atmosphere at low levels, but this is not adequate to support [[nitrogen fixation]] for biological incorporation.<ref name='Icebreaker2018'>{{cite journal | title = The ''Icebreaker Life'' Mission to Mars: A Search for Biomolecular Evidence for Life | journal = Astrobiology | date = April 5, 2013 | first = Christopher P. McKay | coauthors = Carol R. Stoker, Brian J. Glass, Arwen I. Davé, Alfonso F. Davila, Jennifer L. Heldmann, Margarita M. Marinova, Alberto G. Fairen, Richard C. Quinn, Kris A. Zacny, Gale Paulsen, Peter H. Smith, Victor Parro, Dale T. Andersen, Michael H. Hecht, Denis Lacelle, and Wayne H. Pollard. | volume = 13 | issue = 4 | pages = 334–353 | doi = 10.1089/ast.2012.0878 | url = http://online.liebertpub.com/doi/full/10.1089/ast.2012.0878 | accessdate = 2013-06-30|bibcode = 2013AsBio..13..334M }}</ref> Nitrogen in the form of [[nitrate]], if present, could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed in shock and electrical processes. Currently there is no data on its availability.<ref name='Icebreaker2018' />
+After carbon, [[nitrogen]] is arguably the most important element needed for life. Thus, measurements of [[nitrate]] over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N<sub>2</sub>) in the atmosphere at low levels, but this is not adequate to support [[nitrogen fixation]] for biological incorporation.<ref name='Icebreaker2018'>{{cite journal | title = The ''Icebreaker Life'' Mission to Mars: A Search for Biomolecular Evidence for Life | journal = Astrobiology | date = April 5, 2013 | first = Christopher P. McKay | coauthors = Carol R. Stoker, Brian J. Glass, Arwen I. Davé, Alfonso F. Davila, Jennifer L. Heldmann, Margarita M. Marinova, Alberto G. Fairen, Richard C. Quinn, Kris A. Zacny, Gale Paulsen, Peter H. Smith, Victor Parro, Dale T. Andersen, Michael H. Hecht, Denis Lacelle, and Wayne H. Pollard. | volume = 13 | issue = 4 | pages = 334–353 | doi = 10.1089/ast.2012.0878 | pmid = 23560417 |bibcode = 2013AsBio..13..334M | last1 = McKay }}</ref> Nitrogen in the form of [[nitrate]], if present, could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed in shock and electrical processes. Currently there is no data on its availability.<ref name='Icebreaker2018' />
 === Low pressure ===
-Further complicating estimates of the habitability of the Martian surface is the fact that very little is known on the growth of microorganisms at pressures close to the conditions found on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar).<ref name=Serratia>{{cite journal | title = Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres | journal = Astrobiology | date = February 2013 | first = Andrew C. Schuerger | coauthors = Richard Ulrich, Bonnie J. Berry, and Wayne L. Nicholson. | volume = 13 | issue = 2 | pages = 115–131 | doi = 10.1089/ast.2011.0811 | url = http://online.liebertpub.com/doi/full/10.1089/ast.2011.0811 | accessdate = 2013-07-04|bibcode = 2013AsBio..13..115S }}</ref> In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only ''[[Serratia liquefaciens]]'' strain ATCC 27592 exhibited growth at 7 mbar, 0°C, and CO<sub>2</sub>-enriched anoxic atmospheres.<ref name=Serratia />
+Further complicating estimates of the habitability of the Martian surface is the fact that very little is known on the growth of microorganisms at pressures close to the conditions found on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar).<ref name=Serratia>{{cite journal | title = Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres | journal = Astrobiology | date = February 2013 | first = Andrew C. Schuerger | coauthors = Richard Ulrich, Bonnie J. Berry, and Wayne L. Nicholson. | volume = 13 | issue = 2 | pages = 115–131 | doi = 10.1089/ast.2011.0811 | pmid = 23289858 |bibcode = 2013AsBio..13..115S | last1 = Schuerger | pmc = 3582281 }}</ref> In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only ''[[Serratia liquefaciens]]'' strain ATCC 27592 exhibited growth at 7 mbar, 0°C, and CO<sub>2</sub>-enriched anoxic atmospheres.<ref name=Serratia />
 == Liquid water ==
@@ Line 82: / Line 82: @@
 In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.<ref name=underground>{{cite journal |bibcode=2000Sci...288.2330M |title=Evidence for Recent Groundwater Seepage and Surface Runoff on Mars |last1=Malin |first1=Michael C. |last2=Edgett |first2=Kenneth S. |volume=288 |year=2000 |pages=2330–5 |journal=Science |doi=10.1126/science.288.5475.2330 |pmid=10875910 |issue=5475}}</ref><ref>{{cite conference |url=http://www.planets.ucla.edu/wp-content/form-data/mars-abstracts-2013/37-Martinez_2013_UCLA_Mars_Habitability.pdf |title=Present Day Liquid Water On Mars: Theoretical Expectations, Observational Evidence And Preferred Locations |first1=G. M. |last1=Martínez |first2=N. O. |last2=Renno |first3=H. M. |last3=Elliott |first4=E. |last4=Fischer |year=2013 |conference=The Present-day Mars Habitability Conference |location=Los Angeles}}</ref>  Additional similar images were published in 2006, taken by the [[Mars Global Surveyor]], that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in steep crater walls and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago.
-There is  disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.<ref>{{cite journal | doi = 10.1016/j.icarus.2009.09.009 | last1 = Kolb | first1 = K. | last2 = Pelletier | year = 2010 | first2 = Jon D. | last3 = McEwen | first3 = Alfred S. | title = Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water | url = | journal=Icarus | volume = 205 | pages = 113–137 |bibcode = 2010Icar..205..113K }}</ref><ref name="ReferenceB">{{cite web| url=http://www.sciencedirect.com/science/journal/00191035 |title=Icarus &#124; Vol 218, Iss 1, In Progress, (March, 2012) |publisher=ScienceDirect.com |date=2004-06-08 |accessdate=2012-01-16}}{{page needed|date=June 2013}}</ref><ref name=moon>[http://uanews.org/cgi-bin/WebObjects/UANews.woa/1/wa/SRStoryDetails?ArticleID=12376 "University of Arizona Press Release"] March 16, 2006.</ref><ref>{{cite journal| title=Mars Orbiter's Swan Song: The Red Planet Is A-Changin' |journal=Science |date=2006-12-08|first=Richard| last=Kerr| volume=314|issue=5805 |pages=1528–1529 |doi=10.1126/science.314.5805.1528| pmid=17158298}}</ref> Others suggest it may be liquid [[brine]] near the surface,<ref name=voanews>{{cite web| url= http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html| title=NASA Finds Possible Signs of Flowing Water on Mars| publisher=voanews.com| accessdate=August 5, 2011}}</ref><ref name=Ames>{{cite web|author=Source: Ames Research Center Posted Saturday, June 6, 2009 |url=http://www.spaceref.com/news/viewpr.html?pid=28377 |title=NASA Scientists Find Evidence for Liquid Water on a Frozen Early Mars |publisher=SpaceRef |date=June 6, 2009 |accessdate=December 19, 2010}}</ref><ref>{{cite web|url=http://www.space.com/scienceastronomy/mars-phoenix-water-salt-data-100831.html |title=Dead Spacecraft on Mars Lives on in New Study |publisher=SPACE.com |date=June 10, 2008 |accessdate=December 19, 2010}}</ref> but the exact source of the water and the mechanism behind its motion are not understood.<ref name=hirise>{{cite journal |bibcode=2011Sci...333..740M |title=Seasonal Flows on Warm Martian Slopes |last1=McEwen |first1=Alfred S. |last2=Ojha |first2=Lujendra |last3=Dundas |first3=Colin M. |last4=Mattson |first4=Sarah S. |last5=Byrne |first5=Shane |last6=Wray |first6=James J. |last7=Cull |first7=Selby C. |last8=Murchie |first8=Scott L. |last9=Thomas |first9=Nicolas |volume=333 |year=2011 |pages=740–3 |journal=Science |doi=10.1126/science.1204816 |pmid=21817049 |issue=6043}}</ref>
+There is  disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.<ref>{{cite journal | doi = 10.1016/j.icarus.2009.09.009 | last1 = Kolb | first1 = K. | last2 = Pelletier | year = 2010 | first2 = Jon D. | last3 = McEwen | first3 = Alfred S. | title = Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water | url = | journal=Icarus | volume = 205 | pages = 113–137 |bibcode = 2010Icar..205..113K }}</ref><ref name="ReferenceB">{{cite web| url=http://www.sciencedirect.com/science/journal/00191035 |title=Icarus &#124; Vol 218, Iss 1, In Progress, (March, 2012) |publisher=ScienceDirect.com |date=2004-06-08 |accessdate=2012-01-16}}{{page needed|date=June 2013}}</ref><ref name=moon>[http://uanews.org/cgi-bin/WebObjects/UANews.woa/1/wa/SRStoryDetails?ArticleID=12376 "University of Arizona Press Release"] March 16, 2006.</ref><ref>{{cite journal| title=Mars Orbiter's Swan Song: The Red Planet Is A-Changin' |journal=Science |date=2006-12-08|first=Richard| last=Kerr| volume=314|issue=5805 |pages=1528–1529 |doi=10.1126/science.314.5805.1528| pmid=17158298}}</ref> Others suggest it may be liquid [[brine]] near the surface,<ref name=voanews>{{cite web| url= http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html| title=NASA Finds Possible Signs of Flowing Water on Mars| publisher=voanews.com| accessdate=August 5, 2011}}</ref><ref name=Ames>{{cite web|author=Source: Ames Research Center Posted Saturday, June 6, 2009 |url=http://www.spaceref.com/news/viewpr.html?pid=28377 |title=NASA Scientists Find Evidence for Liquid Water on a Frozen Early Mars |publisher=SpaceRef |date=June 6, 2009 |accessdate=December 19, 2010}}</ref><ref>{{cite web|url=http://www.space.com/scienceastronomy/mars-phoenix-water-salt-data-100831.html |title=Dead Spacecraft on Mars Lives on in New Study |publisher=SPACE.com |date=June 10, 2008 |accessdate=December 19, 2010}}</ref> but the exact source of the water and the mechanism behind its motion are not understood.<ref name=hirise>{{cite journal |bibcode=2011Sci...333..740M |title=Seasonal Flows on Warm Martian Slopes |last1=McEwen |first1=Alfred S. |last2=Ojha |first2=Lujendra |last3=Dundas |first3=Colin M. |last4=Mattson |first4=Sarah S. |last5=Byrne |first5=Shane |last6=Wray |first6=James J. |last7=Cull |first7=Selby C. |last8=Murchie |first8=Scott L. |last9=Thomas |first9=Nicolas |last10=Gulick |first10=V. C. |volume=333 |year=2011 |pages=740–3 |journal=Science |doi=10.1126/science.1204816 |pmid=21817049 |issue=6043|display-authors=8 }}</ref>
 === Silica ===
@@ Line 88: / Line 88: @@
 In May 2007, the [[Spirit rover]] disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in [[silica]] (90%).<ref>{{cite press release |title=Mars Rover Spirit Unearths Surprise Evidence of Wetter Past |date=May 21, 2007 |publisher=[[Jet Propulsion Laboratory]] |url=http://www.nasa.gov/mission_pages/mer/mer-20070521.html |accessdate=June 27, 2013}}</ref> The feature is reminiscent of the effect of [[hot spring]] water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life, and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.<ref Name="20071210a">{{cite press release |title=Mars Rover Investigates Signs of Steamy Martian Past |publisher=[[Jet Propulsion Laboratory]] |date=December 10, 2007 |url=http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20071210a.html |accessdate=June 27, 2013}}</ref>
-Based on Earth analogs, [[Hydrothermal vent|hydrothermal systems]] on Mars would be highly attractive for their potential for preserving [[Organic compound|organic]] and [[Inorganic compound|inorganic]] [[biosignature]]s.<ref name='Leveille'>{{cite journal |bibcode=2010AGUFM.P12A..07L |title=Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars |last1=Leveille |first1=R. J. |volume=12 |year=2010 |pages=07 |journal=American Geophysical Union}}</ref><ref>{{cite journal |bibcode=1993Icar..101..129W |title=Preservation of Biological Information in Thermal Spring Deposits: Developing a Strategy for the Search for Fossil Life on Mars |last1=Walter |first1=M. R. |last2=Des Marais |first2=David J. |volume=101 |year=1993 |pages=129–43 |journal=Icarus |doi=10.1006/icar.1993.1011 |pmid=11536937 |issue=1}}</ref><ref>{{cite journal |bibcode=2000Icar..147...49A |title=Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars |last1=Allen |first1=Carlton C. |last2=Albert |first2=Fred G. |last3=Chafetz |first3=Henry S. |last4=Combie |first4=Joan |last5=Graham |first5=Catherine R. |last6=Kieft |first6=Thomas L. |last7=Kivett |first7=Steven J. |last8=McKay |first8=David S. |last9=Steele |first9=Andrew |volume=147 |year=2000 |pages=49–67 |journal=Icarus |doi=10.1006/icar.2000.6435 |pmid=11543582 |issue=1}}</ref>  For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.<ref>{{cite journal |bibcode=1999JGR...104.8489W |title=A Mössbauer investigation of iron-rich terrestrial hydrothermal vent systems: Lessons for Mars exploration |last1=Wade |first1=Manson L. |last2=Agresti |first2=David G. |last3=Wdowiak |first3=Thomas J. |last4=Armendarez |first4=Lawrence P. |last5=Farmer |first5=Jack D. |volume=104 |year=1999 |pages=8489–507 |journal=Journal of Geophysical Research |doi=10.1029/1998JE900049 |pmid=11542933 |issue=E4}}</ref><ref>{{cite journal |bibcode=1995LPI....26....7A |title=A Mossbauer Investigation of Hot Springs Iron Deposits |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |last5=Farmer |first5=J. D. |volume=26 |year=1995 |pages=7 |journal=Abstracts of the Lunar and Planetary Science Conference}}</ref><ref>{{cite journal |bibcode=1997LPICo.916....1A |title=Mössbauer Spectroscopy of Thermal Springs Iron Deposits as Martian Analogs |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |volume=916 |year=1997 |pages=1 |journal=Early Mars: Geologic and Hydrologic Evolution}}</ref>
+Based on Earth analogs, [[Hydrothermal vent|hydrothermal systems]] on Mars would be highly attractive for their potential for preserving [[Organic compound|organic]] and [[Inorganic compound|inorganic]] [[biosignature]]s.<ref name='Leveille'>{{cite journal |bibcode=2010AGUFM.P12A..07L |title=Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars |last1=Leveille |first1=R. J. |volume=12 |year=2010 |pages=07 |journal=American Geophysical Union}}</ref><ref>{{cite journal |bibcode=1993Icar..101..129W |title=Preservation of Biological Information in Thermal Spring Deposits: Developing a Strategy for the Search for Fossil Life on Mars |last1=Walter |first1=M. R. |last2=Des Marais |first2=David J. |volume=101 |year=1993 |pages=129–43 |journal=Icarus |doi=10.1006/icar.1993.1011 |pmid=11536937 |issue=1}}</ref><ref>{{cite journal |bibcode=2000Icar..147...49A |title=Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars |last1=Allen |first1=Carlton C. |last2=Albert |first2=Fred G. |last3=Chafetz |first3=Henry S. |last4=Combie |first4=Joan |last5=Graham |first5=Catherine R. |last6=Kieft |first6=Thomas L. |last7=Kivett |first7=Steven J. |last8=McKay |first8=David S. |last9=Steele |first9=Andrew |last10=Taunton |first10=A. E. |last11=Taylor |first11=M. R. |last12=Thomas-Keprta |first12=K. L. |last13=Westall |first13=F |volume=147 |year=2000 |pages=49–67 |journal=Icarus |doi=10.1006/icar.2000.6435 |pmid=11543582 |issue=1|display-authors=8 }}</ref>  For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.<ref>{{cite journal |bibcode=1999JGR...104.8489W |title=A Mössbauer investigation of iron-rich terrestrial hydrothermal vent systems: Lessons for Mars exploration |last1=Wade |first1=Manson L. |last2=Agresti |first2=David G. |last3=Wdowiak |first3=Thomas J. |last4=Armendarez |first4=Lawrence P. |last5=Farmer |first5=Jack D. |volume=104 |year=1999 |pages=8489–507 |journal=Journal of Geophysical Research |doi=10.1029/1998JE900049 |pmid=11542933 |issue=E4}}</ref><ref>{{cite journal |bibcode=1995LPI....26....7A |title=A Mossbauer Investigation of Hot Springs Iron Deposits |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |last5=Farmer |first5=J. D. |volume=26 |year=1995 |pages=7 |journal=Abstracts of the Lunar and Planetary Science Conference}}</ref><ref>{{cite journal |bibcode=1997LPICo.916....1A |title=Mössbauer Spectroscopy of Thermal Springs Iron Deposits as Martian Analogs |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |volume=916 |year=1997 |pages=1 |journal=Early Mars: Geologic and Hydrologic Evolution}}</ref>
 == Possible biosignatures ==
@@ Line 104: / Line 104: @@
 In June 2012, scientists reported that measuring the ratio of [[hydrogen]] and [[methane]] levels on Mars may help determine the likelihood of life on Mars.<ref name="PNAS-20120607">{{cite journal |last1=Oze |first1=Christopher |last2=Jones |first2=Camille |last3=Goldsmith |first3=Jonas I. |last4=Rosenbauer |first4=Robert J. |title=Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces |date=June 7, 2012 |journal=[[PNAS]] |volume=109| issue =  25 |pages=9750–9754 |doi=10.1073/pnas.1205223109 |bibcode=2012PNAS..109.9750O |pmid=22679287 |pmc=3382529}}</ref><ref name="Space-20120625">{{cite web |author=Staff |title=Mars Life Could Leave Traces in Red Planet's Air: Study |url=http://www.space.com/16284-mars-life-atmosphere-hydrogen-methane.html |date=June 25, 2012 |publisher=[[Space.com]] |accessdate=June 27, 2012 }}</ref> According to the scientists, "...low H<sub>2</sub>/CH<sub>4</sub> ratios (less than approximately 40) indicate that life is likely present and active."<ref name="PNAS-20120607" /> Other scientists have recently reported methods of detecting hydrogen and methane in [[extraterrestrial atmospheres]].<ref name="Nature-20120627">{{cite journal |last1=Brogi |first1=Matteo |last2=Snellen |first2=Ignas A. G. |last3=de Krok |first3=Remco J. |last4=Albrecht |first4=Simon |last5=Birkby |first5=Jayne |last6=de Mooij |first6=Ernest J. W. |title=The signature of orbital motion from the dayside of the planet τ Boötis b |date=June 28, 2012 |journal=[[Nature (journal)|Nature]] |volume=486 |pages=502–504 |doi=10.1038/nature11161 |arxiv=1206.6109 |bibcode=2012Natur.486..502B |issue=7404 }}</ref><ref name="Wired-20120627">{{cite web |last=Mann |first=Adam |title=New View of Exoplanets Will Aid Search for E.T. |url=http://www.wired.com/wiredscience/2012/06/tau-bootis-b/ |date=June 27, 2012 |publisher=[[Wired (magazine)]] |accessdate=June 28, 2012 }}</ref>
-In contrast to the findings described above, studies by Kevin Zahnle, a planetary scientist at NASA's [[Ames Research Center]], and two colleagues, conclude that "there is as yet no compelling evidence for methane on Mars". They argue that the strongest reported observations of the gas to date have been taken at frequencies where interference from methane in [[atmosphere of Earth|Earth's atmosphere]] is particularly difficult to remove, and are thus unreliable. Additionally, they claim that the published observations most favorable to interpretation as indicative of Martian methane are also consistent with no methane being present on Mars.<ref>{{citation | first = Kevin Zahnle | coauthors = Richard Freedman and David Catling | contribution = 42nd Lunar and Planetary Science Conference | title = Is there Methane on Mars? Part II | publisher = Lunar and Planetary Institute | year = 2011| id = | contribution-url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2427.pdf | format = PDF | accessdate = 2011-08-06}}</ref><ref>{{cite news | title = Methane on Mars. Now you see it. Now you don't... | date = 29 December 2010 | url = http://www.economist.com/node/17797286 | work = The Economist | accessdate = 2011-08-06}}</ref><ref>{{cite journal|author=Kevin Zahnle, Richard S. Freedman and David C. Catling |title=Is there methane on Mars?| journal=Icarus |volume=212 | issue=2 |pages= 493–503 |year=2011| url=http://www.sciencedirect.com/science/article/pii/S001910351000446X |doi= 10.1016/j.icarus.2010.11.027 |pmid= |bibcode =  2011Icar..212..493Z}}</ref>
+In contrast to the findings described above, studies by Kevin Zahnle, a planetary scientist at NASA's [[Ames Research Center]], and two colleagues, conclude that "there is as yet no compelling evidence for methane on Mars". They argue that the strongest reported observations of the gas to date have been taken at frequencies where interference from methane in [[atmosphere of Earth|Earth's atmosphere]] is particularly difficult to remove, and are thus unreliable. Additionally, they claim that the published observations most favorable to interpretation as indicative of Martian methane are also consistent with no methane being present on Mars.<ref>{{Cite document| first = Kevin Zahnle | coauthors = Richard Freedman and David Catling | contribution = 42nd Lunar and Planetary Science Conference | title = Is there Methane on Mars? Part II | publisher = Lunar and Planetary Institute | year = 2011| id = | contribution-url = http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2427.pdf | format = PDF | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->&#123;&#123;inconsistent citations&#125;&#125; }}</ref><ref>{{cite news | title = Methane on Mars. Now you see it. Now you don't... | date = 29 December 2010 | url = http://www.economist.com/node/17797286 | work = The Economist | accessdate = 2011-08-06}}</ref><ref>{{cite journal|author=Kevin Zahnle, Richard S. Freedman and David C. Catling |title=Is there methane on Mars?| journal=Icarus |volume=212 | issue=2 |pages= 493–503 |year=2011| url=http://www.sciencedirect.com/science/article/pii/S001910351000446X |doi= 10.1016/j.icarus.2010.11.027 |pmid= |bibcode =  2011Icar..212..493Z}}</ref>
 The [[Curiosity (rover)|''Curiosity'' rover]], which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane;<ref>{{cite web |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2765&mode=thread&order=0&thold=0 |title=Making Sense of Mars Methane |accessdate=October 8, 2008 |last=Tenenbaum |first=David |date=June 9, 2008 |work=Astrobiology Magazine|archiveurl= http://web.archive.org/web/20080923195833/http://astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2765&mode=thread&order=0&thold=0 |archivedate= 23 September 2008 <!--DASHBot-->| deadurl= no}}</ref> but even if the mission is to determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside of the rover's reach.<ref>{{cite news|first=Bill|last=Steigerwald|authorlink=|title=Martian Methane Reveals the Red Planet is not a Dead Planet|date=January 15, 2009|publisher=NASA|url=http://www.nasa.gov/mission_pages/mars/news/marsmethane.html|work=NASA's Goddard Space Flight Center|pages=| accessdate=January 24, 2009|language=| archiveurl=http://web.archive.org/web/20090117141425/http://www.nasa.gov/mission_pages/mars/news/marsmethane.html|archivedate= 17 January 2009 <!--DASHBot-->| deadurl= no}}</ref> The first measurements with the [[Sample Analysis at Mars|Tunable Laser Spectrometer (TLS)]] in the ''Curiosity'' rover indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.<ref>{{cite web |url=http://www.ustream.tv/nasajpl |title=Mars Curiosity Rover News Telecon -November 2, 2012}}</ref><ref name="Science-20121102">{{cite web |last=Kerr |first=Richard A. |title=Curiosity Finds Methane on Mars, or Not|url=http://news.sciencemag.org/sciencenow/2012/11/curiosity-finds-methane-on-mars-.html |date=November 2, 2012|publisher=[[Science (journal)]] |accessdate=November 3, 2012 }}</ref><ref name="Space-20121102">{{cite web|last=Wall |first=Mike |title=Curiosity Rover Finds No Methane on Mars — Yet|url=http://www.space.com/18333-mars-rover-curiosity-methane-measurements.html |date=November 2, 2012|publisher=[[Space.com]] |accessdate=November 3, 2012 }}</ref><ref name="NYT-20121102">{{cite news |last=Chang|first=Kenneth |title=Hope of Methane on Mars Fades|url=http://www.nytimes.com/2012/11/03/science/space/hopes-for-methane-on-mars-deflated.html |date=November 2, 2012 |publisher=[[New York Times]] |accessdate=November 3, 2012 }}</ref>  On July 19, 2013, NASA scientists published the results of a new analysis of the atmosphere of Mars, reporting a lack of [[methane]] around the [[Gale Crater|landing site]] of the [[Curiosity (rover)|''Curiosity'' rover]].<ref name="Wired-20130718">{{cite web |last=Mann |first=Adam |title=Mars Rover Finds Good News for Past Life, Bad News for Current Life on Mars |url=http://www.wired.com/wiredscience/2013/07/curiosity-mars-atmosphere |date=18 July 2013 |work=[[Wired (magazine)]] |accessdate=19 July 2013 }}</ref><ref name="SCI-20130719a">{{cite journal |author=Webster Chris R. et al. |title=Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere |url=http://www.sciencemag.org/content/341/6143/260 |date=19 July 2013 |journal=[[Science (journal)|Science]] |volume=341 |number=6143 |pages=260–263 |doi=10.1126/science.1237961 |accessdate=19 July 2013 }}</ref><ref name="SCI-20130719b">{{cite journal |author=Mahaffy, Paul R. et al. |title=Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover |url=http://www.sciencemag.org/content/341/6143/263 |date=19 July 2013 |journal=[[Science (journal)|Science]] |volume=341 |number=6143 |pages=263–266 |doi=10.1126/science.1237966 |accessdate=19 July 2013 }}</ref> On September 19, 2013, NASA again reported no detection of [[atmospheric methane]] with a measured value of {{val|0.18|0.67}} ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, conclude that the probability of current methanogenic microbial activity on Mars is reduced.<ref name="SJ-20130919">{{cite journal |last1=Webster |first1=Christopher R. |last2=Mahaffy |first2=Paul R. |last3=Atreya |first3=Sushil K. |last4=Flesch |first4=Gregory J. |last5=Farley |first5=Kenneth A. |title=Low Upper Limit to Methane Abundance on Mars |url=http://www.sciencemag.org/content/early/2013/09/18/science.1242902.abstract |date=September 19, 2013 |journal=[[Science (journal)|Science]] |doi=10.1126/science.1242902 |accessdate=September 19, 2013 }}</ref><ref name="SCI-20130919">{{cite journal |last=Cho |first=Adrian |title=Mars Rover Finds No Evidence of Burps and Farts |url=http://news.sciencemag.org/space/2013/09/mars-rover-finds-no-evidence-burps-and-farts