Life on Mars
|This article is one of a series on:|
|Life in the Universe|
|Life in the Solar System|
|Life on Venus|
|Life on Earth|
|Life on Mars|
|Life on Europa|
|Life on Titan|
Life outside the Solar System
|Circumstellar habitable zone|
The possibility of life on Mars is a subject of significant interest to astrobiology due to the planet's proximity and similarities to Earth. To date no proof has been found of past or present life on Mars. However, cumulative evidence is now building that the ancient surface environment of Mars had liquid water and may have been habitable for microorganisms. The existence of habitable conditions does not necessarily indicate the presence of life.
Scientific 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. On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.
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.
On January 24, 2014, NASA reported that the Curiosity and Opportunity rovers started searching for evidence of past life, including a biosphere based on autotrophic, chemotrophic, or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
- 1 Early speculation
- 2 Habitability
- 3 Liquid water
- 4 Possible biosignatures
- 5 Geysers on Mars
- 6 Forward contamination
- 7 Life under simulated Martian conditions
- 8 Missions
- 9 Interactive Mars map
- 10 See also
- 11 References
- 12 External links
Mars' polar ice caps were discovered in the mid-17th century. In the latter part of the 18th century, William Herschel proved they grow and shrink alternately, in the summer and winter of each hemisphere. 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.[original research?]
In 1853, 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. 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. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.
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. 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.
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. Similarly, for each group of parameters, the habitability threshold for each is to be determined. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly. There are no full-Mars simulations published yet that include all of the biocidal factors combined.
|Water|| · liquid water activity (aw)
· Past/future liquid (ice) inventories
· Salinity, pH, and Eh of available water
· C, H, N, O, P, S, essential metals, essential micronutrients
· Fixed nitrogen
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)
· Redox gradients
| · 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., O2−, 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)
Recent models have shown that, even with a dense CO2 atmosphere, early Mars was, in fact, colder than Earth has ever been. However, transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid–late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars. Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid Hesperian onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both.
The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability. 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. The loss of the atmosphere was accompanied by decreasing temperatures. A part of the liquid water inventory sublimed and was transported to the poles, while the rest became trapped in a subsurface ice layer.
Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long lasting hydrothermal system when ice is present in the crust. For example, a 130 km large crater could sustain an active hydrothermal system for up to 2 million years, that is, long enough for microscopic life to emerge.
Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors. 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. 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. 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, together strongly suggest that Mars could have had the environmental factors to support life. 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.
Present day life on Mars could occur kilometers below the surface in the hydrosphere, or in subsurface geothermal hot spots, or it could occur on or near the surface. The permafrost layer on Mars is only a couple of centimeters below the surface. Salty brines can be liquid a few centimeters below that but not far down. Most of the proposed surface habitats are within centimeters of the surface. Any life deeper than that is likely to be dormant. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except when covered in ice or after a sudden release of water. The Mars atmosphere varies in pressure over geological timescales, and may have been able to host liquid water in the recent geological past.
So far, NASA has pursued a "Follow the water" strategy on Mars and has not searched for biosignatures for life there directly since Viking. The observations by Phoenix in 2008 of potential drops of liquid brines forming on its legs led to a renewed interest in the potential habitability of the surface of Mars. Nilton Renno and his team recently found a way that these droplets could form rapidly when salt and ice touch each other so may have formed when salt and ice from the surface got thrown up onto the legs during the landing. Since then, experiments have led to many suggestions for potential habitats on the surface of Mars. However, though liquid water is now confirmed to occur there in brine layers, it's not yet known whether any of the liquid water on Mars is habitable. This depends on factors such as the exact mix of salts and the local conditions on Mars.
Dormant subsurface life
Curiosity measured ionizing radiation levels of 76 mGy a year. This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. However, it varies considerably in habitability depending on its orbital eccentricity and the tilt of its axis. If the surface life has been reanimated as recently as 450,000 years ago, which is possible, then our rovers on Mars could find dormant but still viable life at a depth of only one meter below the surface, according to an estimate in the paper that published the Curiosity ionizing radiation measurements.
Habitability factors for non-dormant surface life
Modern researchers do not consider that ionizing radiation is a limiting factor in habitability assessments for present-day non-dormant surface life. The level of 76 mGy a year measured by Curiosity is similar to levels inside the ISS. In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:
- "From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars."
Here a SPE is a Solar Proton Event (solar storm) and a GCR is a Galactic Cosmic Ray. A "Special Region" is defined as a region on the Mars surface where Earth life could potentially survive.
On UV radiation, the report finds 
- "The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms."
Though the superoxidizing conditions are harmful to some microbes, there are many microbes that actually metabolize perchlorates on Earth. See Perchlorates - Biology. Nowadays perchlorates on Mars are generally thought as boosting habitability. Even when Phoenix discovered perchlorates in 2008, NASA said that the perchlorates do not rule out life on Mars. For a modern view on them, Cassie Conley, planetary protection officer for NASA is quoted in the New York times as saying:
"The salts known as perchlorates that lower the freezing temperature of water at the R.S.L.s, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said."
Recurrent Slope Lineae - potentially habitable
These features form on sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. This is hard to model in any other way except as involving liquid water in some form, though the streaks themselves are thought to be a secondary effect and not a direct indication of dampness of the regolith. Although these features are now confirmed to involve liquid water in some form, the water could be either too cold or too salty for life. At present they are treated as potentially habitable, as "Uncertain Regions, to be treated as Special Regions".
The "Special Regions" assessment says of them:
- "Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at > 250 K, with [water activity] unknown and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely"
They were first reported in the paper by McEwan in Science, August 5, 2011. They were already suspected as involving flowing brines back then, as all the other models available involved liquid water in some form. Finally proven pretty much conclusively to involve liquid water in some form, possibly habitable if temperatures and salinity are right - after detection of hydrated salts that change their hydration state rapidly, reported in a paper published on 28 September 2015 along with a press conference . The brines were not detected directly, because the resolution of the spectrometer isn't high enough for this, and also the brines probably flow in the morning. MRO is in a slowly precessing sun-synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3:00 pm, in the local solar time on the surface, all year round. This is the worst time of day to spot brines from orbit.
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. 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. As a result, the planet has been vulnerable to radiation from space for about 4 billion years. Currently, ionizing radiation on Mars is typically two orders of magnitude (or 100 times) higher than on Earth. Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars for that long. 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. 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.
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 a million years, depending on the type of rock.
Data collected by the Radiation assessment detector (RAD) instrument on board the Curiosity rover revealed that the actual absorbed dose measured is 76 mGy/year at the surface, and that "ionizing radiation strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic matter." Regardless of the source of Martian organic compounds (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfigurating with surrounding elements by ionizing charged particle radiation. 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. 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."
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. Nitrogen in the form of nitrate 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 by thermal shock from impact or volcanic plume lightning on ancient Mars.
On 24 March 2015, NASA reported that the SAM instrument on the Curiosity rover detected nitrates by heating surface sediments. The nitrogen in nitrate is in a "fixed" state, meaning that it is in an oxidized form that can be used by living organisms. The discovery supports the notion that ancient Mars may have been hospitable for life. Modeling indicates that the transient condensed water films on the surface should be transported to lower depths (≈10 m), where subsurface microorganisms could thrive.
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). 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.
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. Liquid water does not appear at the surface itself, but it could form in minuscule amounts around dust particles in snow heated by the Sun. Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.
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. A small amount of water vapor is present in the atmosphere. 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, there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface, including large oceans.
It has been estimated that the primordial oceans on Mars would have covered between 36% and 75% of the planet. On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior. 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, aw ≤ 0.78 to 0.86—a level fatal to most Terrestrial life. Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.
In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies. 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. Others suggest it may be liquid brine near the surface, but the exact source of the water and the mechanism behind its motion are not understood.
In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in silica (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.
Based on Earth analogs, hydrothermal systems on Mars would be highly attractive for their potential for preserving organic and inorganic biosignatures. For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.
Possible trace amounts of methane in the atmosphere of Mars were first discovered in 2003 with earth based telescopes and fully verified in 2004 by the ESA Mars Express spacecraft in orbit around Mars. 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, 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 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, research published in 2012 suggest that a source may be organic compounds on meteorites that are converted to methane by ultraviolet radiation.
The existence of life in the form of microorganisms such as methanogens is among possible, but as yet unproven sources. Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source and carbon dioxide (CO2) as their carbon source, so they could exist in subsurface environments on Mars. 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.
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. The results reported indicate that the perchlorates discovered by the Phoenix Lander would not rule out the possible presence of methanogens on Mars.
Research at the University of Arkansas presented in June 2015 suggested that some methanogens could survive on Mars's low pressure. Rebecca Mickol found that in her laboratory, four species of methanogens survived low-pressure conditions that were similar to a subsurface liquid aquifer on Mars. The four species that she tested were Methanothermobacter wolfeii, Methanosarcina barkeri, Methanobacterium formicicum, and Methanococcus maripaludis. 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. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.
The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane, and in 2014, Curiosity detected a "tenfold spike" in the level of methane in the Martian atmosphere compared to the usual background readings. However, 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 the rover's reach. 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. 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. On September 19, 2013, NASA again reported no detection of atmospheric methane with a measured value of ±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, concluded that the probability of current methanogenic microbial activity on Mars is reduced. 0.18 On 16 December 2014, NASA reported that Curiosity had detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7 ppb, suggesting that methane is released at intervals.
India's Mars Orbiter Mission, launched on November 5, 2013, is searching for methane in the atmosphere of Mars using its Methane Sensor for Mars (MSM). The orbiter has been orbiting Mars since September 24, 2014. The ExoMars Trace Gas Orbiter planned to launch in 2016 would further study the methane, as well as its decomposition products such as formaldehyde and methanol.
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. NASA scientists consider the preliminary findings well worth a follow-up, but have also rejected the claims of life.
Viking lander biological experiments
The 1970s Viking program placed two identical landers on the surface of Mars tasked to look for biosignatures of microbial life on the surface. Of the four experiments performed by each Viking lander, only the 'Labeled Release' (LR) experiment gave a positive result for metabolism, while the other three did not detect organic compounds. The LR was a specific experiment designed to test only a narrowly defined critical aspect of the theory concerning the possibility of life on Mars; therefore, the overall results were declared, officially, inconclusive. Further, no Mars lander mission has found meaningful traces of biomolecules or biosignatures. The claim of extant microbial life on Mars is based on old data collected by the Viking landers, currently reinterpreted as sufficient evidence of life, mainly by Gilbert Levin, Joseph D. Miller, Navarro, Giorgio Bianciardi and Patricia Ann Straat, that the Viking LR experiments did, indeed, detect extant microbial life on Mars.
Relatively recent assessments published in December 2010 by Rafael Navarro–Gonzáles indicate that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2. The study determined that perchlorate —discovered in 2008 by Phoenix lander— can destroy organic compounds when heated, and produce chloromethane and dichloromethane as byproduct, the identical chlorine compounds discovered by both Viking landers when they performed the same tests on Mars. Because perchlorate would have broken down any Martian organics, the question of whether or not Viking found organic compounds is still wide open.
The Labeled Release evidence was not generally accepted initially, and, to this day lacks the consensus of the scientific community.
NASA maintains a catalog of 34 Mars meteorites. 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.
Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:
- 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.
As of 2010, reexaminations of the biomorphs found in the three Martian meteorites are underway with more advanced analytical instruments than previously available.
In 1996, the Martian meteorite ALH84001, a specimen that is much older than the majority of Martian meteorites that have been recovered so far, received considerable attention when a group of NASA scientists led by David S. McKay reported microscopic features and geochemical anomalies that they considered to be best explained by the rock having hosted Martian bacteria in the distant past. Some of these features resembled terrestrial bacteria, aside from their being much smaller than any known form of life. Much controversy arose over this claim, and ultimately all of the evidence McKay's team cited as evidence of life were found to be explainable by non-biological processes. Although the scientific community has largely rejected the claim ALH 84001 contains evidence of ancient Martian life, the controversy associated with it is now seen as a historically significant moment in the development of exobiology.
In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects of the size and shape consistent with Earthly fossilized nanobacteria. 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".
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 in 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.
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. 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. Work is in progress on searching for magnetites within alteration phases.
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. 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.
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. 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. 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. Since their discovery, fiction writer Arthur C. Clarke promoted these formations as deserving of study from an astrobiological perspective.
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. 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. However, they also remark that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis. It has been proposed to develop the Mars Geyser Hopper lander to study the geysers up close.
Planetary protection of Mars aims to prevent biological contamination of the planet. 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.
The prime concern of hardware contaminating Mars derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (extremophiles) despite best efforts. 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 survival rates of radiation-resistant microorganisms including the species Deinococcus radiodurans and genera Brevundimonas, Rhodococcus, and Pseudomonas under simulated Martian conditions. Results from one of these 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. Surprisingly, the diurnal Mars-like cycles in temperature and relative humidity affected the viability of Deinococcus radiodurans cells quite severely. In other simulations, Deinococcus radiodurans also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.
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). 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.
Mars-1 was the first spacecraft launched to Mars in 1962, but communication was lost while en route to Mars. With Mars-2 and Mars-3 in 1971-1972, information was obtained on the nature of the surface rocks and altitude profiles of the surface density of the soil, its thermal conductivity, and thermal anomalies detected on the surface of Mars. The Program found that its northern polar cap has a temperature below -110 °C and that the water vapor content in the atmosphere of Mars is five thousand times less than on Earth. No signs of life were found.
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. 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.
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.
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. 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 ] 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."
One of the designers of the Labeled Release experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars. Levin's interpretation is disputed by many scientists. 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." Other scientists argue that superoxides in the soil could have produced this effect without life being present. 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. More recently, high levels of organic chemicals, particularly chlorobenzene, were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover. Nonetheless, the results of the Viking mission concerning life are considered by the general expert community, at best, as inconclusive.
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. Levin still maintains that his original data were correct, as the positive and negative control experiments were in order. 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." 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.
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. Klaus Biemann, the principal investigator of the GCMS experiment on Viking wrote a rebuttal. 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. 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." Biemann has written a commentary critical of this Navarro-González paper as well, to which the latter have replied; the exchange was published in December 2011.
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. Phoenix's preliminary data revealed that Mars soil contains perchlorate, and thus may not be as life-friendly as thought earlier. 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. A recent analysis of martian meteorite EETA79001 found 0.6 ppm ClO4−, 1.4 ppm ClO3−, and 16 ppm NO3−, most likely of martian origin. The ClO3− suggests presence of other highly oxidizing oxychlorines such as ClO2− or ClO, produced both by UV oxidation of Cl and X-ray radiolysis of ClO4−. Thus only highly refractory and/or well-protected (sub-surface) organics are likely to survive. In addition, recent analysis of the Phoenix WCL showed that the Ca(ClO4)2 in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO4)2 in contact with liquid water would have formed only CaSO4. This suggests a severely arid environment, with minimal or no liquid water interaction.
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. The Curiosity rover landed on Mars on Aeolis Palus in Gale Crater, near Aeolis Mons (a.k.a. Mount Sharp), on August 6, 2012.
On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of methane in the Martian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level. In addition, low levels of chlorobenzene (C
5Cl), were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover.
Future astrobiology 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 2020. 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 (or organic biosignatures) might survive cosmic radiation.
- Mars 2020 – The Mars 2020 rover is a Mars planetary rover mission by NASA with a planned 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.
- 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.
Interactive Mars map
- Circumstellar habitable zone
- Colonization of Mars
- Extraterrestrial life
- Hypothetical types of biochemistry
- Life on Callisto
- Life on Enceladus
- Life on Europa
- Life on Ganymede
- Life on Titan
- Life on Venus
- Planetary habitability
- Terraforming of Mars
- Water on Mars
- Modern Mars habitability
- Present day Mars habitability analogue environments on Earth
- Mumma, Michael J. (January 8, 2012). The Search for Life on Mars. Origin of Life Gordon Research Conference. Galveston, TX.
- Chang, Kenneth (September 12, 2016). "Visions of Life on Mars in Earth's Depths". New York Times. Retrieved September 12, 2016.
- Staff (November 22, 2016). "Scalloped Terrain Led to Finding of Buried Ice on Mars". NASA. Retrieved November 23, 2016.
- "Lake of frozen water the size of New Mexico found on Mars – NASA". The Register. November 22, 2016. Retrieved November 23, 2016.
- "Mars Ice Deposit Holds as Much Water as Lake Superior". NASA. November 22, 2016. Retrieved November 23, 2016.
- 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: . Bibcode:2007prpl.conf..929G.
- 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. Bibcode:2014Sci...343..386G. doi:10.1126/science.1249944. PMID 24458635.
- Various (January 24, 2014). "Special Issue - Table of Contents - Exploring Martian Habitability". Science. 343 (6169): 345–452.
- Various (January 24, 2014). "Special Collection - Curiosity - Exploring Martian Habitability". Science.
- Grotzinger, J. P.; Sumner, D. Y.; Kah, L. C.; Stack, K.; Gupta, S.; Edgar, L.; Rubin, D.; Lewis, K.; Schieber, J.; et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169, number 6169): 1242777. Bibcode:2014Sci...343G.386A. doi:10.1126/science.1242777. PMID 24324272.
- Dunlap, David W. (October 1, 2015). "Life on Mars? You Read It Here First.". New York Times. Retrieved October 1, 2015.
- 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, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.[page needed]
- 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.
- 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.
- Q. Choi,, Charles (May 17, 2010). "Mars Contamination Dust-Up". Astrobiology Magazine.
Whenever multiple biocidal factors are combined, the survival rates plummet quickly,
- Westall, Frances; Loizeau, Damien; Foucher, Frederic; Bost, Nicolas; Betrand, Marylene; Vago, Jorge; Kminek, Gerhard (2013). "Habitability on Mars from a Microbial Point of View". Astrobiology. 13 (18): 887–897. Bibcode:2013AsBio..13..887W. doi:10.1089/ast.2013.1000. PMID 24015806.
- Staff (June 8, 2015). "PIA19673: Spectral Signals Indicating Impact Glass on Mars". NASA. Retrieved June 8, 2015.
- 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. 24. pp. 279–300. doi:10.1007/978-0-387-74288-5_10. ISBN 978-0-387-74287-8.
- "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars". NASA. March 12, 2013.
- Chang, Kenneth (December 9, 2013). "On Mars, an Ancient Lake and Perhaps Life". New York Times.
- Various (December 9, 2013). "Science - Special Collection - Curiosity Rover on Mars". Science.
- Neal-Jones, Nancy; O'Carroll, Cynthia (October 12, 2005). "New Map Provides More Evidence Mars Once Like Earth". Goddard Space Flight Center. NASA.
- "Martian Interior: Paleomagnetism". Mars Express. European Space Agency. January 4, 2007.
- Wall, Mike (March 25, 2011). "Q & A with Mars Life-Seeker Chris Carr". Space.com.
- "Ames Instrument Helps Identify the First Habitable Environment on Mars, Wins Invention Award". Ames Research Center. Space Ref. June 24, 2014. Retrieved 2014-08-11.
- Temming, Maria. "Exotic Glass Could Help Unravel Mysteries of Mars". Retrieved 2015-06-15.
- Gronstal, Aaron L. (Jul 3, 2014). "Liquid Water from Ice and Salt on Mars".
- Donald M. Hassler, Cary Zeitlin, Robert F. Wimmer-Schweingruber, Bent Ehresmann, Scot Rafkin, Jennifer L. Eigenbrode, David E. Brinza, Gerald Weigle, Stephan Böttcher, Eckart Böhm, Soenke Burmeister, Jingnan Guo, Jan Köhler, Cesar Martin, Guenther Reitz, Francis A. Cucinotta, Myung-Hee Kim, David Grinspoon, Mark A. Bullock, Arik Posner, Javier Gómez-Elvira, Ashwin Vasavada, and John P. Grotzinger, and the MSL Science Team (12 November 2013). "Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover" (PDF). Science: 7.
- Donald M. Hassler, Cary Zeitlin, Robert F. Wimmer-Schweingruber, Bent Ehresmann, Scot Rafkin, Jennifer L. Eigenbrode, David E. Brinza, Gerald Weigle, Stephan Böttcher, Eckart Böhm, Soenke Burmeister, Jingnan Guo, Jan Köhler, Cesar Martin, Guenther Reitz, Francis A. Cucinotta, Myung-Hee Kim, David Grinspoon, Mark A. Bullock, Arik Posner, Javier Gómez-Elvira, Ashwin Vasavada, and John P. Grotzinger, and the MSL Science Team (12 November 2013). "Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover" (PDF). Science: 8.
- Joanna Carver and Victoria Jaggard (21 November 2012). "Mars is safe from radiation – but the trip there isn't". New Scientist.
- Rummel, John D.; Beaty, David W.; Jones, Melissa A.; Bakermans, Corien; Barlow, Nadine G.; Boston, Penelope J.; Chevrier, Vincent F.; Clark, Benton C.; de Vera, Jean-Pierre P.; Gough, Raina V.; Hallsworth, John E.; Head, James W.; Hipkin, Victoria J.; Kieft, Thomas L.; McEwen, Alfred S.; Mellon, Michael T.; Mikucki, Jill A.; Nicholson, Wayne L.; Omelon, Christopher R.; Peterson, Ronald; Roden, Eric E.; Sherwood Lollar, Barbara; Tanaka, Kenneth L.; Viola, Donna; Wray, James J. (2014). "A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)" (PDF). Astrobiology. 14 (11): 887–968. doi:10.1089/ast.2014.1227. ISSN 1531-1074.
- Minkel, JR (August 5, 2008). "NASA Says Perchlorate Does Not Rule Out Life on Mars - Unexpected chemical in Martian soil is a food source for some Earthly microbes". Scientific American.
- CHANG, KENNETH (October 5, 2015). "Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way".
- Chang, Kenneth (October 5, 2015). "Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way.". New York Times.
- "Warm-Season Flows on Slope in Newton Crater". NASA Press Release.
- Amos, Jonathan. "Martian salt streaks 'painted by liquid water'". BBC Science.
- Staff (28 September 2015). "Video Highlight - NASA News Conference - Evidence of Liquid Water on Today's Mars". NASA. Retrieved 30 September 2015.
- Staff (28 September 2015). "Video Complete - NASA News Conference - Water Flowing on Present-Day Mars m". NASA. Retrieved 30 September 2015.
- Ojha, L.; Wilhelm, M. B.; Murchie, S. L.; McEwen, A. S.; Wray, J. J.; Hanley, J.; Massé, M.; Chojnacki, M. (2015). "Spectral evidence for hydrated salts in recurring slope lineae on Mars". Nature Geoscience. 8 (11): 829–832. Bibcode:2015NatGe...8..829O. doi:10.1038/ngeo2546.
- "Mars Reconnaissance Orbiter Telecommunications" (PDF). JPL. September 2006.
- Luhmann, J. G.; Russell, C. T. (1997). "Mars: Magnetic Field and Magnetosphere". In Shirley, J. H.; Fainbridge, R. W. Encyclopedia of Planetary Sciences. New York: Chapman and Hall. pp. 454–6.
- Phillips, Tony (January 31, 2001). "The Solar Wind at Mars". NASA.
- "What makes Mars so hostile to life?". BBC News. January 7, 2013.
- Keating, A.; Goncalves, P. (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.
- Than, Ker (January 29, 2007). "Study: Surface of Mars Devoid of Life". Space.com.
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.
- Dartnell, Lewis R.; Storrie-Storrie-Lombardi, Michael C.; Muller, Jan-Peter; Griffiths, Andrew. D.; Coates, Andrew J.; Ward, John M. (2011). "Implications of Cosmic Radiation on the Martian Surface for Microbial Survival and Detection of Fluorescent Biosignatures" (PDF). Lunar and Planetary Institute. 42: 1977. Bibcode:2011LPI....42.1977D.
- 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.
- Lovet, Richard A. (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.
- Lovet, Richard A. (February 2, 2007). "Mars Life May Be Too Deep to Find, Experts Conclude". National Geographic News.
- Hassler, Donald M.; Zeitlin, C; et al. (January 24, 2014). "Mars' Surface Radiation Environment Measured with the Mars ScienceLaboratory's Curiosity Rover" (PDF). Science. 343 (6169): 1244797. Bibcode:2014Sci...343D.386H. doi:10.1126/science.1244797. PMID 24324275.
- McKay, Christopher P.; Stoker, Carol R.; Glass, Brian J.; Davé, Arwen I.; Davila, Alfonso F.; Heldmann, Jennifer L.; Marinova, Margarita M.; Fairen, Alberto G.; Quinn, Richard C.; et al. (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.
- Stern, Jennifer C. (24 March 2015). "Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale crater, Mars". Proceedings of the National Academy of Sciences of the United States of America. 112 (14): 4245–4250. Bibcode:2015PNAS..112.4245S. doi:10.1073/pnas.1420932112. PMC . PMID 25831544. Retrieved 2015-03-25.
- Neal-Jones, Nancy; Steigerwald, William; Webster, Guy; Brown, Dwayne (24 March 2015). "Curiosity Rover Finds Biologically Useful Nitrogen on Mars". NASA. Retrieved 25 March 2015.
- "Curiosity Mars rover detects 'useful nitrogen'". NASA. BBC News. 25 March 2015. Retrieved 2015-03-25.
- Boxe, C. S.; Hand, K.P.; Nealson, K.H.; Yung, Y.L.; Saiz-Lopez, A. (2012). "An active nitrogen cycle on Mars sufficient to support a subsurface biosphere". International Journal of Astrobiology. 11 (2): 109–115. Bibcode:2012IJAsB..11..109B. doi:10.1017/S1473550411000401. Retrieved 2015-05-10.
- Schuerger, Andrew C.; Ulrich, Richard; Berry, Bonnie J.; Nicholson, Wayne L. (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 . PMID 23289858.
- 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: 83–96. Bibcode:2006IJMSE...2...83H. doi:10.1555/mars.2006.0006.
- Shiga, David (December 7, 2009). "Watery niche may foster life on Mars". New Scientist.
- Vieru, Tudor (December 7, 2009). "Greenhouse Effect on Mars May Be Allowing for Life". Softpedia.[unreliable source?]
- Mellon, Michael T. (May 10, 2011). "Subsurface Ice at Mars: A review of ice and water in the equatorial regions" (PDF). Planetary Protection Subcommittee Meeting. University of Colorado.
- Britt, Robert Roy (February 22, 2005). "Ice Packs and Methane on Mars Suggest Present Life Possible". space.com.
- 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.
- 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 2009-03-07.
- 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): 589–594. Bibcode:1991Natur.352..589B. doi:10.1038/352589a0.
- "Flashback: Water on Mars Announced 10 Years Ago". SPACE.com. June 22, 2000.
- "The Case of the Missing Mars Water". Science@NASA. Retrieved 2009-03-07.
- "Mars Rover Opportunity Examines Clay Clues in Rock". NASA. Jet Propulsion Laboratory. May 17, 2013.
- "NASA Rover Helps Reveal Possible Secrets of Martian Life". NASA. November 29, 2005.
- ISBN 0-312-24551-3[page needed][full citation needed]
- "PSRD: Ancient Floodwaters and Seas on Mars". Psrd.hawaii.edu. July 16, 2003.
- "Gamma-Ray Evidence Suggests Ancient Mars Had Oceans". SpaceRef. November 17, 2008.
- 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.
- 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. Lay summary – ScienceDaily (June 14, 2010).
- 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. June 8, 2004.[page needed]
- "Press Release". University of Arizona. March 16, 2006.
- Kerr, Richard (December 8, 2006). "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.
- 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.
- "Dead Spacecraft on Mars Lives on in New Study". SPACE.com. June 10, 2008.
- 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.
- "Mars Rover Investigates Signs of Steamy Martian Past" (Press release). Jet Propulsion Laboratory. December 10, 2007.
- 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.
- 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". Mars Express. ESA.[verification needed]
- "Hunting for young lava flows". Geophysical Research Letters. Red Planet. June 1, 2011.
- 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. Lay summary – Phys.org (December 8, 2009).
- 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.
- "Earth organisms survive under low-pressure Martian conditions". University of Arkansas. 2 June 2015. Retrieved 2015-06-04.
- 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 2009-01-16.
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
- 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, 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. Instruments and Methods for Astrobiology and Planetary Missions XII. 7441. pp. 12–27. Bibcode:2009SPIE.7441E..12L. doi:10.1117/12.829183. ISBN 978-0-8194-7731-6.
- 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 . PMID 22679287.
- Staff (June 25, 2012). "Mars Life Could Leave Traces in Red Planet's Air: Study". Space.com.
- 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: . Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. PMID 22739313.
- Mann, Adam (June 27, 2012). "New View of Exoplanets Will Aid Search for E.T.". Wired (magazine).
- Webster, Guy; Neal-Jones, Nancy; Brown, Dwayne (December 16, 2014). "NASA Rover Finds Active and Ancient Organic Chemistry on Mars". NASA. Retrieved December 16, 2014.
- Chang, Kenneth (December 16, 2014). "'A Great Moment': Rover Finds Clue That Mars May Harbor Life". New York Times. Retrieved December 16, 2014.
- Tenenbaum, David (June 9, 2008). "Making Sense of Mars Methane". Astrobiology Magazine. Archived from the original on 2008-09-23.
- 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 2009-01-17.
- "Mars Curiosity Rover News Telecon -November 2, 2012".
- Kerr, Richard A. (November 2, 2012). "Curiosity Finds Methane on Mars, or Not". Science (journal).
- Wall, Mike (November 2, 2012). "Curiosity Rover Finds No Methane on Mars — Yet". Space.com.
- Chang, Kenneth (November 2, 2012). "Hope of Methane on Mars Fades". New York Times.
- Mann, Adam (July 18, 2013). "Mars Rover Finds Good News for Past Life, Bad News for Current Life on Mars". Wired (magazine).
- Webster, Chris R.; Mahaffy, Paul R.; Flesch, Gregory J.; Niles, Paul B.; Jones, John H.; Leshin, Laurie A.; Atreya, Sushil K.; Stern, Jennifer C.; Christensen, Lance E.; Owen, Tobias; Franz, Heather; Pepin, Robert O.; Steele, Andrew; aff9 (July 19, 2013). "Isotope Ratios of H, C, and O in CO2 and H2O of the Martian Atmosphere". Science. 341 (6143): 260–263. Bibcode:2013Sci...341..260W. doi:10.1126/science.1237961. PMID 23869013.
- Mahaffy, Paul R.; Atreya, Sushil K.; Franz, Heather; Wong, Michael; Conrad, Pamela G.; Harpold, Dan; Jones, John J.; Leshin, Laurie A.; Manning, Heidi; Owen, Tobias; Pepin, Robert O.; Squyres, Steven; Trainer, Melissa; Kemppinen, Osku; Bridges, Nathan; Johnson, Jeffrey R.; Minitti, Michelle; Cremers, David; Bell, James F.; Edgar, Lauren; Farmer, Jack; Godber, Austin; Wadhwa, Meenakshi; Wellington, Danika; McEwan, Ian; Newman, Claire; Richardson, Mark; Charpentier, Antoine; et al. (July 19, 2013). "Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover". Science. 341 (6143): 263–266. Bibcode:2013Sci...341..263M. doi:10.1126/science.1237966. PMID 23869014.
- Webster, Christopher R.; Mahaffy, Paul R.; Atreya, Sushil K.; Flesch, Gregory J.; Farley, Kenneth A.; Kemppinen, O.; Bridges, N.; Johnson, J. R.; Minitti, M.; Cremers, D.; Bell, J. F.; Edgar, L.; Farmer, J.; Godber, A.; Wadhwa, M.; Wellington, D.; McEwan, I.; Newman, C.; Richardson, M.; Charpentier, A.; Peret, L.; King, P.; Blank, J.; Weigle, G.; Schmidt, M.; Li, S.; Milliken, R.; Robertson, K.; Sun, V.; et al. (September 19, 2013). "Low Upper Limit to Methane Abundance on Mars". Science. 342 (6156): 355–357. Bibcode:2013Sci...342..355W. doi:10.1126/science.1242902. PMID 24051245.
- Cho, Adrian (September 19, 2013). "Mars Rover Finds No Evidence of Burps and Farts". Science.
- Chang, Kenneth (September 19, 2013). "Mars Rover Comes Up Empty in Search for Methane". New York Times.
- Webster, Christopher R. (23 January 2015). "Mars methane detection and variability at Gale crater". Science. 347 (6220): 415–417. Bibcode:2015Sci...347..415W. doi:10.1126/science.1261713. PMID 25515120.
- Rincon, Paul (July 9, 2009). "Agencies outline Mars initiative". BBC News.
- "NASA orbiter to hunt for source of Martian methane in 2016". Thaindian News. Asian News International. March 6, 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.
- 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.
- Levin, Gilbert V. (2007). "Analysis of evidence of Mars life". Electroneurobiología. 15 (2): 39–47. arXiv: . Bibcode:2007arXiv0705.3176L.
- Klotz, Irene (Apr 12, 2012). "Mars Viking Robots 'Found Life'" (Press release). Discovery Communications, LLC.
- Crocco, Mario; Contreras, N- C. (2008). Folia Neurobiológica Argentina Vol. XI, "Un palindrome: las criaturas vivas conscientes como instrumentos de la naturaleza; la naturaleza como instrumento de las criaturas vivas conscientes". Ediciones Análisis, Buenos Aires–Rosario–Bahía Blanca. p. 70. ISBN 978-987-29362-0-4.
- 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.
- Navarro–Gonzáles, Rafael; Vargas, Edgar; de la Rosa, José; Raga, Alejandro C.; McKay, Christopher P. (2010-12-15). "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars". Journal of Geophysical Research: Planets. 115 (E12010): E12010. Bibcode:2010JGRE..11512010N. doi:10.1029/2010JE003599. Retrieved 2011-01-07.
- Navarro-González, Rafael; Vargas, Edgar; de la Rosa, José; Raga, Alejandro C.; McKay, Christopher P. (2011). "Correction to "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars"". Journal of Geophysical Research. 116: E08011. Bibcode:2011JGRE..116.8011N. doi:10.1029/2011JE003854.
- "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars". Bibcode:2010JGRE..11512010N.
- Johnson, John (2008-08-06). "Perchlorate found in Martian soil". Los Angeles Times.
- "Martian Life Or Not? NASA's Phoenix Team Analyzes Results". Science Daily. 2008-08-06.
- "Did Viking Mars Landers Find Life's Building Blocks? Missing Piece Inspires New Look at Puzzle". ScienceDaily. 2010-09-05. Retrieved 2010-09-23.
- Navarro-González, Rafael; et al. (2011). "Comment on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars". Journal of Geophysical Research. 116: E12001. Bibcode:2011JGRE..11612001B. doi:10.1029/2011JE003869.
- Levin, Gilbert V.; Straat, Patricia Ann. MARS: Dead or Alive? (PDF). Mars Society Convention.
- "Mars Meteorites". NASA. Retrieved 2010-02-16.
- Gibson Jr., E. K.; Westall, F.; McKay, D. S.; Thomas-Keprta, K.; Wentworth, S.; Romanek, C. S. "Evidence for ancient Martian life" (PDF). Mail Code SN2, NASA Johnson Space Center, Houston TX 77058, USA: NASA.
- Crenson, Matt (August 6, 2006). "After 10 years, few believe life on Mars". Space.com. Associated Press. Archived from the original on 2006-08-09.
- 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.
- Baalke, Ron (1995). "The Nakhla Meteorite". Jet Propulsion Lab. NASA. Retrieved 2008-08-17.
- "Rotating image of a Nakhla meteorite fragment". London: Natural History Museum. 2008.
- Rincon, Paul (February 8, 2006). "Space rock re-opens Mars debate". BBC News.
- Meyer, C. (2004). "Mars Meteorite Compendium" (PDF). NASA.
- Whitehouse, David (August 27, 1999). "Life on Mars – new claims". BBC News.
- Compilation of scientific research references on the Nakhla meteorite: http://curator.jsc.nasa.gov/antmet/marsmets/nakhla/references.cfm
- "Shergoti Meteorite". JPL, NASA.
- Webster, Guy (February 27, 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA.
- 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. Bibcode:2014AsBio..14..170W. doi:10.1089/ast.2011.0733.
- Gannon, Megan (February 28, 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com.
- "NASA Findings Suggest Jets Bursting From Martian Ice Cap". Jet Propulsion Laboratory. NASA. August 16, 2006.
- 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.
- 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. Bibcode:2003OLEB...33..515G. 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 2009-09-04. (Audio interview, MP3 6 min.)
- Orme, Greg M.; Ness, Peter K. (June 9, 2003). "Martian Spiders" (PDF). Marsbugs. 10 (23): 5–7. Archived (PDF) from the original on 2007-09-27.
- 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, 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.
- 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. 95–98. ISBN 978-0-309-10851-5.
- Cowing, Keith (April 11, 2013). "Planetary Protection: A Work in Progress". Astrobiology.
- 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.
- 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.
- Schuerger, Andrew C.; Ulrich, Richard; Berry, Bonnie J.; Nicholson., Wayne L. (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 . PMID 23289858.
- 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. April 26, 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.
- de la Torre Noetzel, R.; Sanchez Inigo, F.J.; Rabbow, E.; Horneck, G.; de Vera, J. P.; Sancho, L.G. (June 2007). "Lichens Survive in Space: Results from the 2005 LICHENS Experiment" (PDF). Astrobiology. 7 (3): 443–454. Bibcode:2007AsBio...7..443S. doi:10.1089/ast.2006.0046. PMID 17630840.
- 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.
- Robbins, Stuart (2008). ""Journey Through the Galaxy" Mars Program: Mars ~ 1960-1974". SJR Design. Retrieved 2014-01-26.
- Mihos, Chris (11 January 2006). "Mars (1960-1974): Mars 1". Department of Astronomy, Case Western Reserve University. Retrieved 2014-01-26.
- Strom, R.G.; Croft, Steven K.; Barlow, Nadine G. (1992). The Martian Impact Cratering Record. University of Arizona Press. ISBN 0-8165-1257-4.[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. NY, NY.: Mitchell Beazley Publishers.[page needed]
- "Astrobiology". Biology Cabinet. September 26, 2006.
- Plaxco, Kevin W.; Gross, Michael (2011). Astrobiology: A Brief Introduction. JHU Press. pp. 282–283. ISBN 978-1-4214-0194-2.
- Stenger, Richard (November 7, 2000). "Mars sample return plan carries microbial risk, group warns". CNN.
- Plaxco, Kevin W.; Gross, Michael (2006). Astrobiology: A Brief Introduction. JHU Press. p. 223. ISBN 978-0-8018-8366-8.
- Plaxco, Kevin W.; Gross, Michael (2011). Astrobiology: A Brief Introduction (2nd ed.). JHU Press. pp. 285–286. ISBN 978-1-4214-0194-2.
- 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.
- "Life on Mars Found by NASA's Viking Mission?".
- Klotz, Irene (April 12, 2012). "Mars Viking Robots 'Found Life'". DiscoveryNews.
- 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 . PMID 17548829.
- Webster, Guy; Hoover, Rachel; Marlaire, Ruth; Frias, Gabriela (September 3, 2010). "Missing Piece Inspires New Look at Mars Puzzle". Jet Propulsion Laboratory, NASA. Retrieved 2010-10-24.
- Plaxco, Kevin W.; Gross, Michael (2011). Astrobiology: A Brief Introduction (2nd ed.). JHU Press. pp. 282–283. ISBN 978-1-4214-0194-2.
- Biemann, K.; Bada, J. L. (2011). "Comment on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars" by Rafael Navarro-González et al". Journal of Geophysical Research. 116: E12001. Bibcode:2011JGRE..11612001B. doi:10.1029/2011JE003869.
- Navarro-González, R.; McKay, C. P. (2011). "Reply to comment by Biemann and Bada on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars"". Journal of Geophysical Research. 116: E12002. Bibcode:2011JGRE..11612002N. doi:10.1029/2011JE003880.
- "Piecing Together Life's Potential". Mars Daily.
- "NASA Spacecraft Confirms Perchlorate on Mars". NASA. NASA. August 5, 2008.
- 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.
- Kounaves, S. P. et al., Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics, Icarus, 2014, 229, 206-213, doi:10.1016/j.icarus.2013.11.012,
- Kounaves, S. P.; et al. (2014). "Identification of the perchlorate parent salts at the Phoenix Mars landing site and implications". Icarus. 232: 226–231. Bibcode:2014Icar..232..226K. doi:10.1016/j.icarus.2014.01.016.
- "Mars Science Laboratory Launch". November 26, 2011.
- "NASA Launches Super-Size Rover to Mars: 'Go, Go!'". New York Times. Associated Press. November 26, 2011.
- USGS (May 16, 2012). "Three New Names Approved for Features on Mars". USGS.
- NASA Staff (March 27, 2012). "'Mount Sharp' on Mars Compared to Three Big Mountains on Earth". NASA.
- Agle, D. C. (March 28, 2012). "'Mount Sharp' On Mars Links Geology's Past and Future". NASA.
- Staff (March 29, 2012). "NASA's New Mars Rover Will Explore Towering 'Mount Sharp'". Space.com.
- Webster, Guy; Brown, Dwayne (July 22, 2011). "NASA's Next Mars Rover To Land At Gale Crater". NASA JPL.
- Chow, Dennis (July 22, 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com.
- Amos, Jonathan (July 22, 2011). "Mars rover aims for deep crater". BBC News.
- mars.nasa.gov. "Volatiles Released by Heating Sample Powder from Martian Rock "Cumberland" | Mars Image". mars.nasa.gov. Retrieved 2017-02-23.
- "ExoMars: ESA and Roscosmos set for Mars missions". European Space Agency (ESA). March 14, 2013.
- Cowing, Keith (December 21, 2012). "Science Definition Team for the 2020 Mars Rover". NASA. Science Ref.
- Study Reveals Young Mars Was A Wet World
- NASA – The Mars Exploration Program
- Scientists have discovered that Mars once had saltwater oceans
- BBC News: Ammonia on Mars could mean life
- Scientist says that life on Mars is likely today
- Ancient salty sea on Mars wins as the most important scientific achievement of 2004 – Journal Science
- Mars meteor found on Earth provides evidence that suggests microbial life once existed on Mars
- Scientific American Magazine (November 2005 Issue) Did Life Come from Another World?
- Audio interview about "Dark Dune Spots"