Life on Mars

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For other uses, see Life on Mars (disambiguation).
An artist's impression of a living Mars.

Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Although fictional Martians have been a recurring feature of popular entertainment, it remains an open question whether life currently exists on Mars, or has existed there in the past.

Early speculation

Historical map of Mars from Giovanni Schiaparelli.
Mars canals, as seen by astronomer P. Lowell, 1898.

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

In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms. Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were however soon 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.[1] 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.[2] By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal theory.

Mariner 4

Main article: Mariner 4

The photographs taken by the Mariner 4 probe fly-by in 1965 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 also 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.[2] After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.

Viking experiments

Main article: Viking biological experiments
File:Sagan Viking.jpg
Carl Sagan poses next to a replica of the Viking landers.

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil. The tests were formulated to look for life similar to that found on Earth. Of the four experiments, only the Labeled Release experiment returned a positive result, 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 GC-MS detected no organic molecules. However, there are vastly different interpretations of what those results imply.

One of the designers of the LR experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars.[2] However, this result is disputed by many scientists, who argue that superoxidant chemicals 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.[3] The results of the Viking mission concerning life, are considered by the general expert community, at best, as inconclusive.[2][4]

Since Mars lost most of its magnetic field about 4 billion years ago, the Martian ionosphere is unable to stop the solar wind or radiation, and it interacts directly with exposed soil, making life, as we know it, impossible to exist. Also, liquid water, necessary for life and for metabolism, cannot exist on the surface of Mars with its present low atmospheric pressure and temperature, except at the lowest shaded elevations for short periods[5][6] and liquid water never appears at the surface itself.[7]

On 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, USA), Gilbert Levin's investigation was assessed once more.[3] Levin maintains that his original data were correct, as the positive and negative control experiments were in order.

Ronald Paepe, an edaphologist (soil scientist), communicated to the European Geosciences Union Congress that the discovery of the recent detection of phyllosilicate clays on Mars may indicate pedogenesis, or soil development processes, extended over the entire surface of Mars.[8] Paepe's interpretation views most of Mars surface as active soil, colored red by eons of widespread wearing by water, vegetation and microbial activity.[8]

A research team from the Salk Institute for Biological Studies headed by Rafael Navarro-González, concluded that the equipment used (TV-GC-MS) by the Viking program to search for organic molecules, may not be sensitive enough to detect low levels of organics.[9] Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection.

Gillevinia straata

File:Gillevinia straata.jpg
Newly proposed taxonomic system.

The claim for life on Mars, in the form of Gillevinia straata, is based on old data reinterpreted as sufficient evidence of life, mainly by professors Gilbert Levin,[3] Rafael Navarro-González[9] and Ronalds Paepe.[8] The evidence supporting the existence of Gillevinia straata microorganisms relies on the data collected by the two Mars Viking landers that searched for biosignatures of life, but the analytical results were, officially, inconclusive.[2]

In 2006, Mario Crocco, a neurobiologist at the Neuropsychiatric Hospital Borda in Buenos Aires, Argentina, proposed the creation of a new nomenclatural rank that classified these results as 'metabolic' and therefore belonging to a form of life. Crocco proposed to create new biological ranking categories (taxa), in the new kingdom system of life, in order to be able to accommodate the genus of Martian microorganisms. Crocco proposed the following taxonomical entry:[10]

  • Organic life system: Solaria
  • Biosphere: Marciana
  • kingdom: Jakobia (named after neurobiologist Christfried Jakob)
  • Genus et species: Gillevinia straata

As a result, the Gillevinia straata would not be a bacterium (which rather is a terrestrial taxon) but a member of the kingdom 'Jakobia' in the biosphere 'Marciana' of the 'Solaria' system.

The intended effect of the new nomenclature was to reverse the burden of proof concerning the life issue, but the taxonomy proposed by Crocco has not been accepted by the scientific community; and is considered a single Nomen nudum. Further, no Mars mission has found traces of biomolecules.

Meteorites

The interpretation of whether meteorite deposits are really proof of (ancient) life on Mars are controversial but of enormous interest to biologists. Single celled life on Mars, even if extinct today, would corroborate origin of life theories. NASA maintains a catalog[11] of at least 57 Mars meteorites, which are extremely valuable since these are the only physical samples available of Mars. Speculation has grown as a result that studies show that at least three of them may have evidence of possible past life on Mars. Although the scientific evidence collected is reliable, its interpretation varies. To date, no fatal strikes have been made to any of the original lines of scientific evidence despite several misconstrued press releases.[12]

Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:[12]

  1. Is the geologic context of the sample compatible with past life?
  2. Is the age of the sample and its stratigraphic location compatible with possible life?
  3. Does the sample contain evidence of cellular morphology and colonies?
  4. Is there any evidence of biominerals showing chemical or mineral disequilibria?
  5. Is there any evidence of stable isotope patterns unique to biology?
  6. Are there any organic biomarkers present?
  7. 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.

ALH84001 meteorite

An electron microscope reveals bacteria-like structures in meteorite fragment ALH84001

The ALH84001 meteorite was found on December 1984 on Antarctica by members of the ANSMET project; the meteorite weighs 1.93 kg.[13] The sample was ejected from Mars about 17 million years ago and spent 11,000 years in or on the Antarctic ice sheets. Composition analysis by NASA revealed a kind of magnetite that on Earth, is only found in association with certain microorganisms;[12] Then, in August 2002, another NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH 84001 occurs as small, uniform-sized crystals that, on Earth, is associated only with biologic activity, and that the remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biologic magnetite was organized into chains as would be expected. The meteorite displays indication of relatively low temperature secondary mineralization by water and show evidence of preterrestrial aqueous alteration. Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface.

Some structures resembling the mineralized casts of terrestrial bacteria and their appendages (fibrils) or by-products (extracellular polymeric substances) occur in the rims of carbonate globules and preterrestrial aqueous alteration regions.[14][15] The size and shape of the objects is consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial.

Nakhla Meteorite

Nakhla meteorite

The Nakhla meteorite fell on Earth on June 28, 1911 on the locality of Nakhla, Alexandria, Egypt.[16][17]

In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects[18] of the size and shape consistent with Earthly fossilized nanobacteria, but the existence of nanobacteria itself is controversial. Analysis with gas chromatography and mass spectrometry (GC-MS) studied its high molecular weight polycyclic aromatic hydrocarbons in 2000, and NASA scientists concluded that as much as 75% of the organic matter in Nakhla "may not be recent terrestrial contamination".[12][19]

This caused additional interest in this meteorite, so on 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic carbon content was observed. When the results and evidence were published on 2006, some independent researchers claimed that the carbon deposits are of biologic origin. However, it was remarked that since carbon is the fourth most aboundant element in the Universe, finding it in curious patterns is not indicative or suggestive of biological origin.[20][21]

Shergotty meteorite

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. [22] This meteorite is relatively young, calculated to have been formed in Mars only 165 million years ago from volcanic origin. It is composed mostly of pyroxene and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest to be remnants of biofilm and their associated microbial communities.[12] Work is in progress on searching for magnetites within alteration phases.

Liquid water

Main article: Water on Mars
A series of artist's conceptions of hypothetical past water coverage on Mars.

No Mars probe since Viking has tested the Martian regolith directly for signs of life. NASA's recent missions have focused on another question: whether Mars held lakes or oceans of liquid water on its surface in the ancient past. Scientists have found hematite, a mineral that forms in the presence of water. Many scientists have long held this to be almost self-evident based on various geological landforms on the planet, but others have proposed different explanations—wind erosion, oxygen oceans, etc. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for present or past life, but for evidence of liquid water on the surface of Mars in the planet's ancient past.

In June 2000, evidence for water currently under the surface of Mars was discovered in the form of flood-like gullies.[23] Deep subsurface water deposits near the planet's liquid core might form a present-day habitat for life. However, in March 2006, astronomers announced the discovery of similar gullies on the Moon,[24] which is believed never to have had liquid water on its surface. The astronomers suggest that the gullies could be the result of micrometeorite impacts.

In March 2004, NASA announced that its rover Opportunity had discovered evidence that Mars was, in the ancient past, a wet planet.[25] This had raised hopes that evidence of past life might be found on the planet today.

In December 2006, NASA showed images 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 craters and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago, and is perhaps doing so even now. Some researchers were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said other materials such as sand or dust can flow like a liquid and produce similar results.[26]

Recent 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.[27] Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.

Methane

See also: Atmosphere of Mars § Methane

Trace amounts of methane in the atmosphere of Mars were discovered in 2003 and verified in 2004.[28][29][30][31][32][33] The presence of methane on Mars is very intriguing, since as an unstable gas, it indicates that it must have a 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,[34][35] 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. The existence of life in the form of microorganisms such as methanogens are among possible, but as yet unproven sources. If microscopic Martian life is producing the methane, it likely resides far below the surface, where it's still warm enough for liquid water to exist.[36]

Formaldehyde

In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter, 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.[37][38] NASA scientists consider the preliminary findings are well worth a follow-up, but have also rejected the claims of life.[39][40]

Silica

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area extremely rich in silica (90%).[41] 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 must have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water. Another could have been from water in a hot spring environment.[42]

Dark dune spots

Dark dune spots appear every spring.
Image obtained by the Mars Global Surveyor of dark dune spots.

Dark dune spots are features that can be seen mainly in the southern polar region (between 60°-80° latitudes) of Mars, on or under the polar ice sheet. The spots were discovered by images obtained by the Mars Global Surveyor in 1998 - 1999. The spots appear at the beginning of the Martian spring and disappear by the beginning of the winter. A theory of the spots' possible biological origin is made by a Hungarian team; they propose that the spots are colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap.[43] As the Sun returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesise 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 it has completely melted, they rapidly desiccate and turn black surrounded by a grey aureole.[44][45] The 'salt-loving' bacteria haloarchaea has been proposed to be used as a "model" to study hypothetic extremophiles on Mars.[46]

NASA's current theory is that the patches are composed of either basaltic ash fragments or aggregates of a minor, dark dust component of the layered deposits that forms as a sublimation residue.[47] While the European Space Agency (ESA) has not yet formulated a theory, it has stated that the location and shape of the spots is at odds with a physical explanation.[48]

Cosmic radiation

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field, that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery [49]. 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.

In 2007, it was calculated that DNA and RNA damage by cosmic radiation was limiting life on Mars to depths below 7.5 metres.[50] Therefore, the best hopes for a story of life on Mars are at environments that haven't been studied yet, subsurface.[51]

Missions

Phoenix lander, 2008

Main article: Phoenix (spacecraft)
An artist's concept of the Phoenix spacecraft.

The Phoenix mission landed a telerobot 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 is capable of digging a 0.5 meter trench in the regolith. There is an electrochemistry experiment which analysed the ions in the regolith and the amount and type of antioxidants on Mars. The Viking program data indicates 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.[52] Phoenix's preliminary data revealed that Mars soil contains perchlorate, and thus may not be as life-friendly as thought earlier.[53][54][55] 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.[56]

Future missions

  • Mars Science Laboratory, a NASA project planned for launch in late 2011, will contain instruments and experiments designed to look for past or present conditions relevant to biological activity.
  • NASA is planning to launch the Astrobiology Field Laboratory in 2016, to help answer questions about life on Mars. The Mars Exploration and Payload Analysis Group is responsible for deciding what experiments will fly on the mission.[52]
  • Deep drills have been advocated for future missions to sample various depths beneath the surface where, some believe, liquid water may be found and where microorganisms might survive cosmic radiation.[50]
  • 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 life forms be found in a sample, it would be difficult to conclude that those organisms were alive when obtained. This also raises ethical questions. Carl Sagan said in COSMOS chapter "Blues For a Red Planet" that "If there is life on Mars, then I believe we should do nothing to disturb that life. Mars then, belongs to the Martians, even if they are microbes."

See also

References

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External links

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