The possibility oflife on Mars is a subject of interest inastrobiology due to theplanet's proximity and similarities toEarth. To date, no conclusive evidence of past or presentlife has been found onMars. Cumulative evidence suggests that during the ancientNoachian time period, the surface environment ofMars had liquid water and may have beenhabitable for microorganisms, but habitable conditions do not necessarily indicate life.[1][2] Scientific investigations for potential life on Mars began in the 19th century and continue today with telescopes and robotic probes searching for water, chemicalbiosignatures in the soil and rocks at the planet's surface, andbiomarker gases in the atmosphere.[3]
Mars is of particular interest for the study of theorigins of life because of its similarity to the early Earth. This is especially true since Mars has a cold climate and lacksplate tectonics orcontinental drift, so it has remained almost unchanged since the end of theHesperian period. At least two-thirds of Mars's surface is more than 3.5 billion years old, and it could have been habitable 4.48 billion years ago, 500 million years before the earliest known Earth lifeforms;[4] Mars may thus hold the best record of the prebiotic conditions leading to life, even if life does not or has never existed there.[5][6]
Following the confirmation of the past existence of surface liquid water, theCuriosity,Perseverance andOpportunity rovers started searching for evidence of past life, including a pastbiosphere based onautotrophic,chemotrophic, orchemolithoautotrophicmicroorganisms, as well as ancient water, includingfluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable.[7][8][9][10] The search for evidence of habitability,fossils, andorganic compounds on Mars is now a primary objective forspace agencies. The discovery of organic compounds inside sedimentary rocks and ofboron on Mars are of interest as they are precursors forprebiotic chemistry. Such findings, along with previous discoveries that liquid water was clearly present on ancient Mars, further supports the possible early habitability ofGale Crater on Mars.[11][12] Currently, the surface of Mars is bathed withionizing radiation, andMartian soil is rich inperchlorates toxic tomicroorganisms.[13][14] Therefore, the consensus is that if life exists—or existed—on Mars, it could be found or is best preserved in the subsurface, away from present-day harsh surface processes.
In June 2018, NASA announced the detection of seasonal variation ofmethane levels on Mars. Methane could be produced by microorganisms or by geological means.[15] The EuropeanExoMars Trace Gas Orbiter started mapping the atmospheric methane in April 2018, and the 2022ExoMars roverRosalind Franklin was planned to drill and analyze subsurface samples before the programme's indefinite suspension, while the NASAMars 2020 roverPerseverance, having landed successfully, will cache dozens of drill samples for their potential transport to Earth laboratories in the late 2020s or 2030s. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms onVenus (viaphosphine) and Mars (viamethane) was reported.[16] In October 2024, NASA announced that it may be possible for photosynthesis to occur within dusty water ice exposed[17] in the mid-latitude regions of Mars.[18] On 10 September 2025, NASA reportedPerseverance found a possiblebiosignature in aJezero Crater rock the previous year, hinting at ancientmicrobial activity.[19][20]
Mars's polar ice caps were discovered in the mid-17th century.[citation needed] In the late 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 thatMars had certain other similarities toEarth, for example that thelength of a day on Mars was almost the same as a day on Earth. They also knew that itsaxial tilt was similar to Earth's, which meant it experienced seasons just as Earth does—but of nearly double the length owing to itsmuch longer year. These observations led to increasing speculation that the darkeralbedo features were water and the brighter ones were land, whence followed speculation on whether Mars may be inhabited by some form of life.[21]
In 1854,William Whewell, a fellow ofTrinity College, Cambridge, theorized that Mars had seas, land and possibly life forms.[22] Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparentMartian canals—which were later found to be optical illusions. Despite this, in 1895, American astronomerPercival Lowell published his bookMars, followed byMars and its Canals in 1906,[23] proposing that the canals were the work of a long-gone civilization.[24] This idea led British writerH. G. Wells to writeThe War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet'sdesiccation.[25]
The 1907 bookIs Mars Habitable? by British naturalistAlfred Russel Wallace was a reply to, and refutation of, Lowell'sMars and Its Canals. Wallace's book concluded that Mars "is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely uninhabitable."[26] HistorianCharles H. Smith refers to Wallace's book as one of the first works in the field ofastrobiology.[27]
Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomerWilliam Wallace Campbell showed that neither water nor oxygen were present in theMartian atmosphere.[28] The influential observerEugène Antoniadi used the 83-cm (32.6 inch) aperture telescope atMeudon Observatory at the 1909opposition of Mars and saw no canals, the outstanding photos of Mars taken at the new Baillaud dome at thePic du Midi observatory also brought formal discredit to the Martian canals theory in 1909,[29] and the notion of canals began to fall out of favor.[28]
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.[30] The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with an emphasis on water availability, temperature, the presence of nutrients, an energy source, and protection from solar ultraviolet andgalactic cosmic radiation.[31][32]
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.[30] Similarly, for each group of parameters, the habitability threshold for each is to be determined.[30] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.[33] There are no full-Mars simulations published yet that include all of the biocidal factors combined.[33] Furthermore, the possibility of Martian life having a far different biochemistry and habitability requirements than the terrestrial biosphere is an open question. A common hypothesis is methanogenic Martian life, and while such organisms exist on Earth too, they are exceptionally rare (while in itself numerous, there are not many environments on Earth where life commonly known to humans exists, where methanogenic life also can) and cannot survive in the majority of terrestrial environments that contain oxygen.[34]
Recentmodels have shown that, even with a denseCO2 atmosphere, early Mars was colder than Earth has ever been.[35][36][37][38] Transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the lateNoachian 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.[38] Both the mineralogical and the morphological evidence indicates a degradation of habitability from the midHesperian 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.[38] Billions of years ago, before this degradation, the surface of Mars was apparently fairly habitable, consisted of liquid water and clement weather, though it is unknown if life existed on Mars.[39]
The loss of the Martianmagnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.[41] When there was a magnetic field, the atmosphere would have been protected from erosion by thesolar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.[42] The loss of the atmosphere was accompanied by decreasing temperatures. Part of the liquid water inventory sublimed and was transported to the poles, while the rest becametrapped inpermafrost, a subsurface ice layer.[38]
Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long-lastinghydrothermal 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,[38] but unlikely to have progressed any further down the evolutionary path.[43]
Soil and rock samples studied in 2013 by NASA'sCuriosity rover's onboard instruments brought about additional information on several habitability factors.[44] The rover team identified some of the key chemical ingredients for life in this soil, includingsulfur,nitrogen,hydrogen, oxygen,phosphorus and possiblycarbon, as well as clay minerals, suggesting a long-ago aqueous environment—perhaps a lake or an ancient streambed—that had neutral acidity and low salinity.[44] On December 9, 2013, NASA reported that, based on evidence fromCuriosity studyingAeolis Palus,Gale Crater contained an ancientfreshwater lake which could have been a hospitable environment formicrobial life.[45][46] The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a pastmagnetic field that protected the planet from cosmic and solar radiation,[47][48] together strongly suggest that Mars could have had the environmental factors to support life.[49][50] The assessment of past habitability is not in itself evidence thatMartian life has ever actually existed. If it did, it was probablymicrobial, existing communally in fluids or on sediments, either free-living or asbiofilms, respectively.[41] The exploration ofterrestrial analogues provide clues as to how and where best look for signs of life on Mars.[51]
Impactite, shown to preserve signs of life on Earth, was discovered on Mars and could contain signs of ancient life, if life ever existed on the planet.[52]
On June 7, 2018, NASA announced that theCuriosity rover had discovered organic molecules in sedimentary rocks dating to three billion years old.[53][54] The detection of organic molecules in rocks indicate that some of the building blocks for life were present.[55][56]
Research into how the conditions for habitability ended is ongoing. On October 7, 2024, NASA announced that the results of the previous three years of sampling onboardCuriosity suggested that based on highcarbon-13 andoxygen-18 levels in the regolith, the early Martian atmosphere was less likely than previously thought, to be stable enough to support surface water hospitable to life, with rapid wetting-drying cycles and very high-salinity cryogenic brines providing potential explanations.[57][58]
Conceivably, if life exists (or existed) on Mars, evidence of life could be found, or is best preserved, in the subsurface, away from present-day harsh surface conditions.[59] Present-day life on Mars, or its biosignatures, could occur kilometers below the surface, or in subsurface geothermal hot spots, or it could occur a few meters below the surface. Thepermafrost layer on Mars is only a couple of centimeters below the surface, and saltybrines can be liquid a few centimeters below that but not far down. 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 after a sudden release of underground water.[60][61][62]
So far, NASA has pursued a "follow the water" strategy on Mars and has not searched for biosignatures for life there directly since theViking missions. The consensus by astrobiologists is that it may be necessary to access the Martian subsurface to find currently habitable environments.[59]
In 1965, theMariner 4 probe discovered that Mars had noglobal magnetic field that would protect the planet from potentially life-threateningcosmic radiation andsolar radiation; observations made in the late 1990s by theMars Global Surveyor confirmed this discovery.[63] Scientists speculate that the lack of magnetic shielding helped thesolar wind blow away much ofMars's atmosphere over the course of several billion years.[64] As a result, the planet has been vulnerable to radiation from space for about 4 billion years.[65]
Recentin-situ data fromCuriosity rover indicates thationizing radiation fromgalactic cosmic rays (GCR) andsolar particle events (SPE) may not be a limiting factor in habitability assessments for present-day surface life on Mars. The level of 76 mGy per year measured byCuriosity is similar to levels inside the ISS.[66]
Curiosity rover measured ionizing radiation levels of 76 mGy per year.[67] This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. 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, then rovers on Mars could find dormant but still viable life at a depth of one meter below the surface, according to an estimate.[68] Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars since Mars lost its protective magnetosphere and atmosphere.[69][70] After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that over time, any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.[69][71][72] The team calculated that the cumulative damage toDNA andRNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 meters below the planet's surface.[71]Even the most radiation-tolerant terrestrial bacteria would survive in dormantspore state only 18,000 years at the surface; at 2 meters—the greatest depth at which theExoMars rover will be capable of reaching—survival time would be 90,000 to half a million years, depending on the type of rock.[73]
Data collected by theRadiation assessment detector (RAD) instrument on board theCuriosity rover revealed that the absorbed dose measured is 76mGy/year at the surface,[74] and that "ionizing radiation strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic molecules."[74] Regardless of the source of Martianorganic compounds (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfiguring with surrounding elements by ionizing charged particle radiation.[74] These improved subsurface radiation estimates give insight into the potential for the preservation of possible organicbiosignatures as a function of depth as well as survival times of possible microbial or bacterial life forms left dormant beneath the surface.[74] The report concludes that thein 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."[74]
In September 2017, NASA reportedradiation levels on the surface of the planetMars were temporarily doubled and were associated with anaurora 25 times brighter than any observed earlier, due to a major, and unexpected,solar storm in the middle of the month.[75]
On UV radiation, a 2014 report concludes[76] that "[T]he 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." In addition, laboratory research published in July 2017 demonstrated that UV irradiated perchlorates cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure.[77][78] The penetration depth of UV radiation into soils is in the sub-millimeter to millimeter range and depends on the properties of the soil.[78] A recent study found that photosynthesis could occur within dusty ice exposed in the Martian mid-latitudes because the overlying dusty ice blocks the harmful ultraviolet radiation at Mars's surface.[79]
The Martian regolith is known to contain a maximum of 0.5% (w/v)perchlorate (ClO4−) that is toxic for most living organisms,[80] but since they drastically lower the freezing point of water and a few extremophiles can use it as an energy source (seePerchlorates - Biology) and grow at concentrations of up to 30% (w/v)sodium perchlorate[81] by physiologically adapting to increasing perchlorate concentrations,[82] it has prompted speculation of what their influence would be on habitability.[77][81][83][84][85]
Research published in July 2017 shows that when irradiated with a simulated Martian UV flux, perchlorates become even more lethal to bacteria (bactericide). Even dormant spores lost viability within minutes.[77] In addition, two other compounds of the Martian surface,iron oxides andhydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure.[77][78] It was also found that abraded silicates (quartz and basalt) lead to the formation of toxicreactive oxygen species.[86] The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable."[87] This research demonstrates that the present-day surface is more uninhabitable than previously thought,[77][88] and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.[88][89]
However, researcherKennda Lynch discovered the first-known instance of a habitat containing perchlorates and perchlorates-reducing bacteria in an analog environment: a paleolake in Pilot Valley,Great Salt Lake Desert, Utah, United States.[90] She has been studying thebiosignatures of these microbes, and is hoping that theMars Perseverance rover will find matching biosignatures at itsJezero Crater site.[91][92]
Recurrent slope lineae (RSL) 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 the 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".).[93][94] They were suspected as involving flowing brines back then.[95][96][97][98]
The thermodynamic availability of water (water activity) strictly limits microbial propagation on Earth, particularly in hypersaline environments, and there are indications that the brine ionic strength is a barrier to the habitability of Mars. Experiments show that highionic strength, driven to extremes on Mars by the ubiquitous occurrence of divalent ions, "renders these environments uninhabitable despite the presence of biologically available water."[99]
After carbon,nitrogen is arguably the most important element needed for life. Thus, measurements ofnitrate 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 supportnitrogen fixation for biological incorporation.[100] Nitrogen in the form ofnitrate 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.[101]
On March 24, 2015, NASA reported that theSAM instrument on theCuriosity 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 byliving organisms. The discovery supports the notion that ancient Mars may have been hospitable for life.[101][102][103] It is suspected that all nitrate on Mars is a relic, with no modern contribution.[104] Nitrate abundance ranges from non-detection to 681 ± 304 mg/kg in the samples examined until late 2017.[104] Modeling indicates that the transient condensed water films on the surface should be transported to lower depths (≈10 m) potentially transporting nitrates, where subsurface microorganisms could thrive.[105]
In contrast, phosphate, one of the chemical nutrients thought to be essential for life, is readily available on Mars.[106]
Further complicating estimates of the habitability of the Martian surface is the fact that very little is known about the growth of microorganisms at pressures close to those 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).[107] In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and onlySerratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0 °C, and CO2-enriched anoxic atmospheres.[107]
On 10 September 2025 NASA announced that the Perseverance Rover had discovered the best possible signs of life on Mars to date: rings ofiron phosphateorganic molecules à la ancient Terran microbial life.[108]
Liquid water is a necessary but not sufficient condition for life as humans know it, as habitability is a function of a multitude of environmental parameters.[109] Liquid water cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.[110][111] Liquid water does not appear at the surface itself,[112] but it could form in minuscule amounts around dust particles in snow heated by the Sun.[113][114][unreliable source?] Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.[115][116][117][118]
Water on Mars exists almost exclusively as water ice, located in theMartian polar ice caps and under the shallow Martian surface even at more temperate latitudes.[122][123] A small amount of water vapor is present in theatmosphere.[124] There are no bodies of liquid water on the Martian surface because the water vapor pressure is less than 1 Pa,[125] the 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,[126] there was a denseratmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,[127][128][129][130] including large oceans.[131][132][133][134][135]
It has been estimated that the primordial oceans on Mars would have covered between 36%[136] and 75% of the planet.[137] On November 22, 2016, NASA reported finding a large amount ofunderground ice in theUtopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water inLake Superior.[119][120][121]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. Toscaet al. found that the Martian water in the locations they studied all hadwater activity, aw ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.[138]Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.[139]
In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.[140][141] Additional similar images were published in 2006, taken by theMars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images 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.[142][143][144] Others suggest it may be liquidbrine near the surface,[145][146][147] but the exact source of the water and the mechanism behind its motion are not understood.[148]
In July 2018, scientists reported the discovery of asubglacial lake on Mars, 1.5 km (0.93 mi) below thesouthern polar ice cap, and extending sideways about 20 km (12 mi), the first known stable body of water on the planet.[149][150][151][152] The lake was discovered using theMARSIS radar on board theMars Express orbiter, and the profiles were collected between May 2012 and December 2015.[153] The lake is centered at 193°E, 81°S, a flat area that does not exhibit any peculiar topographic characteristics but is surrounded by higher ground, except on its eastern side, where there is a depression.[149] However, subsequent studies disagree on whether any liquid can be present at this depth without anomalous heating from the interior of the planet.[154][155] Instead, some studies propose that other factors may have led to radar signals resembling those containing liquid water, such as clays, or interference between layers of ice and dust.[156][157][158]
In May 2007, theSpirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area 90% rich insilica.[159] The feature is reminiscent of the effect ofhot 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.[160]
Methane (CH4) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases. Therefore, a persistent presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas.
Trace amounts of methane, at the level of severalparts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASAGoddard Space Flight Center in 2003.[169][170] Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.[171] On June 7, 2018, NASA announced it has detected a seasonal variation of methane levels on Mars.[15][172][55][56][173][174][175][54]
TheExoMars Trace Gas Orbiter (TGO), launched in March 2016, began on April 21, 2018, to map the concentration and sources of methane in the atmosphere,[176][177] as well as its decomposition products such asformaldehyde andmethanol. As of May 2019, the Trace Gas Orbiter showed that the concentration of methane is under detectable level (< 0.05 ppbv).[178][179]
Curiosity detected a cyclical seasonal variation in atmospheric methane.
The principal candidates for the origin of Mars's methane include non-biological processes such aswater-rock reactions,radiolysis of water, andpyrite formation, all of which produceH2 that could then generate methane and other hydrocarbons viaFischer–Tropsch synthesis withCO and CO2.[180] It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineralolivine, which is known to be common on Mars.[181] Although geologic sources of methane such asserpentinization are possible, the lack of currentvolcanism,hydrothermal activity orhotspots[182] are not favorable for geologic methane.
Livingmicroorganisms, such asmethanogens, are another possible source, but no evidence for the presence of such organisms has been found on Mars,[183][184][185] until June 2019 as methane was detected by theCuriosity rover.[186] 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.[187] 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.[188]
Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models andin vitro experiments testing the growth ofmethanogenic 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.[189]
A team led by Levin suggested that both phenomena—methane production and degradation—could be accounted for by an ecology of methane-producing and methane-consuming microorganisms.[190][191]
Distribution ofmethane in the atmosphere of Mars in the Northern Hemisphere during summer
Research at the University of Arkansas presented in June 2015 suggested that some methanogens could survive in 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 wereMethanothermobacter wolfeii,Methanosarcina barkeri,Methanobacterium formicicum, andMethanococcus maripaludis.[187] In June 2012, scientists reported that measuring the ratio ofhydrogen andmethane levels on Mars may help determine the likelihood of life on Mars.[183][184] According to the scientists, "low H2/CH4 ratios (less than approximately 40)" would "indicate that life is likely present and active".[183] The observed ratios in the lower Martian atmosphere were "approximately 10 times" higher "suggesting that biological processes may not be responsible for the observed CH4".[183] The scientists suggested measuring the H2 and CH4 flux at the Martian surface for a more accurate assessment. Other scientists have recently reported methods of detecting hydrogen and methane inextraterrestrial atmospheres.[192][193]
Even if rover missions determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside of the rover's reach.[194]
In February 2005, it was announced that thePlanetary Fourier Spectrometer (PFS) on theEuropean Space Agency'sMars Express Orbiter had detected traces offormaldehyde in theatmosphere 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 harboring colonies of microbial life.[195][196] NASA scientists consider the preliminary findings well worth a follow-up but have also rejected the claims of life.[197][198]
The 1970sViking program placed two identical landers on the surface of Mars tasked to look forbiosignatures of microbial life on the surface. The 'Labeled Release' (LR) experiment gave a positive result formetabolism, while thegas chromatograph–mass spectrometer did not detectorganic 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 inconclusive.[28] No Mars lander mission has found meaningful traces ofbiomolecules orbiosignatures. 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 byGilbert Levin,[199][200] Joseph D. Miller,[201] Navarro,[202] Giorgio Bianciardi andPatricia Ann Straat.
Assessments published in December 2010 by Rafael Navarro-Gonzáles[203][204][205][206] indicate that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2. The study determined thatperchlorate—discovered in 2008 byPhoenix lander[207][208]—can destroy organic compounds when heated, and producechloromethane anddichloromethane as a 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.[209][210]
The Labeled Release evidence was not generally accepted initially, and, to this day lacks the consensus of the scientific community.[211]
As of 2018, there are 224 knownMartian meteorites (some of which were found in several fragments).[212] These are valuable because they are the only physical samples of Mars available to Earth-bound laboratories. Some researchers have argued that microscopicmorphological features found inALH84001 arebiomorphs, however this interpretation has been highly controversial and is not supported by the majority of researchers in the field.[213]
Seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:[213]
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.[213]
An electron microscope reveals bacteria-like structures in meteorite fragmentALH84001
In 1996, the Martian meteoriteALH84001, 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 byDavid 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 was 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.[214][215]
In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects[218] of the size and shape consistent with Earthlyfossilizednanobacteria.Analysis withgas chromatography andmass spectrometry (GC-MS) studied its high molecular weightpolycyclic aromatic hydrocarbons in 2000, and NASA scientists concluded that as much as 75% of the organic compounds in Nakhla "may not be recent terrestrial contamination".[213][219]
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 dendriticcarbon content was observed. When the results and evidence were published in 2006, some independent researchers claimed that the carbon deposits are of biologic origin. It was remarked that since carbon is the fourth most abundant element in theUniverse, finding it in curious patterns is not indicative or suggestive of biological origin.[220][221]
TheShergotty meteorite, a Martian meteorite weighing 4 kilograms (8.8 lb), fell on Earth onShergotty, India on August 25, 1865, and was retrieved by witnesses almost immediately.[222] It is composed mostly ofpyroxene 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.[213]
Yamato 000593 has areas with spheres (red) that have twice thecarbon as areas without spheres (blue) (February 27, 2014).
Yamato 000593 is thesecond largestmeteorite fromMars found on Earth. Studies suggest theMartian meteorite was formed about 1.3 billion years ago from alava flow onMars. Animpact occurred on Mars about 12 million years ago and ejected the meteorite from the Martian surface intospace. The meteorite landed on Earth inAntarctica about 50,000 years ago. Themass of the meteorite is 13.7 kg (30 lb) and it has been found to contain evidence of pastwater movement.[223][224][225] At a microscopic level,spheres are found in the meteorite that are rich incarbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed bybiotic activity according to NASA scientists.[223][224][225]
Organism–substrate interactions and their products are important biosignatures on Earth as they represent direct evidence of biological behaviour.[226] It was the recovery of fossilized products of life-substrate interactions (ichnofossils) that has revealed biological activities in the early history of life on the Earth, e.g., Proterozoic burrows, Archean microborings and stromatolites.[227][228][229][230][231][232] Two major ichnofossil-like structures have been reported from Mars, i.e. the stick-like structures from Vera Rubin Ridge and the microtunnels from Martian Meteorites.
Observations at Vera Rubin Ridge by the Mars Space Laboratory roverCuriosity show millimetric, elongate structures preserved in sedimentary rocks deposited in fluvio-lacustrine environments within Gale Crater. Morphometric and topologic data are unique to the stick-like structures among Martian geological features and show that ichnofossils are among the closest morphological analogues of these unique features.[233] Nevertheless, available data cannot fully disprove two major abiotic hypotheses, that are sedimentary cracking and evaporitic crystal growth as genetic processes for the structures.
Microtunnels have been described from Martian meteorites. They consist of straight to curved microtunnels that may contain areas of enhanced carbon abundance. The morphology of the curved microtunnels is consistent with biogenic traces on Earth, including microbioerosion traces observed in basaltic glasses.[234][235][232] Further studies are needed to confirm biogenicity.
On 10 September 2025, NASA reported that thePerseverance rover had identified a "potential biosignature" in aJezero Crater rock core ("Sapphire Canyon") from the Bright Angel formation, named theCheyava Falls rock; features consistent with possible ancient microbial activity.[19][236][237][238] Perseverance's survey of revealed organic-carbon-bearingmudstones with mineral signatures from low-temperatureredox reactions; returned samples may be able to clarify the origins of these minerals, organics, and textures.[20]
If confirmed, this biosignature would mean that there was microbial life on Mars around 3.5 billion years ago. According to geologistMichael Tice:[239]
If the Cheyava Falls results ultimately do lead to the proof of ancient life on Mars ... that means two different planets hosted microbes getting their energy through the same means at about the same time in the distant past. That could suggest that early life learns how to survive in this way regardless of where it originated.
The same organic materials can be produced by non-biological processes which require "hot conditions" like volcanic activity; the rock location suggests that it was underwater, and there is no detected past volcanic activity in that region.[239]
NASA-ESA Mars Sample Return mission was designed to collect the samples collected by Perseverance and deliver them to Earth. The mission was deemed "financially unsustainable" and was proposed to be cancelled by the Trump administration.[240]
Artist's concept showing sand-laden jets erupt from geysers on Mars
Close up of dark dune spots, probably created by cold geyser-like eruptions
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.[241][242][243][244] 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.[245]
A team of Hungarian scientists propose that the geysers' most visible features, dark dune spots and spider channels, may be colonies ofphotosynthetic Martian microorganisms, which over-winter beneath the ice cap, and as thesunlight 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.[246][247][248] 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.[249][250] Since their discovery, fiction writerArthur C. Clarke promoted these formations as deserving of study from anastrobiological perspective.[251]
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.[252] A British team also considers the possibility thatorganic matter,microbes, or even simple plants might co-exist with these inorganic formations, especially if the mechanism includes liquid water and ageothermal energy source.[245] They also remark that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis.[245] It has been proposed to develop theMars Geyser Hopper lander to study the geysers up close.[253]
Planetary protection of Mars aims to prevent biological contamination of the planet.[254] A major goal is to preserve the planetary record of natural processes by preventing human-caused microbial introductions, also calledforward 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.[255]
The prime concern of hardware contaminating Mars derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (extremophiles) despite best efforts.[32][256] Hardware includes landers, crashed probes, end-of-mission disposal of hardware, and the hard landing of entry, descent, and landing systems. This has prompted research on survival rates ofradiation-resistant microorganisms including the speciesDeinococcus radiodurans and generaBrevundimonas,Rhodococcus, andPseudomonas under simulated Martian conditions.[257] Results from one of these experimental irradiation experiments, combined with previous radiation modeling, indicate thatBrevundimonas 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.[257] The diurnal Mars-like cycles in temperature and relative humidity affected the viability ofDeinococcus radiodurans cells quite severely.[258] In other simulations,Deinococcus radiodurans also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.[259]
Since the 1950s, researchers have used containers that simulate environmental conditions on Mars to determine the viability of a variety of lifeforms on Mars. Such devices, called "Mars jars" or "Mars simulation chambers", were first described and used in U.S. Air Force research in the 1950s byHubertus Strughold, and popularized in civilian research byJoshua Lederberg andCarl Sagan.[260]
Although numerous studies point to resistance to some of Mars conditions, they do so separately, and none has considered the full range of Martian surface conditions, including temperature, pressure, atmospheric composition, radiation, humidity, oxidizing regolith including perchlorates,[267] and others, all at the same time and in combination.[268] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.[33]
Astrobiologists funded by NASA are researching the limits of microbial life in solutions with high salt concentrations at low temperature.[269] Any body of liquid water under the polar ice caps or underground is likely to exist under high hydrostatic pressure and have a significant salt concentration. They know that the landing site ofPhoenix lander was found to be regolith cemented with water ice and salts, and the soil samples likely contained magnesium sulfate, magnesium perchlorate, sodium perchlorate, potassium perchlorate, sodium chloride and calcium carbonate.[269][270][271] Earth bacteria capable of growth and reproduction in the presence of highly salted solutions, calledhalophile or "salt-lover", were tested for survival using salts commonly found on Mars and at decreasing temperatures.[269] The species tested includeHalomonas,Marinococcus,Nesterenkonia, andVirgibacillus.[269] Laboratory simulations show that whenever multiple Martian environmental factors are combined, the survival rates plummet quickly,[33] however, halophile bacteria were grown in a lab in water solutions containing more than 25% of salts common on Mars, and starting in 2019[needs update], the experiments will incorporate exposure to low temperature, salts, and high pressure.[269]
Mars-1 was the first spacecraft launched to Mars in 1962,[274] but communication was lost while en route to Mars. WithMars-2 andMars-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 (−166 °F) 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.[275]
Signs of life of the Mars space program AMS from orbit were not found. The descent vehicle Mars-2 crashed on landing, the descent vehicleMars-3 launched 1.5 minutes after landing in thePtolemaeus crater, but worked for only 14.5 seconds.[276]
Mariner Crater, as seen by Mariner 4 in 1965. Pictures like this suggested that Mars is too dry for any kind of life.
Streamlined Islands seen by Viking orbiter showed that large floods occurred on Mars. The image is located inLunae Palus quadrangle.
Mariner 4 probe performed the first successfulflyby 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 noglobal magnetic field that would protect the planet from potentially life-threateningcosmic rays. The probe was able to calculate theatmospheric 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.[28] 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.[28][277][278]
Liquid water is necessary for known life andmetabolism, so if water was present on Mars, the chances of it having supported life may have been determinant. TheViking orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams.[279][280][281]
The primary mission of theViking 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.[282] The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result,[dubious –discuss] showing increased14CO2 production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled14CO2 was evolved in the Labeled Release experiment, and that theGCMS detected no organic molecules. There are vastly different interpretations of what those results imply: A 2011astrobiology textbook notes that the GCMS was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that theViking missions failed to detect life in the Martian soil."[283]
Norman Horowitz was the head of theJet Propulsion Laboratory bioscience section for theMariner andViking missions from 1965 to 1976. Horowitz considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival of life on other planets.[284] However, he also considered that the conditions found on Mars were incompatible with carbon based life.
One of the designers of the Labeled Release experiment,Gilbert Levin, believes his results are a definitive diagnostic for life on Mars.[28] Levin's interpretation is disputed by many scientists.[285] A 2006astrobiology 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."[286] Other scientists argue thatsuperoxides in the soil could have produced this effect without life being present.[287] An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph and mass spectrometer, designed to identifynatural organic matter, did not detect organic molecules.[199] More recently, high levels oforganic chemicals, particularlychlorobenzene, weredetected in powder drilled from one of the rocks, named "Cumberland", analyzed by theCuriosity rover.[288][289] The results of the Viking mission concerning life are considered by the general expert community as inconclusive.[28][287][290]
In 2007, during a Seminar of the Geophysical Laboratory of theCarnegie Institution (Washington, D.C., US),Gilbert Levin's investigation was assessed once more.[199] Levin still maintains that his original data were correct, as the positive and negative control experiments were in order.[291] Moreover, Levin's team, on April 12, 2012, reported a statistical speculation, based on old data—reinterpreted mathematically throughcluster analysis—of theLabeled Release experiments, that may suggest evidence of "extant microbial life on Mars".[291][292] 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.[293]
A research team from theNational Autonomous University of Mexico headed byRafael Navarro-González concluded that the GCMS equipment (TV-GC-MS) used by theViking program to search for organic molecules, may not be sensitive enough to detect low levels of organics.[206]Klaus Biemann, the principal investigator of the GCMS experiment onViking wrote a rebuttal.[294] 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.[206]
After the discovery ofperchlorates on Mars by thePhoenix lander, practically the same team of Navarro-González published a paper arguing that the Viking GCMS results were compromised by the presence of perchlorates.[295] 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."[296] Biemann has written a commentary critical of this Navarro-González paper as well,[297] to which the latter have replied;[298] the exchange was published in December 2011.
ThePhoenix 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 Martianregolith 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 theions in the regolith and the amount and type ofantioxidants on Mars. TheViking program data indicate that oxidants on Mars may vary with latitude, noting thatViking 2 saw fewer oxidants thanViking 1 in its more northerly position.Phoenix landed further north still.[299]Phoenix's preliminary data revealed that Mars soil containsperchlorate, and thus may not be as life-friendly as thought earlier.[300][301][208] ThepH and salinity level were viewed as benign from the standpoint of biology. The analysers also indicated the presence of bound water and CO2.[302] 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.[303] In addition, recent analysis of thePhoenix WCL showed that the Ca(ClO4)2 in thePhoenix 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.[304]
On December 16, 2014, NASA reported theCuriosity rover detected a "tenfold spike", likely localized, in the amount ofmethane in theMartian 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.[288][289] In addition, low levels ofchlorobenzene (C 6H 5Cl), were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by theCuriosity rover.[288][289]
The NASAMars 2020 mission includes thePerseverance rover. Launched on July 30, 2020 it is intended to investigate anastrobiologically relevant ancient environment on Mars. This includes its surfacegeological processes and history, and an assessment of its pasthabitability and the potential for preservation ofbiosignatures within accessible geological materials.[315]Perseverance has been on Mars for 4 years, 280 days.
TheCheyava Falls rock discovered on Mars in June 2024 has been designated by NASA as a "potentialbiosignature" and was core sampled by thePerseverance rover for possible return to Earth and further examination. Although highly intriguing, no definitive final determination on a biological or abiotic origin of this rock can be made with the data currently available.
ExoMars is a European-led multi-spacecraft programme currently under development by the European Space Agency (ESA) and theRoscosmos for launch in 2016 and 2020.[316] Its primary scientific mission will be to search for possiblebiosignatures on Mars, past or present. Arover with a 2 m (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 survivecosmic radiation.[49] The program was suspended in 2022, and is unlikely to launch before 2028.[317]
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, so it was concluded that "investigating carbon-based organic compounds would be one of the more fruitful approaches for seeking potential signs of life in returned samples as opposed to culture-based approaches."[318]
Mars, withits presence of water, has arguably the mosthospitable, but still very inhospitable, surface conditions of the planets in theSolar System, other than Earth. Surface conditions on the Saturnian moonTitan and conditions at altitudes off the ground onVenus, and possibly some gas giants might provide better conditions, such as atmospheric pressure, temperature, radiation and gravity more similar to Earth's surface conditions.[319]
Human habitation on Mars would requirein situ resource utilization (ISRU); A NASA report states that "applicable frontier technologies include robotics, machine intelligence, nanotechnology, synthetic biology,3-D printing/additive manufacturing, and autonomy. These technologies combined with the vast natural resources should enable, pre- and post-human arrival ISRU to greatly increase reliability and safety and reduce cost for human colonization of Mars."[320][321][322]
^Mumma, Michael J. (8 January 2012).The Search for Life on Mars. Origin of Life Gordon Research Conference. Galveston, TX.Archived from the original on 4 June 2016.
^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.OCLC263175453.[page needed]
^Dollfus, A. (2010) "The first Pic du Midi photographs of Mars, 1909"[1]
^abcConrad, 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 (1719): 2185.Bibcode:2013LPI....44.2185C.
^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.
^abcdMEPAG 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.PMID17067257.
^abcdeQ. Choi, Charles (17 May 2010)."Mars Contamination Dust-Up". Astrobiology Magazine. Archived from the original on 20 August 2011.Whenever multiple biocidal factors are combined, the survival rates plummet quickly,
^"Mars rover detects carbon signature that hints at past life source".AAAS Articles DO Group. 17 January 2022.doi:10.1126/science.ada0209.
^Fairén, A. G.; et al. (2011). "Cold glacial oceans would have inhibited phyllosilicate sedimentation on early Mars".Nature Geoscience.4 (10):667–670.Bibcode:2011NatGe...4..667F.doi:10.1038/ngeo1243.
^Dehant, V.; Lammer, H.; Kulikov, Y. N.; Grießmeier, J. -M.; Breuer, D.; Verhoeven, O.; Karatekin, Ö.; Hoolst, T.; et al. (2007). "Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars".Geology and Habitability of Terrestrial Planets. Space Sciences Series of ISSI. Vol. 24. pp. 279–300.doi:10.1007/978-0-387-74288-5_10.ISBN978-0-387-74287-8.
^abChang, Kenneth (7 June 2018)."Life on Mars? Rover's Latest Discovery Puts It 'On the Table'".The New York Times.Archived from the original on 8 June 2018. Retrieved8 June 2018.The identification of organic molecules in rocks on the red planet does not necessarily point to life there, past or present, but does indicate that some of the building blocks were present.
^ab"NASA Astrobiology Strategy"(PDF).NASA. 2015. Archived fromthe original(PDF) on 22 December 2016. Retrieved12 November 2017.Subsurface: Conceivably, if life exists (or existed) on Mars, an icy moon, or some other planetary body, evidence of that life could be found, or is best preserved, in the subsurface, away from present-day harsh surface processes.
^Carr, Michael H.The Surface of Mars. Cambridge Planetary Science Series (No. 6).ISBN978-0-511-26688-1.
^Luhmann, J. G.; Russell, C. T. (1997)."Mars: Magnetic Field and Magnetosphere". In Shirley, J. H.; Fainbridge, R. W. (eds.).Encyclopedia of Planetary Sciences. New York: Chapman and Hall. pp. 454–6.Archived from the original on 5 March 2018. Retrieved5 March 2018.
^abThan, Ker (29 January 2007)."Study: Surface of Mars Devoid of Life".Space.com.Archived from the original on 29 April 2014.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.
^abDartnell, 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.S2CID59046908.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. (2 February 2007)."Mars Life May Be Too Deep to Find, Experts Conclude".National Geographic News. Archived fromthe original on 21 February 2014.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.
^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.
^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. (5 April 2013). "TheIcebreaker 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.PMID23560417.
^abNitrogen on Mars: Insights from Curiosity (PDF). J. C. Stern, B. Sutter, W. A. Jackson, Rafael Navarro-González, Christopher P. McKay, Douglas W. Ming, P. Douglas Archer, D. P. Glavin1, A. G. Fairen, andPaul R. Mahaffy. Lunar and Planetary Science XLVIII (2017).
^Adcock, C. T.; Hausrath, E. M.; Forster, P. M. (2013). "Readily available phosphate from minerals in early aqueous environments on Mars".Nature Geoscience.6 (10):824–827.Bibcode:2013NatGe...6..824A.doi:10.1038/ngeo1923.
^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..110.5004H.doi:10.1029/2004JE002261.hdl:2060/20050169988.S2CID1578727.
^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.
^Arfstrom, J. D. (2012). "A Conceptual Model of Equatorial Ice Sheets on Mars".Comparative Climatology of Terrestrial Planets.1675: 8001.Bibcode:2012LPICo1675.8001A.
^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.S2CID4321529.
^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.
^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 (1):113–137.Bibcode:2010Icar..205..113K.doi:10.1016/j.icarus.2009.09.009.
^"Press Release". University of Arizona. 16 March 2006. Archived from the original on 21 July 2006.
^Leveille, R. J. (2010). "Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars".AGU Fall Meeting Abstracts.12: P12A–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.PMID11536937.
^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.PMID11543582.
^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".Bulletin of the American Astronomical Society.35: 937.Bibcode:2003DPS....35.1418M.
^Svedhem, Hakan; Vago, Jorge L.; Bruinsma, Sean; Müller-Wodarg, Ingo; et al. (2017).ExoMars Trace Gas Orbiter provides atmospheric data during Aerobraking into its final orbit. 49th Annual Division for Planetary Sciences Meeting. October 15–20, 2017. Provo, Utah.Bibcode:2017DPS....4941801S. 418.01.
^Krasnopolsky, Vladimir A.; Maillard, Jean Pierre; Owen, Tobias C. (December 2004). "Detection of methane in the martian atmosphere: evidence for life?".Icarus.172 (2):537–547.Bibcode:2004Icar..172..537K.doi:10.1016/j.icarus.2004.07.004.
^Steigerwald, Bill (15 January 2009)."Martian Methane Reveals the Red Planet is not a Dead Planet".NASA's Goddard Space Flight Center. NASA.Archived from the original on 16 January 2009.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 (eds.).Instruments and Methods for Astrobiology and Planetary Missions XII. Vol. 7441. pp. 12–27.Bibcode:2009SPIE.7441E..0DL.doi:10.1117/12.829183.ISBN978-0-8194-7731-6.S2CID73595154.
^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.ISBN978-987-29362-0-4.
^Navarro-González, Rafael; Vargas, Edgar; de la Rosa, José; Raga, Alejandro C.; McKay, Christopher P. (2010). "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars".Journal of Geophysical Research. Vol. 115.Bibcode:2010JGRE..11512010N.doi:10.1029/2010JE003599.
^abcdeGibson, E. K. Jr.; Westall, F.; McKay, D. S.; Thomas-Keprta, K.; Wentworth, S.; Romanek, C. S. (1999)."Evidence for ancient Martian life"(PDF).The Fifth International Conference on Mars, July 19–24, 1999, Pasadena, California, a Lunar and Planetary Science Conference (Abstract). NASA. p. 6142.Bibcode:1999ficm.conf.6142G.Archived(PDF) from the original on 19 March 2015.
^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.PMID8688069.S2CID40690489.
^Baalke, Ron (1995)."The Nakhla Meteorite".Jet Propulsion Lab. NASA.Archived from the original on 14 September 2008. Retrieved17 August 2008.
^Compilation of scientific research references on the Nakhla meteorite:"Nakhla References". Archived fromthe original on 4 September 2008. Retrieved21 August 2008.
^Kieffer, H. H. (2000). "Annual Punctuated CO2 Slab-Ice and Jets on Mars".International Conference on Mars Polar Science and Exploration (1057): 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.
^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:2004ESASP.545..265P.
^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.
^Orme, Greg M.; Ness, Peter K. (9 June 2003)."Martian Spiders"(PDF).Marsbugs.10 (23):5–7. Archived fromthe original(PDF) on 27 September 2007.
^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. Vol. 545. pp. 77–80.Bibcode:2004ESASP.545...77M.ISBN978-92-9092-856-0.
^Landis, Geoffrey; Oleson, Steven; McGuire, Melissa (2012). "Design Study for a Mars Geyser Hopper".50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition.doi:10.2514/6.2012-631.hdl:2060/20120004036.ISBN978-1-60086-936-5.
^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.ISBN978-0-309-10851-5.
^abDartnell, 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.PMID20950171.
^de la Vega, U. Pogoda; Rettberg, P.; Reitz, G. (2007). "Simulation of the environmental climate conditions on martian surface and its effect onDeinococcus radiodurans".Advances in Space Research.40 (11):1672–7.Bibcode:2007AdSpR..40.1672D.doi:10.1016/j.asr.2007.05.022.
^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.PMID20402583.
^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.
^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 lichenCircinaria 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.
^Mihos, Chris (11 January 2006)."Mars (1960–1974): Mars 1".Department of Astronomy, Case Western Reserve University. Archived fromthe original on 13 October 2013. Retrieved26 January 2014.
^Webster, Guy; Hoover, Rachel; Marlaire, Ruth; Frias, Gabriela (3 September 2010)."Missing Piece Inspires New Look at Mars Puzzle". Jet Propulsion Laboratory, NASA.Archived from the original on 3 November 2010. Retrieved24 October 2010.
^Kounaves, S. P.; et al. (2014). "Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics".Icarus.2014 (229):206–213.Bibcode:2014Icar..229..206K.doi:10.1016/j.icarus.2013.11.012.
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