View source for Astronomy:Life on Mars

{{Short description|Scientific assessments on the microbial habitability of Mars}}
The possibility of life on [[Astronomy:Mars|Mars]] is a subject of interest in [[Astronomy:Astrobiology|astrobiology]] due to the [[Astronomy:Planet|planet]]'s proximity and similarities to [[Astronomy:Earth|Earth]]. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ancient [[Astronomy:Noachian|Noachian]] time period, the surface environment of [[Astronomy:Water on Mars|Mars had liquid water]] and may have been [[Astronomy:Planetary habitability|habitable]] for microorganisms, but habitable conditions do not necessarily indicate life.<ref name="NYT-20200724">{{cite news |last=Ferreira |first=Becky |title=3 Great Mysteries About Life on Mars - How habitable was early Mars? Why did it become less hospitable? And could there be life there now? |url=https://www.nytimes.com/2020/07/24/science/mars-life-water.html |date=July 24, 2020 |work={{wipe|The New York Times}} |access-date=July 24, 2020 }}</ref><ref name="NYT-20160912">{{cite news|last=Chang |first=Kenneth |title=Visions of Life on Mars in Earth's Depths |url=https://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |date=September 12, 2016 |work=[[Social:Financial Times|Financial Times]] |access-date=September 12, 2016 |url-status=live |archive-url=https://web.archive.org/web/20160912225220/http://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |archive-date=September 12, 2016 }}</ref>

Scientific searches for evidence of life began in the 19th century and continue today via telescopic investigations and deployed probes, searching for water, chemical [[Astronomy:Biosignature|biosignature]]s in the soil and rocks at the planet's surface, and [[Biology:Biomarker|biomarker]] gases in the atmosphere.<ref>{{cite conference|url=https://ntrs.nasa.gov/search.jsp?R=20120003707 |title=The Search for Life on Mars |last=Mumma |first=Michael J. |date=January 8, 2012 |conference=Origin of Life Gordon Research Conference |location=Galveston, TX |url-status=live |archive-url=https://web.archive.org/web/20160604111239/https://ntrs.nasa.gov/search.jsp?R=20120003707 |archive-date=June 4, 2016 }}</ref>

Mars is of particular interest for the study of the [[Biology:Abiogenesis|origins of life]] because of its similarity to the early Earth. This is especially true since Mars has a cold climate and lacks [[Earth:Plate tectonics|plate tectonics]] or [[Earth:Continental drift|continental drift]], so it has remained almost unchanged since the end of the [[Earth:Hesperian|Hesperian]] period. At least two-thirds of Mars's surface is more than 3.5&nbsp;billion years old, and it could have been habitable  4.48 billion years ago, 500 million years before the earliest known Earth lifeforms;<ref>{{cite journal |title=Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability |date=2019 |last1=Moser |first1=D. E. |last2=Arcuri |first2=G. A. |last3=Reinhard |first3=D. A. |last4= White |first4=L. F. |last5=Darling |first5=J. R. |last6=Barker |first6=I. R. |last7=Larson |first7=D. J. |last8=Irving |first8=A. J. |last9=McCubbin |first9=F. M. |last10=Tait |first10=K. T. |last11=Roszjar |first11=J. |last12=Wittmann |first12=A. |last13=Davis |first13=C. |journal=Nature Geoscience |volume= 12|issue= 7|pages= 522–527|bibcode = 2019NatGe..12..522M|doi=10.1038/s41561-019-0380-0 |url=https://researchportal.port.ac.uk/portal/en/publications/decline-of-giant-impacts-on-mars-by-448-billion-years-ago-and-an-early-opportunity-for-habitability(87d6a7fa-4a74-4507-9f84-feb6b8492c2f).html |doi-access=free }}</ref> Mars may thus hold the best record of the prebiotic conditions leading to life, even if life does not or has never existed there.<ref>{{cite journal |doi=10.1029/RG027i002p00189 |title=The early environment and its evolution on Mars: Implication for life |date=1989 |last1=McKay |first1=Christopher P. |last2=Stoker |first2=Carol R. |journal=Reviews of Geophysics |volume=27 |issue=2 |pages=189–214|bibcode = 1989RvGeo..27..189M |type=Submitted manuscript |url=https://zenodo.org/record/1231452 }}</ref><ref name="Fromproto">{{cite journal |bibcode=2007prpl.conf..929G |arxiv=astro-ph/0602008 |title=From Protoplanets to Protolife: The Emergence and Maintenance of Life |url=https://archive.org/details/arxiv-astro-ph0602008 |last1=Gaidos |first1=Eric |last2=Selsis |first2=Franck |date=2007 |pages=929–44 |journal=Protostars and Planets V}}</ref>

Following the confirmation of the past existence of surface liquid water, the [[Engineering:Curiosity (rover)|''Curiosity'']], [[Engineering:Perseverance (rover)|''Perseverance'']] and [[Engineering:Opportunity (rover)|''Opportunity'']] rovers started searching for evidence of past life, including a past [[Biology:Biosphere|biosphere]] based on [[Biology:Autotroph|autotroph]]ic, [[Biology:Chemotroph|chemotroph]]ic, or chemolithoautotrophic [[Biology:Microorganism|microorganism]]s, as well as ancient water, including [[Earth:Lacustrine plain|fluvio-lacustrine environments]] ([[Earth:Plain|plain]]s related to ancient rivers or lakes) that may have been habitable.<ref name="SCI-20140124a">{{cite journal|last=Grotzinger |first=John P. |title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars |journal=Science |date=January 24, 2014 |volume=343 |issue=6169 |pages=386–387 |doi=10.1126/science.1249944 |bibcode=2014Sci...343..386G |pmid=24458635 |doi-access=free }}</ref><ref name="SCI-20140124special">{{cite journal|author=Various |title=Special Issue - Table of Contents - Exploring Martian Habitability |url=https://www.science.org/toc/science/343/6169 |date=January 24, 2014 |journal=Science |volume=343 |number=6169 |pages=345–452 |url-status=live |archive-url=https://web.archive.org/web/20140129042127/http://www.sciencemag.org/content/343/6169.toc |archive-date=January 29, 2014 }}</ref><ref name="SCI-20140124">{{cite journal|author=Various |title=Special Collection - Curiosity - Exploring Martian Habitability |url=https://www.science.org/action/doSearch?AllField=Curiosity+Mars |date=January 24, 2014 |journal=Science |url-status=live |archive-url=https://web.archive.org/web/20140128102653/http://www.sciencemag.org/site/extra/curiosity/ |archive-date=January 28, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal|title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |date=January 24, 2014 |journal=Science |volume=343 |issue=6169 |page=1242777 |doi=10.1126/science.1242777 |last1=Grotzinger |first1=J. P. |last2=Sumner |first2=D. Y. |last3=Kah |first3=L. C. |last4=Stack |first4=K. |last5=Gupta |first5=S. |last6=Edgar |first6=L. |last7=Rubin |first7=D. |last8=Lewis |first8=K. |last9=Schieber |first9=J. |last10=Mangold |first10=N. |last11=Milliken |first11=R. |last12=Conrad |first12=P. G. |last13=Desmarais |first13=D. |last14=Farmer |first14=J. |last15=Siebach |first15=K. |last16=Calef |first16=F. |last17=Hurowitz |first17=J. |last18=McLennan |first18=S. M. |last19=Ming |first19=D. |last20=Vaniman |first20=D. |last21=Crisp |first21=J. |last22=Vasavada |first22=A. |last23=Edgett |first23=K. S. |last24=Malin |first24=M. |last25=Blake |first25=D. |last26=Gellert |first26=R. |last27=Mahaffy |first27=P. |last28=Wiens |first28=R. C. |last29=Maurice |first29=S. |last30=Grant |first30=J. A. |display-authors=9 |bibcode=2014Sci...343A.386G |pmid=24324272 |citeseerx=10.1.1.455.3973 |s2cid=52836398 }}</ref> The search for evidence of habitability, [[Earth:Taphonomy|taphonomy]] (related to fossils), and organic compounds on Mars is now a primary objective for space agencies.

The findings of organic compounds inside sedimentary rocks and of [[Chemistry:Boron|boron]] on Mars are of interest as they are precursors for prebiotic chemistry. Such findings, along with previous discoveries that liquid water was clearly present on ancient Mars, further supports the possible early habitability of [[Astronomy:Gale (crater)|Gale Crater]] on Mars.<ref name="GPL-20170905">{{cite journal |author=Gasda, Patrick J.|display-authors=0|title=In situ detection of boron by ChemCam on Mars |date=September 5, 2017 |journal=[[Earth:Geophysical Research Letters|Geophysical Research Letters]] |doi=10.1002/2017GL074480 |bibcode=2017GeoRL..44.8739G |volume=44 |issue=17 |pages=8739–8748|url=https://authors.library.caltech.edu/82117/2/grl56315-sup-0001-2017GL074480-SI.pdf |doi-access=free }}</ref><ref name="GZ-20170906">{{cite news |last=Paoletta |first=Rae |title=Curiosity Has Discovered Something That Raises More Questions About Life on Mars |url=https://gizmodo.com/curiosity-has-discovered-something-that-raises-more-que-1800879035 |date=September 6, 2017 |work=Gizmodo |access-date=September 6, 2017 |archive-url=https://web.archive.org/web/20170906222116/http://gizmodo.com/curiosity-has-discovered-something-that-raises-more-que-1800879035 |archive-date=September 6, 2017 |url-status=live }}</ref> Currently, the surface of Mars is bathed with [[Physics:Ionizing radiation|ionizing radiation]], and [[Astronomy:Martian soil|Martian soil]] is rich in perchlorates toxic to [[Biology:Microorganism|microorganism]]s.<ref name="SM-20170706">{{cite news |last=Daley |first=Jason
 |title=Mars Surface May Be Too Toxic for Microbial Life - The combination of UV radiation and perchlorates common on Mars could be deadly for bacteria
 |url=http://www.smithsonianmag.com/smart-news/mars-surface-may-be-toxic-bacteria-180963966/
 |date=July 6, 2017 |work=Smithsonian |access-date=July 8, 2017
 |archive-url=https://web.archive.org/web/20170709052855/http://www.smithsonianmag.com/smart-news/mars-surface-may-be-toxic-bacteria-180963966/
 |archive-date=July 9, 2017 |url-status=live }}</ref><ref name="NAT-20170706">{{cite journal|last1=Wadsworth |first1=Jennifer |last2=Cockell |first2=Charles S.
 |title=Perchlorates on Mars enhance the bacteriocidal effects of UV light
 |date=July 6, 2017 |journal=Scientific Reports |volume=7 |page=4662 |number=4662
 |doi=10.1038/s41598-017-04910-3 |bibcode = 2017NatSR...7.4662W |pmid=28684729 |pmc=5500590}}</ref> 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 of [[Chemistry:Methane|methane]] levels on Mars. Methane could be produced by microorganisms or by geological means.<ref name="NASA-20180607">{{cite web |last1=Brown |first1=Dwayne |last2=Wendel |first2=JoAnna |last3=Steigerwald |first3=Bill |last4=Jones |first4=Nancy |last5=Good |first5=Andrew |title=Release 18-050 - NASA Finds Ancient Organic Material, Mysterious Methane on Mars |url=https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars |date=June 7, 2018 |work=[[Organization:NASA|NASA]] |access-date=June 7, 2018 |archive-url=https://web.archive.org/web/20180607181653/https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars/ |archive-date=June 7, 2018 |url-status=live }}</ref> The European [[Astronomy:ExoMars Trace Gas Orbiter|ExoMars Trace Gas Orbiter]] started mapping the atmospheric methane in April 2018, and the 2022 [[Astronomy:ExoMars|ExoMars]] rover ''[[Engineering:Rosalind Franklin (rover)|Rosalind Franklin]]'' was planned to drill and analyze subsurface samples before the programme's indefinite suspension, while the NASA [[Astronomy:Mars 2020|Mars 2020]] rover ''[[Engineering:Perseverance (rover)|Perseverance]]'', having landed successfully, will cache dozens of drill samples for their potential transport to Earth laboratories in the late 2020s or 2030s. As of February 8, 2021, an updated status of studies considering the possible detection of lifeforms on [[Astronomy:Venus|Venus]] (via [[Chemistry:Phosphine|phosphine]]) and Mars (via [[Chemistry:Methane|methane]]) was reported.<ref name="NYT-20210208">{{cite news |last1=Chang |first1=Kenneth |last2=Stirone |first2=Shannon |title=Life on Venus? The Picture Gets Cloudier - Despite doubts from many scientists, a team of researchers who said they had detected an unusual gas in the planet's atmosphere were still confident of their findings. |url=https://www.nytimes.com/2021/02/08/science/venus-life-phosphine.html |date=February 8, 2021 |work={{wipe|The New York Times}} |access-date=February 8, 2021 }}</ref>

==Early speculation==
{{multiple image
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| image1 = Karte Mars Schiaparelli MKL1888.png
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| caption1 = Historical map of Mars from [[Biography:Giovanni Schiaparelli|Giovanni Schiaparelli]]
| image2 = Lowell Mars channels.jpg
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| caption2 = Mars canals illustrated by astronomer [[Biography:Percival Lowell|Percival Lowell]], 1898
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[[Astronomy:Martian polar ice caps|Mars's polar ice caps]] were discovered in the mid-17th century.{{citation needed|date=March 2021}} 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 that [[Astronomy:Mars|Mars]] had certain other similarities to Earth, for example that the [[Astronomy:Timekeeping on Mars|length of a day on Mars]] was almost the same as a day on Earth. They also knew that its [[Astronomy:Axial tilt|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 [[Astronomy:Darian calendar|much longer year]]. These observations led to increasing speculation that the darker [[Astronomy:Albedo feature|albedo feature]]s were water and the brighter ones were land, whence followed speculation on whether Mars may be inhabited by some form of life.<ref name="Basalla">{{cite book|last1=Basalla|first1=George|title=Civilized life in the universe: scientists on intelligent extraterrestrials|date=2006|publisher=Oxford University Press|location=New York|isbn=9780195171815|page=[https://archive.org/details/civilizedlifeinu0000basa/page/52 52]|url=https://archive.org/details/civilizedlifeinu0000basa/page/52}}</ref>

In 1854, [[Biography:William Whewell|William Whewell]], a fellow of Trinity College, Cambridge, theorized that Mars had seas, land and possibly life forms.<ref>{{cite web|url=https://mars.nasa.gov/allaboutmars/mystique/history/1800/|title=1800s {{!}} Mars Exploration Program|last=mars.nasa.gov|website=mars.nasa.gov|access-date=March 23, 2018|archive-url=https://web.archive.org/web/20190110150909/https://mars.nasa.gov/allaboutmars/mystique/history/1800/|archive-date=January 10, 2019|url-status=live}}</ref> Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent [[Astronomy:Martian canals|Martian canals]]—which were later found to be optical illusions. Despite this, in 1895, American astronomer [[Biography:Percival Lowell|Percival Lowell]] published his book ''Mars,'' followed by ''Mars and its Canals'' in 1906,<ref name="NYT-20151001">{{cite news|last=Dunlap |first=David W. |title=Life on Mars? You Read It Here First. |url=https://www.nytimes.com/2015/09/30/insider/life-on-mars-you-read-it-here-first.html |date=October 1, 2015 |work={{wipe|New York Times}} |access-date=October 1, 2015 |url-status=live |archive-url=https://web.archive.org/web/20151001163353/http://www.nytimes.com/2015/09/30/insider/life-on-mars-you-read-it-here-first.html |archive-date=October 1, 2015 }}</ref> proposing that the canals were the work of a long-gone civilization.<ref>{{cite book |title=Is Mars habitable?: A critical examination of Professor Percival Lowell's book 'Mars and its canals,' with an alternative explanation |first=Alfred Russel |last=Wallace |location=London |publisher=Macmillan |date=1907 |oclc=263175453}}{{page needed|date=June 2013}}</ref> This idea led British writer [[Biography:H. G. Wells|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.<ref>Philip Ball, {{cite web | url = https://www.newstatesman.com/2018/07/war-of-the-worlds-2018-bbc-hg-wells | title = What the War of the Worlds means now| date = July 18, 2018}} ''New Statesman (America Edition)'' July 18, 2018</ref>

[[Physics:Spectroscopy|Spectroscopic]] analysis of Mars's 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.<ref name="chambers">{{cite book |first=Paul |last=Chambers |title=Life on Mars; The Complete Story |place=London |publisher=Bland ford |date=1999 |isbn=978-0-7137-2747-0 |url-access=registration |url=https://archive.org/details/lifeonmarscomple00cham }}{{page needed|date=June 2013}}</ref> The influential observer Eugène Antoniadi used the 83-cm (32.6&nbsp;inch) aperture telescope at Meudon Observatory at the 1909 opposition of Mars and saw no canals, the outstanding photos of Mars taken at the new Baillaud dome at the Pic du Midi observatory also brought formal discredit to the Martian canals theory in 1909,<ref>Dollfus, A. (2010) "The first Pic du Midi photographs of Mars, 1909" [http://adsbit.harvard.edu//full/2010JBAA..120..240D/0000241.000.html]</ref> and the notion of canals began to fall out of favor.<ref name="chambers"/>

==Habitability==

Chemical, physical, geological, and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.<ref name="2013 LPS">{{cite journal |bibcode=2013LPI....44.2185C |title=Habitability Assessment at Gale Crater: Implications from Initial Results |last1=Conrad |first1=P. G. |last2=Archer |first2=D. |last3=Coll |first3=P. |last4=De La Torre |first4=M. |last5=Edgett |first5=K. |last6=Eigenbrode |first6=J. L. |last7=Fisk |first7=M. |last8=Freissenet |first8=C. |last9=Franz |first9=H. |last10=Glavin |first10=D. P. |last11=Gómez |first11=F. |last12=Haberle |first12=R. |last13=Hamilton |first13=V. |last14=Jones |first14=J. H. |last15=Kah |first15=L. C. |last16=Leshin |first16=L. A. |last17=Mahaffy |first17=P. M. |last18=McAdam |first18=A. |last19=McKay |first19=C. P. |last20=Navarro-González |first20=R. |last21=Steele |first21=A. |last22=Stern |first22=J. |last23=Sumner |first23=D. |last24=Treiman |first24=A. H. |last25=Wong |first25=M. H. |last26=Wray |first26=J. |last27=Yingst |first27=R. A. |author28=MSL Science Team |display-authors=9 |volume=1719 |issue=1719 |date=2013 |page=2185 |journal=44th Lunar and Planetary Science Conference }}</ref> The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with an emphasis on water availability, temperature, the presence of nutrients, an energy source, and protection from solar ultraviolet and [[Astronomy:Cosmic ray|galactic cosmic radiation]].<ref name="D.C.Golden">{{cite journal |bibcode=2012P&SS...72...91S |title=Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions |last1=Schuerger |first1=Andrew C. |last2=Golden |first2=D. C. |last3=Ming |first3=Doug W. |volume=72 |issue=1 |date=2012 |pages=91–101 |journal=Planetary and Space Science |doi=10.1016/j.pss.2012.07.026}}</ref><ref name="Beaty">{{cite journal |bibcode=2006AsBio...6..677M |title=Findings of the Mars Special Regions Science Analysis Group | author1=MEPAG Special Regions-Science Analysis Group | last2=Beaty | first2=D. | last3=Buxbaum | first3=K. | last4=Meyer | first4=M. | last5=Barlow | first5=N. | last6=Boynton | first6=W. | last7=Clark | first7=B. | last8=Deming | first8=J. | last9=Doran | first9=P. T. | last10=Edgett | first10=K. | last11=Hancock | first11=S. | last12=Head | first12=J. | last13=Hecht | first13=M. | last14=Hipkin | first14=V. | last15=Kieft | first15=T. | last16=Mancinelli | first16=R. | last17=McDonald | first17=E. | last18=McKay | first18=C. | last19=Mellon | first19=M. | last20=Newsom | first20=H. | last21=Ori | first21=G. | last22=Paige | first22=D. | last23=Schuerger | first23=A. C. | last24=Sogin | first24=M. | last25=Spry | first25=J. A. | last26=Steele | first26=A. | last27=Tanaka | first27=K. | last28=Voytek | first28=M. | display-authors=9 | volume=6 | date=2006 | pages=677–732 | journal=Astrobiology | doi=10.1089/ast.2006.6.677 | pmid=17067257 | issue=5 }}</ref>

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below.<ref name="2013 LPS" /> Similarly, for each group of parameters, the habitability threshold for each is to be determined.<ref name="2013 LPS" /> Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.<ref name="dust-up">{{cite web|url=http://www.astrobio.net/exclusive/3495/mars-contamination-dust-up |title=Mars Contamination Dust-Up |first=Charles |last=Q. Choi |date=May 17, 2010 |publisher=Astrobiology Magazine |quote=Whenever multiple biocidal factors are combined, the survival rates plummet quickly, |url-status=usurped |archive-url=https://web.archive.org/web/20110820212814/http://www.astrobio.net/exclusive/3495/mars-contamination-dust-up |archive-date=August 20, 2011 }}</ref> There are no full-Mars simulations published yet that include all of the biocidal factors combined.<ref name="dust-up" /> 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 and cannot survive in the majority of terrestrial environments that contain oxygen. <ref>{{Cite web |date=2022-01-17 |title=Mars rover detects carbon signature that hints at past life source |url=http://dx.doi.org/10.1126/science.ada0209 |access-date=2023-11-14 |website=AAAS Articles DO Group}}</ref>

{| class="wikitable" 
|-
! style="align: center; background: lavender;" colspan="2" | '''Habitability factors'''<ref name="Beaty" />
|-
|[[Astronomy:Water on Mars|Water]] || {{unordered list| [[Physics:Water activity|liquid water activity]] (a<sub>w</sub>) | Past/future liquid (ice) inventories | [[Earth:Salinity|Salinity]], [[Chemistry:PH|pH]], and [[Chemistry:Reduction potential|Eh]] of available water}}
|-
|Chemical environment|| {{unbulleted list| '''Nutrients:''' {{unordered list| C, H, N, O, P, S, essential metals, essential micronutrients | [[Biology:Nitrogen fixation|Fixed nitrogen]] | Availability/mineralogy}} | '''Toxin abundances and lethality:''' {{unordered list| Heavy metals (e.g., Zn, Ni, Cu, Cr, As, Cd, etc., some essential, but toxic at high levels) | Globally distributed oxidizing soils}}}}
|-
|Energy for [[Biology:Metabolism|metabolism]]|| {{unbulleted list| '''Solar''' (surface and near-surface only) | '''Geochemical''' (subsurface) {{unordered list| [[Chemistry:Oxidizing agent|Oxidants]] | [[Chemistry:Reducing agent|Reductants]] | [[Biology:Redox gradient|Redox gradient]]s}}}}
|-
|Conducive <br />physical conditions || {{unordered list| Temperature | Extreme diurnal temperature fluctuations | Low pressure (Is there a low-pressure threshold for terrestrial [[Biology:Anaerobic organism|anaerobes]]?) | Strong [[Physics:Ultraviolet germicidal irradiation|ultraviolet germicidal irradiation]] | [[Astronomy:Cosmic ray|Galactic cosmic radiation]] and solar particle events (long-term accumulated effects) | Solar UV-induced volatile oxidants, e.g., [[Chemistry:Superoxide|O<sub>2</sub><sup>−</sup>]], O<sup>−</sup>, [[Chemistry:Hydrogen peroxide|H<sub>2</sub>O<sub>2</sub>]], O<sub>3</sub> | Climate/variability (geography, seasons, diurnal, and eventually, obliquity variations) | Substrate (soil processes, rock microenvironments, dust composition, shielding) | High [[Chemistry:Carbon dioxide|CO<sub>2</sub>]] concentrations in the global atmosphere | Transport ([[Earth:Aeolian processes|aeolian]], groundwater flow, surface water, glacial)}}
|}

===Past===
Recent models have shown that, even with a dense [[Chemistry:Carbon dioxide|CO<sub>2</sub>]] atmosphere, early Mars was colder than Earth has ever been.<ref>{{cite journal | last = Fairén | first = A. G. | year = 2010 | title = A cold and wet Mars Mars | journal = Icarus | volume = 208 | issue = 1| pages = 165–175 | doi=10.1016/j.icarus.2010.01.006 | bibcode = 2010Icar..208..165F }}</ref><ref>{{cite journal | last = Fairén | first = A. G. | display-authors=0 |year = 2009 | title = Stability against freezing of aqueous solutions on early Mars | url = https://zenodo.org/record/1233311| journal = Nature | volume = 459 | issue = 7245 | pages = 401–404 | doi=10.1038/nature07978 | pmid = 19458717 | bibcode = 2009Natur.459..401F | s2cid = 205216655 }}</ref><ref>{{cite journal | last = Fairén | first = A. G. | display-authors=0 |year = 2011 | title = Cold glacial oceans would have inhibited phyllosilicate sedimentation on early Mars | journal = Nature Geoscience | volume = 4 | issue = 10 | pages = 667–670 | doi=10.1038/ngeo1243 | bibcode = 2011NatGe...4..667F }}</ref><ref name="habitability 2013">{{cite journal | title=Habitability on Mars from a Microbial Point of View | journal=Astrobiology | date=2013 | last1=Westall | first1=Frances | last2=Loizeau | first2=Damien | last3=Foucher | first3=Frederic | last4=Bost | first4=Nicolas | last5=Betrand | first5=Marylene | last6=Vago | first6=Jorge | last7=Kminek | first7=Gerhard | volume=13 | issue=18 | pages=887–897 | doi=10.1089/ast.2013.1000 | bibcode=2013AsBio..13..887W | pmid=24015806| s2cid=14117893 | url=https://hal-insu.archives-ouvertes.fr/insu-00866015/document }}</ref> Transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late [[Astronomy:Noachian|Noachian]] valley networks, even though the mid-late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars.<ref name="habitability 2013" /> Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid [[Earth:Hesperian|Hesperian]] onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both.<ref name="habitability 2013" /> 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.<ref>{{cite web|url=https://www.scientificamerican.com/article/new-instrument-could-spy-signs-of-alien-life-in-glowing-rocks/|title=New Instrument Could Spy Signs of Alien Life in Glowing Rocks|publisher=Scientific American|date=July 27, 2022}}</ref>

[[File:PIA19673-Mars-AlgaCrater-ImpactGlassDetected-MRO-20150608.jpg|thumb|left|Alga crater is thought to have deposits of impact glass that may have preserved ancient biosignatures, if present during the impact.<ref name="NASA-20150608">{{cite web|author=Staff |title=PIA19673: Spectral Signals Indicating Impact Glass on Mars |url=http://photojournal.jpl.nasa.gov/catalog/PIA19673 |date=June 8, 2015 |work=NASA |access-date=June 8, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150612022335/http://photojournal.jpl.nasa.gov/catalog/PIA19673 |archive-date=June 12, 2015 }}</ref>]]

The loss of the Martian [[Astronomy:Magnetosphere|magnetic field]] strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.<ref name="Biosignatures 2011">{{cite journal | doi=10.1089/ast.2010.0506 | quote=There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance. | title=Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group | date=2011 | last1=Summons | first1=Roger E. | last2=Amend | first2=Jan P. | last3=Bish | first3=David | last4=Buick | first4=Roger | last5=Cody | first5=George D. | last6=Des Marais | first6=David J. | last7=Dromart | first7=Gilles | last8=Eigenbrode | first8=Jennifer L. | last9=Knoll | first9=Andrew H. | last10=Sumner | first10=Dawn Y. | display-authors=8 | journal=Astrobiology | volume=11 | issue=2 | pages=157–81 | pmid=21417945 | bibcode = 2011AsBio..11..157S | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:13041033 | type=Submitted manuscript | hdl=1721.1/66519 | s2cid=9963677 | hdl-access=free }}</ref> When there was a magnetic field, the atmosphere would have been protected from erosion by the [[Astronomy:Solar wind|solar wind]], which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.<ref name="Dehant">{{cite book |doi=10.1007/978-0-387-74288-5_10 |chapter=Planetary Magnetic Dynamo Effect on Atmospheric Protection of Early Earth and Mars |title=Geology and Habitability of Terrestrial Planets |series=Space Sciences Series of ISSI |date=2007 |last1=Dehant |first1=V. |last2=Lammer |first2=H. |last3=Kulikov |first3=Y. N. |last4=Grießmeier |first4=J. -M. |last5=Breuer |first5=D. |last6=Verhoeven |first6=O. |last7=Karatekin |first7=Ö. |last8=Hoolst |first8=T. |last9=Korablev |first9=O. |last10=Lognonné |first10=P. |display-authors=8 |isbn=978-0-387-74287-8 |volume=24 |pages=279–300 }}</ref> 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 became
trapped in [[Earth:Permafrost|permafrost]], a subsurface ice layer.<ref name="habitability 2013" />

Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long-lasting [[Earth:Hot spring|hydrothermal system]] when ice is present in the crust. For example, a 130&nbsp;km large crater could sustain an active hydrothermal system for up to 2&nbsp;million years, that is, long enough for microscopic life to emerge,<ref name="habitability 2013" /> but unlikely to have progressed any further down the evolutionary path.<ref>[https://phys.org/news/2018-01-rover-life-mars-proveit.html Rover could discover life on Mars – here's what it would take to prove it] {{Webarchive|url=https://web.archive.org/web/20180107132731/https://phys.org/news/2018-01-rover-life-mars-proveit.html |date=January 7, 2018 }}. Claire Cousins, ''PhysOrg''. January 5, 2018.</ref>

Soil and rock samples studied in 2013 by NASA's [[Engineering:Curiosity (rover)|''Curiosity'' rover's]] onboard instruments brought about additional information on several habitability factors.<ref name="ancient life">{{cite news|title=NASA Rover Finds Conditions Once Suited for Ancient Life on Mars |date=March 12, 2013 |url=http://www.nasa.gov/mission_pages/msl/news/msl20130312.html |publisher=[[Organization:NASA|NASA]] |url-status=live |archive-url=https://web.archive.org/web/20130703035324/http://www.nasa.gov/mission_pages/msl/news/msl20130312.html |archive-date=July 3, 2013 }}</ref> The rover team identified some of the key chemical ingredients for life in this soil, including [[Chemistry:Sulfur|sulfur]], [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Hydrogen|hydrogen]], oxygen, [[Chemistry:Phosphorus|phosphorus]] and possibly [[Chemistry:Carbon|carbon]], 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.<ref name="ancient life" /> On December 9, 2013, NASA reported that, based on evidence from ''Curiosity'' studying [[Astronomy:Aeolis Palus|Aeolis Palus]], [[Astronomy:Gale (crater)|Gale Crater]] contained an ancient freshwater lake which could have been a hospitable environment for microbial life.<ref name="NYT-20131209">{{cite news|last=Chang |first=Kenneth |title=On Mars, an Ancient Lake and Perhaps Life |url=https://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |date=December 9, 2013 |work={{wipe|New York Times}} |url-status=live |archive-url=https://web.archive.org/web/20131209202521/http://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |archive-date=December 9, 2013 }}</ref><ref name="SCI-20131209">{{cite journal|author=Various |title=Science - Special Collection - Curiosity Rover on Mars |url=https://www.science.org/action/doSearch?AllField=Curiosity+Mars |date=December 9, 2013 |journal=Science |url-status=live |archive-url=https://web.archive.org/web/20140128102653/http://www.sciencemag.org/site/extra/curiosity/ |archive-date=January 28, 2014 }}</ref> The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past [[Astronomy:Magnetosphere|magnetic field]] that protected the planet from cosmic and solar radiation,<ref>{{cite web|url=http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html |title=New Map Provides More Evidence Mars Once Like Earth |publisher=NASA |work=Goddard Space Flight Center |date=October 12, 2005 |last1=Neal-Jones |first1=Nancy |last2=O'Carroll |first2=Cynthia |url-status=live |archive-url=https://archive.today/20120914153601/http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html |archive-date=September 14, 2012 }}</ref><ref>{{cite web |title=Martian Interior: Paleomagnetism |publisher=European Space Agency |work=Mars Express |date=January 4, 2007 |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31028&fbodylongid=645 |access-date=June 6, 2013 |archive-url=https://web.archive.org/web/20120324103626/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31028&fbodylongid=645 |archive-date=March 24, 2012 |url-status=live }}</ref> together strongly suggest that Mars could have had the environmental factors to support life.<ref name="Wall" /><ref>{{cite news |url=http://spaceref.com/news/viewpr.html?pid=43453 |archive-url=https://archive.today/20140812051100/http://spaceref.com/news/viewpr.html?pid=43453 |url-status=dead |archive-date=August 12, 2014 |title=Ames Instrument Helps Identify the First Habitable Environment on Mars, Wins Invention Award |work=Ames Research Center |publisher=Space Ref |date=June 24, 2014 |access-date=August 11, 2014 }}</ref> The assessment of past habitability is not in itself evidence that [[Astronomy:Martian|Martian]] life has ever actually existed. If it did, it was probably [[Biology:Microorganism|microbial]], existing communally in fluids or on sediments, either free-living or as [[Earth:Biofilm|biofilm]]s, respectively.<ref name="Biosignatures 2011" /> The exploration of [[Astronomy:Terrestrial analogue sites|terrestrial analogues]] provide clues as to how and where best look for signs of life on Mars.<ref>{{cite journal | last = Fairén | first = A. G. | display-authors=0 | year = 2010 | title = Astrobiology through the ages of Mars: the study of terrestrial analogues to understand the habitability of Mars | journal = Astrobiology | volume = 10 | issue = 8 | pages = 821–843 | doi=10.1089/ast.2009.0440 | pmid = 21087162 | bibcode = 2010AsBio..10..821F }}</ref>

[[Earth:Impactite|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.<ref>{{cite web|title=Exotic Glass Could Help Unravel Mysteries of Mars |url=http://www.scientificamerican.com/article/exotic-glass-could-help-unravel-mysteries-of-mars/ |access-date=June 15, 2015 |first=Maria |last=Temming |website=Scientific American |url-status=live |archive-url=https://web.archive.org/web/20150615010829/http://www.scientificamerican.com/article/exotic-glass-could-help-unravel-mysteries-of-mars/ |archive-date=June 15, 2015 }}</ref>

On June 7, 2018, NASA announced that the ''Curiosity'' rover had discovered organic molecules in sedimentary rocks dating to three billion years old.<ref>{{cite web |url=https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars |title=NASA Finds Ancient Organic Material, Mysterious Methane on Mars |publisher=NASA |first1=Dwayne |last1=Brown |first2=JoAnna |last2=Wendel |first3=Bill |last3=Steigerwald |first4=Nancy |last4=Jones |first5=Andrew |last5=Good |display-authors=1 |date=June 7, 2018 |access-date=June 12, 2018 |archive-url=https://web.archive.org/web/20180608202435/https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars/ |archive-date=June 8, 2018 |url-status=live }}</ref><ref name="SCI-20180608c">{{cite journal |author=Eigenbrode, Jennifer L. |display-authors=0 |title=Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars |date=June 8, 2018 |journal=Science |volume=360 |issue=6393 |pages=1096–1101 |doi=10.1126/science.aas9185 |pmid=29880683 |bibcode=2018Sci...360.1096E |hdl=10044/1/60810 |s2cid=46983230 |url=https://authors.library.caltech.edu/86910/2/aas9185-Eigenbrode-SM.pdf |doi-access=free }}</ref> The detection of organic molecules in rocks indicate that some of the building blocks for life were present.<ref name="SPC-20180607">{{cite web |last=Wall |first=Mike |title=Curiosity Rover Finds Ancient 'Building Blocks for Life' on Mars |url=https://www.space.com/40819-mars-methane-organics-curiosity-rover.html |date=June 7, 2018 |work=Space.com |access-date=June 7, 2018 |archive-url=https://web.archive.org/web/20180607191720/https://www.space.com/40819-mars-methane-organics-curiosity-rover.html |archive-date=June 7, 2018 |url-status=live }}</ref><ref name="NYT-20180607">{{cite news |last=Chang |first=Kenneth |title=Life on Mars? Rover's Latest Discovery Puts It 'On the Table' - Quote: "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." |url=https://www.nytimes.com/2018/06/07/science/mars-nasa-life.html |date=June 7, 2018 |work={{wipe|The New York Times}} |access-date=June 8, 2018 |archive-url=https://web.archive.org/web/20180608050854/https://www.nytimes.com/2018/06/07/science/mars-nasa-life.html |archive-date=June 8, 2018 |url-status=live }}</ref>

===Present===
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.<ref name="NASA strategy 2015">{{cite web|url=https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|title=NASA Astrobiology Strategy|year=2015|work=NASA|quote=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.|access-date=November 12, 2017|archive-url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|archive-date=December 22, 2016|url-status=dead}}</ref> 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. The [[Earth:Permafrost|permafrost]] layer on Mars is only a couple of centimeters below the surface, and salty [[Earth:Brine|brine]]s 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.<ref name="Floods 2015">{{cite news |url=http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |archive-url=https://archive.today/20150929035120/http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |url-status=dead |archive-date=September 29, 2015 |title=Regional, Not Global, Processes Led to Huge Martian Floods |work=Planetary Science Institute |publisher=SpaceRef |date=September 11, 2015 |access-date=September 12, 2015 }}</ref><ref name=Jakosky2001>{{cite journal | last1=Jakosky | first1=B. M. | last2=Phillips | first2=R. J. | year = 2001 |title=Mars' volatile and climate history | journal=Nature |volume=412| issue=6843| pages=237–244| doi=10.1038/35084184 | pmid=11449285| bibcode=2001Natur.412..237J | doi-access=free }}</ref><ref name="Carr">{{cite book |title=The Surface of Mars |publisher=Cambridge Planetary Science Series (No. 6)|isbn=978-0-511-26688-1 |first=Michael H. |last=Carr <!-- United States Geological Survey, Menlo Park -->}}</ref>

So far, NASA has pursued a "follow the water" strategy on Mars and has not searched for biosignatures for life there directly since the ''Viking'' missions. The consensus by astrobiologists is that it may be necessary to access the Martian subsurface to find currently habitable environments.<ref name="NASA strategy 2015"/>

====Cosmic radiation====
In 1965, the [[Astronomy:Mariner 4|Mariner 4]] probe discovered that Mars had no [[Astronomy:Magnetosphere|global magnetic field]] that would protect the planet from potentially life-threatening cosmic radiation and [[Physics:Solar radiation|solar radiation]]; observations made in the late 1990s by the [[Astronomy:Mars Global Surveyor|Mars Global Surveyor]] confirmed this discovery.<ref>{{cite book |chapter-url=http://ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/ |chapter=Mars: Magnetic Field and Magnetosphere |first1=J. G. |last1=Luhmann |first2=C. T. |last2=Russell |title=Encyclopedia of Planetary Sciences |editor1-first=J. H. |editor1-last=Shirley |editor2-first=R. W. |editor2-last=Fainbridge |pages=454–6 |publisher=Chapman and Hall |location=New York |date=1997 |access-date=March 5, 2018 |archive-url=https://web.archive.org/web/20180305143632/http://ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/ |archive-date=March 5, 2018 |url-status=live }}</ref> Scientists speculate that the lack of magnetic shielding helped the [[Astronomy:Solar wind|solar wind]] blow away much of [[Astronomy:Atmosphere of Mars|Mars's atmosphere]] over the course of several billion years.<ref>{{cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |title=The Solar Wind at Mars |date=January 31, 2001 |first=Tony |last=Phillips |publisher=NASA |url-status=live |archive-url=https://web.archive.org/web/20110818180040/https://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |archive-date=August 18, 2011 }}</ref> As a result, the planet has been vulnerable to radiation from space for about 4&nbsp;billion years.<ref name="hostile to life">{{cite news|title=What makes Mars so hostile to life? |date=January 7, 2013 |url=http://www.bbc.co.uk/science/0/20915340 |work=BBC News |url-status=live |archive-url=https://web.archive.org/web/20130830081628/http://www.bbc.co.uk/science/0/20915340 |archive-date=August 30, 2013 }}</ref>

Recent ''in-situ'' data from ''Curiosity'' rover indicates that [[Physics:Ionizing radiation|ionizing radiation]] from [[Astronomy:Cosmic ray|galactic cosmic rays]] (GCR) and [[Astronomy:Solar particle event|solar particle event]]s (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 by ''Curiosity'' is similar to levels inside the ISS.<ref>{{cite journal|last1=Joanna Carver and Victoria Jaggard |title=Mars is safe from radiation – but the trip there isn't |journal=New Scientist |date=November 21, 2012 |url=https://www.newscientist.com/article/dn22520-mars-is-safe-from-radiation-but-the-trip-there-isnt/ |url-status=live |archive-url=https://web.archive.org/web/20170212165233/https://www.newscientist.com/article/dn22520-mars-is-safe-from-radiation-but-the-trip-there-isnt/ |archive-date=February 12, 2017 }}</ref> <!-- In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:<ref name="RummelBeaty2014">{{cite journal|last1=Rummel |first1=John D. |last2=Beaty |first2=David W. |last3=Jones |first3=Melissa A. |last4=Bakermans |first4=Corien |last5=Barlow |first5=Nadine G. |last6=Boston |first6=Penelope J. |last7=Chevrier |first7=Vincent F. |last8=Clark |first8=Benton C. |last9=de Vera |first9=Jean-Pierre P. |last10=Gough |first10=Raina V. |last11=Hallsworth |first11=John E. |last12=Head |first12=James W. |last13=Hipkin |first13=Victoria J. |last14=Kieft |first14=Thomas L. |last15=McEwen |first15=Alfred S. |last16=Mellon |first16=Michael T. |last17=Mikucki |first17=Jill A. |last18=Nicholson |first18=Wayne L. |last19=Omelon |first19=Christopher R. |last20=Peterson |first20=Ronald |last21=Roden |first21=Eric E. |last22=Sherwood Lollar |first22=Barbara |last23=Tanaka |first23=Kenneth L. |last24=Viola |first24=Donna |last25=Wray |first25=James J. |title=A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2) |journal=Astrobiology |volume=14 |issue=11 |year=2014 |pages=887–968 |issn=1531-1074 |doi=10.1089/ast.2014.1227 |pmid=25401393 |url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf|page=902 |url-status=live |archive-url=https://web.archive.org/web/20170213000635/https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf |archive-date=February 13, 2017 |bibcode = 2014AsBio..14..887R }}</ref> -->

====Cumulative effects====
''Curiosity'' rover measured ionizing radiation levels of 76 mGy per year.<ref>{{cite journal|author1=Donald M Hassler |author2=Cary Zeitlin |author3=Robert F. Wimmer-Schweingruber |author4=Bent Ehresmann |author5=Scot Rafkin |author6=Jennifer L. Eigenbrode |author7=David E. Brinza |author8=Gerald Weigle |author9=Stephan Böttcher |author10=Eckart Böhm |author11=Soenke Burmeister |author12=Jingnan Guo |author13=Jan Köhler |author14=Cesar Martin |author15=Guenther Reitz |author16=Francis A. Cucinotta |author17=Myung-Hee Kim |author18=David Grinspoon |author19=Mark A. Bullock |author20=Arik Posner |author21=Javier Gómez-Elvira |author22=Ashwin Vasavada |author23=John P. Grotzinger |author24=MSL Science Team |title=Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover |journal=Science |volume=343 |issue=6169 |date=November 12, 2013 |page=7 |url=http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |url-status=live |archive-url=https://web.archive.org/web/20140202113404/http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |archive-date=February 2, 2014 |bibcode=2014Sci...343D.386H |doi=10.1126/science.1244797 |pmid=24324275 |hdl=1874/309142 |s2cid=33661472 }}</ref> 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.<ref>{{cite journal|author1=Donald M Hassler |author2=Cary Zeitlin |author3=Robert F. Wimmer-Schweingruber |author4=Bent Ehresmann |author5=Scot Rafkin |author6=Jennifer L. Eigenbrode |author7=David E. Brinza |author8=Gerald Weigle |author9=Stephan Böttcher |author10=Eckart Böhm |author11=Soenke Burmeister |author12=Jingnan Guo |author13=Jan Köhler |author14=Cesar Martin |author15=Guenther Reitz |author16=Francis A. Cucinotta |author17=Myung-Hee Kim |author18=David Grinspoon |author19=Mark A. Bullock |author20=Arik Posner |author21=Javier Gómez-Elvira |author22=Ashwin Vasavada |author23=John P. Grotzinger |author24=MSL Science Team |title=Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover |journal=Science |volume=343 |issue=6169 |date=November 12, 2013 |page=8 |url=http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |url-status=live |archive-url=https://web.archive.org/web/20140202113404/http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |archive-date=February 2, 2014 |bibcode=2014Sci...343D.386H |doi=10.1126/science.1244797 |pmid=24324275 |hdl=1874/309142 |s2cid=33661472 }}</ref> 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.<ref name="cosmic radiation" /><ref>{{cite journal|title=Implications of Cosmic Radiation on the Martian Surface for Microbial Survival and Detection of Fluorescent Biosignatures |journal=Lunar and Planetary Institute |volume=42 |issue=1608 |page=1977 |date=2011 |first1=Lewis R. |last1=Dartnell |first2=Michael C. |last2=Storrie-Storrie-Lombardi |first3=Jan-Peter |last3=Muller |first4=Andrew. D. |last4=Griffiths |first5=Andrew J. |last5=Coates |first6=John M. |last6=Ward |url=http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf |bibcode=2011LPI....42.1977D |url-status=live |archive-url=https://web.archive.org/web/20131006065617/http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1977.pdf |archive-date=October 6, 2013 }}</ref> 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.<ref name="cosmic radiation">{{cite web|url=http://www.space.com/3396-study-surface-mars-devoid-life.html |title=Study: Surface of Mars Devoid of Life |first=Ker |last=Than |date=January 29, 2007 |work=Space.com |quote=After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation. |url-status=live |archive-url=https://web.archive.org/web/20140429215220/http://www.space.com/3396-study-surface-mars-devoid-life.html |archive-date=April 29, 2014 }}</ref><ref name="Dartnell">{{cite journal |doi=10.1029/2006GL027494 |quote=Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2&nbsp;m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m. |title=Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology |date=2007 |last1=Dartnell |first1=L. R. |last2=Desorgher |first2=L. |last3=Ward |first3=J. M. |last4=Coates |first4=A. J. |journal=Geophysical Research Letters |volume=34 |issue=2 |pages=L02207 | bibcode= 2007GeoRL..34.2207D |s2cid=59046908 |url=http://discovery.ucl.ac.uk/134609/ |doi-access=free }}</ref><ref name="Dartnell_Geographic_with_quote">{{cite web|first=Richard A. |last=Lovet |title=Mars Life May Be Too Deep to Find, Experts Conclude |url=http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html |work=National Geographic News |date=February 2, 2007 |quote=That's because any bacteria that may once have lived on the surface have long since been exterminated by cosmic radiation sleeting through the thin Martian atmosphere. |url-status=dead |archive-url=https://web.archive.org/web/20140221095944/http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html |archive-date=February 21, 2014 }}</ref> The team calculated that the cumulative damage to [[Biology:DNA|DNA]] and [[Biology:RNA|RNA]] by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 meters below the planet's surface.<ref name="Dartnell" />
Even the most radiation-tolerant terrestrial bacteria would survive in dormant [[Biology:Spore|spore]] state only 18,000 years at the surface; at 2 meters—the greatest depth at which the [[Astronomy:ExoMars|ExoMars]] rover will be capable of reaching—survival time would be 90,000 to half a million years, depending on the type of rock.<ref name="Dartnell_Geographic">{{cite web|first=Richard A. |last=Lovet |title=Mars Life May Be Too Deep to Find, Experts Conclude |url=http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html |work=National Geographic News |date=February 2, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20140221095944/http://news.nationalgeographic.co.uk/news/2007/02/070202-mars-life.html |archive-date=February 21, 2014 }}</ref>

Data collected by the [[Astronomy:Radiation assessment detector|Radiation assessment detector]] (RAD) instrument on board the [[Engineering:Curiosity (rover)|''Curiosity'' rover]] revealed that the absorbed dose measured is 76 [[Physics:Gray (unit)|mGy]]/year at the surface,<ref name="RAD January 2014" /> and that "[[Physics:Ionizing radiation|ionizing radiation]] strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic molecules."<ref name="RAD January 2014">{{cite journal|title=Mars' Surface Radiation Environment Measured with the Mars ScienceLaboratory's Curiosity Rover |journal=Science |date=January 24, 2014 |first1=Donald M. |last1=Hassler |volume=343 |issue=6169 |page=1244797 |url=http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |doi=10.1126/science.1244797 |pmid=24324275 |display-authors=2 |last2=Zeitlin |first2=C |last3=Ehresmann |first3=B |last4=Rafkin |first4=S |last5=Eigenbrode |first5=J. L. |last6=Brinza |first6=D. E. |last7=Weigle |first7=G |last8=Böttcher |first8=S |last9=Böhm |first9=E |last10=Burmeister |first10=S |last11=Guo |first11=J |last12=Köhler |first12=J |last13=Martin |first13=C |last14=Reitz |first14=G |last15=Cucinotta |first15=F. A. |last16=Kim |first16=M. H. |last17=Grinspoon |first17=D |last18=Bullock |first18=M. A. |last19=Posner |first19=A |last20=Gómez-Elvira |first20=J |last21=Vasavada |first21=A |last22=Grotzinger |first22=J. P. |last23=Msl Science |first23=Team |last24=Kemppinen |first24=O. |last25=Cremers |first25=D. |last26=Bell |first26=J. F. |last27=Edgar |first27=L. |last28=Farmer |first28=J. |last29=Godber |first29=A. |bibcode=2014Sci...343D.386H |url-status=live |archive-url=https://web.archive.org/web/20140202113404/http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf |archive-date=February 2, 2014 |hdl=1874/309142 |s2cid=33661472 }}</ref> Regardless of the source of Martian [[Chemistry:Organic compound|organic compound]]s (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfiguring with surrounding elements by ionizing charged particle radiation.<ref name="RAD January 2014" /> These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic [[Astronomy:Biosignature|biosignature]]s as a function of depth as well as survival times of possible microbial or bacterial life forms left dormant beneath the surface.<ref name="RAD January 2014" /> The report concludes that the ''in situ'' "surface measurements—and subsurface estimates—constrain the preservation window for Martian organic matter following exhumation and exposure to ionizing radiation in the top few meters of the Martian surface."<ref name="RAD January 2014" />

In September 2017, NASA reported [[Physics:Radiation|radiation]] levels on the surface of the planet [[Astronomy:Mars|Mars]] were temporarily doubled and were associated with an [[Astronomy:Aurora|aurora]] 25 times brighter than any observed earlier, due to a major, and unexpected, [[Astronomy:Coronal mass ejection|solar storm]] in the middle of the month.<ref name="PHYS-20170930">{{cite web |last=Scott |first=Jim |title=Large solar storm sparks global aurora and doubles radiation levels on the martian surface |url=https://phys.org/news/2017-09-large-solar-storm-global-aurora.html |date=September 30, 2017 |work=Phys.org |access-date=September 30, 2017 |archive-url=https://web.archive.org/web/20170930222447/https://phys.org/news/2017-09-large-solar-storm-global-aurora.html |archive-date=September 30, 2017 |url-status=live }}</ref>

====UV radiation====
On UV radiation, a 2014 report concludes <ref name="RummelBeaty2014">{{cite journal|last1=Rummel |first1=John D. |last2=Beaty |first2=David W. |last3=Jones |first3=Melissa A. |last4=Bakermans |first4=Corien |last5=Barlow |first5=Nadine G. |last6=Boston |first6=Penelope J. |last7=Chevrier |first7=Vincent F. |last8=Clark |first8=Benton C. |last9=de Vera |first9=Jean-Pierre P. |last10=Gough |first10=Raina V. |last11=Hallsworth |first11=John E. |last12=Head |first12=James W. |last13=Hipkin |first13=Victoria J. |last14=Kieft |first14=Thomas L. |last15=McEwen |first15=Alfred S. |last16=Mellon |first16=Michael T. |last17=Mikucki |first17=Jill A. |last18=Nicholson |first18=Wayne L. |last19=Omelon |first19=Christopher R. |last20=Peterson |first20=Ronald |last21=Roden |first21=Eric E. |last22=Sherwood Lollar |first22=Barbara |last23=Tanaka |first23=Kenneth L. |last24=Viola |first24=Donna |last25=Wray |first25=James J. |title=A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2) |journal=Astrobiology |volume=14 |issue=11 |year=2014 |pages=887–968 |issn=1531-1074 |doi=10.1089/ast.2014.1227 |pmid=25401393 |url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf<!--|page=902--> |url-status=live |archive-url=https://web.archive.org/web/20170213000635/https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf |archive-date=February 13, 2017 |bibcode = 2014AsBio..14..887R }}</ref> 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.<ref name="bacteriocidal"/><ref name="oxides"/> The penetration depth of UV radiation into soils is in the sub-millimeter to millimeter range and depends on the properties of the soil.<ref name="oxides">{{cite journal | doi = 10.1017/S1473550416000331 | volume=16 | title=Shielding biomolecules from effects of radiation by Mars analogue minerals and soils | year=2017 | journal=International Journal of Astrobiology | pages=280–285 | last1 = Ertem | first1 = G. | last2 = Ertem | first2 = M. C. | last3 = McKay | first3 = C. P. | last4 = Hazen | first4 = R. M.| issue=3 | bibcode=2017IJAsB..16..280E | s2cid=125294279 }}</ref>

====Perchlorates====
The Martian regolith is known to contain a maximum of 0.5% (w/v) [[Chemistry:Perchlorate|perchlorate]] (ClO<sub>4</sub><sup>−</sup>) that is toxic for most living organisms,<ref>{{cite journal | doi = 10.1017/S1473550416000458 | volume=16 | title=Earth analogues for past and future life on Mars: isolation of perchlorate resistant halophiles from Big Soda Lake | year=2017 | journal=International Journal of Astrobiology | pages=218–228 | last1 = Matsubara | first1 = Toshitaka | last2 = Fujishima | first2 = Kosuke | last3 = Saltikov | first3 = Chad W. | last4 = Nakamura | first4 = Satoshi | last5 = Rothschild | first5 = Lynn J.| author4-link = Lynn J. Rothschild | issue=3 | bibcode=2017IJAsB..16..218M | doi-access = free }}</ref> but since they drastically lower the freezing point of water and a few extremophiles can use it as an energy source (see [[Chemistry:Perchlorate#Biology|Perchlorates - Biology]]) and grow at concentrations of up to 30% (w/v) [[Chemistry:Sodium perchlorate|sodium perchlorate]]<ref name=":0">{{Cite journal|last1=Heinz|first1=Jacob|last2=Krahn|first2=Tim|last3=Schulze-Makuch|first3=Dirk|date=April 28, 2020|title=A New Record for Microbial Perchlorate Tolerance: Fungal Growth in NaClO4 Brines and its Implications for Putative Life on Mars|journal=Life|language=en|volume=10|issue=5|pages=53|doi=10.3390/life10050053|issn=2075-1729|pmc=7281446|pmid=32353964|bibcode=2020Life...10...53H |doi-access=free}}</ref> by physiologically adapting to increasing perchlorate concentrations,<ref>{{Cite journal |last1=Heinz |first1=Jacob |last2=Doellinger |first2=Joerg |last3=Maus |first3=Deborah |last4=Schneider |first4=Andy |last5=Lasch |first5=Peter |last6=Grossart |first6=Hans‐Peter |last7=Schulze‐Makuch |first7=Dirk |date=August 10, 2022 |title=Perchlorate‐specific proteomic stress responses of Debaryomyces hansenii could enable microbial survival in Martian brines |url=https://onlinelibrary.wiley.com/doi/10.1111/1462-2920.16152 |journal=Environmental Microbiology |volume=24 |issue=11 |language=en |pages=1462–2920.16152 |doi=10.1111/1462-2920.16152 |pmid=35920032 |issn=1462-2912|doi-access=free }}</ref> it has prompted speculation of what their influence would be on habitability.<ref name="bacteriocidal">{{cite journal| pmc=5500590 | pmid=28684729 | doi=10.1038/s41598-017-04910-3 | volume=7 | issue=1 | title=Perchlorates on Mars enhance the bacteriocidal effects of UV light | year=2017 | journal=Sci Rep | page=4662 | last1 = Wadsworth | first1 = J | last2 = Cockell | first2 = CS| bibcode=2017NatSR...7.4662W }}</ref><ref name=":0" /><ref>{{cite journal | doi = 10.1017/S1473550416000434 | volume=16 | title=Bacterial growth tolerance to concentrations of chlorate and perchlorate salts relevant to Mars | year=2017 | journal=International Journal of Astrobiology | pages=229–235 | last1 = Al Soudi | first1 = Amer F. | last2 = Farhat | first2 = Omar | last3 = Chen | first3 = Fei | last4 = Clark | first4 = Benton C. | last5 = Schneegurt | first5 = Mark A.| issue=3 | bibcode=2017IJAsB..16..229A | doi-access = free }}</ref><ref>{{cite news|last1=Chang |first1=Kenneth |title=Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way. |work=The New York Times |url=https://www.nytimes.com/2015/10/06/science/mars-catharine-conley-nasa-planetary-protection-officer.html |agency=New York Times |date=October 5, 2015 |url-status=live |archive-url=https://web.archive.org/web/20151006193649/http://www.nytimes.com/2015/10/06/science/mars-catharine-conley-nasa-planetary-protection-officer.html |archive-date=October 6, 2015 }}</ref><ref>{{Cite journal|last1=Heinz|first1=Jacob|last2=Waajen|first2=Annemiek C.|last3=Airo|first3=Alessandro|last4=Alibrandi|first4=Armando|last5=Schirmack|first5=Janosch|last6=Schulze-Makuch|first6=Dirk|date=November 1, 2019|title=Bacterial Growth in Chloride and Perchlorate Brines: Halotolerances and Salt Stress Responses of Planococcus halocryophilus|journal=Astrobiology|language=en|volume=19|issue=11|pages=1377–1387|doi=10.1089/ast.2019.2069|issn=1531-1074|pmc=6818489|pmid=31386567|bibcode=2019AsBio..19.1377H}}</ref>

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.<ref name="bacteriocidal"/> In addition, two other compounds of the Martian surface, [[Chemistry:Iron oxide|iron oxide]]s and [[Chemistry:Hydrogen peroxide|hydrogen 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.<ref name="bacteriocidal"/><ref name="oxides"/> It was also found that abraded silicates (quartz and basalt) lead to the formation of toxic [[Biology:Reactive oxygen species|reactive oxygen species]].<ref>{{cite journal | last1 = Bak | first1 = Ebbe N. | last2 = Larsen | first2 = Michael G. | last3 = Moeller | first3 = Ralf | last4 = Nissen | first4 = Silas B. | last5 = Jensen | first5 = Lasse R. | last6 = Nørnberg | first6 = Per | last7 = Jensen | first7 = Svend J. K. | last8 = Finster | first8 = Kai | title=Silicates Eroded under Simulated Martian Conditions Effectively Kill Bacteria - A Challenge for Life on Mars | journal=Frontiers in Microbiology | date=September 12, 2017 | volume=8 |pages=1709 |doi=10.3389/fmicb.2017.01709 | pmid = 28955310 | pmc = 5601068 | doi-access = free }}</ref> The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable."<ref>[http://time.com/4845251/mars-life-toxins-microbes/ Why Life on Mars May Be Impossible] {{Webarchive|url=https://web.archive.org/web/20170907210833/http://time.com/4845251/mars-life-toxins-microbes/ |date=September 7, 2017 }}. Jeffrey Kluger. ''Time'' - Science; July 6, 2017.</ref> This research demonstrates that the present-day surface is more uninhabitable than previously thought,<ref name="bacteriocidal"/><ref name="Wall2017">[https://www.space.com/37402-mars-life-soil-toxic-perchlorates-radiation.html Mars Soil May Be Toxic to Microbes] {{Webarchive|url=https://web.archive.org/web/20170911025141/https://www.space.com/37402-mars-life-soil-toxic-perchlorates-radiation.html |date=September 11, 2017 }}. Mike Wall. Space.com. July 6, 2017</ref> and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.<ref name="Wall2017"/><ref>[http://www.abc.net.au/news/2017-07-07/mars-toxic-soil-could-make-growing-vegies-harder/8687626 Mars soil is likely toxic to cells—does this mean humans won't be able to grow vegetables there?] {{Webarchive|url=https://web.archive.org/web/20170911020648/http://www.abc.net.au/news/2017-07-07/mars-toxic-soil-could-make-growing-vegies-harder/8687626 |date=September 11, 2017 }}. David Coady. ''The World Today''. July 7, 2017</ref>

However, researcher [[Biography:Kennda Lynch|Kennda 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.<ref>{{Cite journal|last1=Lynch|first1=Kennda L.|last2=Jackson|first2=W. Andrew|last3=Rey|first3=Kevin|last4=Spear|first4=John R.|last5=Rosenzweig|first5=Frank|last6=Munakata-Marr|first6=Junko|date=March 1, 2019|title=Evidence for Biotic Perchlorate Reduction in Naturally Perchlorate-Rich Sediments of Pilot Valley Basin, Utah|url=https://www.liebertpub.com/doi/10.1089/ast.2018.1864|journal=Astrobiology|volume=19|issue=5|pages=629–641|doi=10.1089/ast.2018.1864|pmid=30822097|bibcode=2019AsBio..19..629L|s2cid=73492950|issn=1531-1074}}</ref> She has been studying the [[Astronomy:Biosignature|biosignature]]s of these microbes, and is hoping that the [[Engineering:Perseverance (rover)|Mars Perseverance]] rover will find matching biosignatures at its [[Astronomy:Jezero (crater)|Jezero Crater]] site.<ref>Chang, Kenneth (July 28, 2020). [https://www.nytimes.com/2020/07/28/science/nasa-jezero-perseverance.html "How NASA Found the Ideal Hole on Mars to Land In"]. ''The New York Times''. ISSN 0362-4331. Retrieved 2021-03-02.</ref><ref>Daines, Gary (August 14, 2020). [https://www.nasa.gov/mediacast/gravity-assist-looking-for-life-in-ancient-lakes "Looking For Life in Ancient Lakes"] (Season 4, Episode 15 ). Gravity Assist.NASA. Podcast. Retrieved 2021-03-02.</ref>

====Recurrent slope lineae====
[[Astronomy:Seasonal flows on warm Martian slopes|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".).<ref name="RummelBeaty2014-2">{{cite journal|last1=Rummel|first1=John D.|last2=Beaty|first2=David W.|last3=Jones|first3=Melissa A.|last4=Bakermans|first4=Corien|last5=Barlow|first5=Nadine G.|last6=Boston|first6=Penelope J.|last7=Chevrier|first7=Vincent F.|last8=Clark|first8=Benton C.|last9=de Vera|first9=Jean-Pierre P.|last10=Gough|first10=Raina V.|last11=Hallsworth|first11=John E.|last12=Head|first12=James W.|last13=Hipkin|first13=Victoria J.|last14=Kieft|first14=Thomas L.|last15=McEwen|first15=Alfred S.|last16=Mellon|first16=Michael T.|last17=Mikucki|first17=Jill A.|last18=Nicholson|first18=Wayne L.|last19=Omelon|first19=Christopher R.|last20=Peterson|first20=Ronald|last21=Roden|first21=Eric E.|last22=Sherwood Lollar|first22=Barbara|last23=Tanaka|first23=Kenneth L.|last24=Viola|first24=Donna|last25=Wray|first25=James J.|title=A New Analysis of liquid "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)|journal=Astrobiology|volume=14|issue=11|year=2014|pages=887–968|issn=1531-1074|doi=10.1089/ast.2014.1227|pmid=25401393|url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf<!--|page=902-->|bibcode=2014AsBio..14..887R}}</ref><ref>{{cite web|title=Warm-Season Flows on Slope in Newton Crater |url=https://www.nasa.gov/mission_pages/MRO/multimedia/pia14472.html |website=NASA Press Release |url-status=live |archive-url=https://web.archive.org/web/20170212164022/https://www.nasa.gov/mission_pages/MRO/multimedia/pia14472.html |archive-date=February 12, 2017 |date=July 23, 2018 }}</ref> They were suspected as involving flowing brines back then.<ref>{{cite news|last1=Amos |first1=Jonathan |title=Martian salt streaks 'painted by liquid water' |url=https://www.bbc.co.uk/news/science-environment-34379284 |publisher=BBC Science |url-status=live |archive-url=https://web.archive.org/web/20161125042041/http://www.bbc.co.uk/news/science-environment-34379284 |archive-date=November 25, 2016 }}</ref><ref name="NASA-20150928b">{{cite web
 |author=Staff 
 |title=Video Highlight - NASA News Conference - Evidence of Liquid Water on Today's Mars 
 |url=https://www.youtube.com/watch?v=bDv4FRHI3J8 
 |date=September 28, 2015
 |work=[[Organization:NASA|NASA]] 
 |access-date=September 30, 2015
 |url-status=live
 |archive-url=https://web.archive.org/web/20151001113935/https://www.youtube.com/watch?v=bDv4FRHI3J8 
 |archive-date=October 1, 2015 }}</ref><ref name="NASA-20150928a">{{cite web|author=Staff |title=Video Complete - NASA News Conference - Water Flowing on Present-Day Mars m |url=https://www.youtube.com/watch?v=MRQ5B_ik2dU |date=September 28, 2015 |work=[[Organization:NASA|NASA]] |access-date=September 30, 2015 |url-status=live |archive-url=https://web.archive.org/web/20151015205144/https://www.youtube.com/watch?v=MRQ5B_ik2dU |archive-date=October 15, 2015 }}</ref><ref name="Ojhaetal2015">{{cite journal |last1=Ojha |first1=L. |last2=Wilhelm |first2=M. B. |last3=Murchie |first3=S. L. |last4=McEwen |first4=A. S. |last5=Wray |first5=J. J. |last6=Hanley |first6=J. |last7=Massé |first7=M. |last8=Chojnacki |first8=M. |date=2015 |title=Spectral evidence for hydrated salts in recurring slope lineae on Mars |journal=Nature Geoscience |doi=10.1038/ngeo2546 |volume=8 |issue=11 |pages=829–832|bibcode = 2015NatGe...8..829O }}</ref>

The thermodynamic availability of water ([[Physics:Water activity|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 high [[Chemistry:Ionic strength|ionic strength]], driven to extremes on Mars by the ubiquitous occurrence of divalent ions, "renders these environments uninhabitable despite the presence of biologically available water."<ref>{{cite journal | doi = 10.1089/ast.2015.1432 | volume=16 | title=Ionic Strength Is a Barrier to the Habitability of Mars | year=2016 | journal=Astrobiology | pages=427–442 | last1 = Fox-Powell | first1 = Mark G. | last2 = Hallsworth | first2 = John E. | last3 = Cousins | first3 = Claire R. | last4 = Cockell | first4 = Charles S.| issue=6 | pmid=27213516 | bibcode=2016AsBio..16..427F | hdl=10023/10912 | s2cid=4314602 | url=http://oro.open.ac.uk/73442/1/Fox-Powell_AST2015-1432_accepted.pdf | hdl-access=free }}</ref>

===Nitrogen fixation===
After carbon, [[Chemistry:Nitrogen|nitrogen]] is arguably the most important element needed for life. Thus, measurements of [[Chemistry:Nitrate|nitrate]] over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N<sub>2</sub>) in the atmosphere at low levels, but this is not adequate to support [[Biology:Nitrogen fixation|nitrogen fixation]] for biological incorporation.<ref name="Icebreaker2018">{{cite journal | title=The ''Icebreaker Life'' Mission to Mars: A Search for Biomolecular Evidence for Life | journal=Astrobiology | date=April 5, 2013 | first1=Christopher P. | last1=McKay | first2=Carol R. | last2=Stoker | first3=Brian J. | last3=Glass | first4=Arwen I. | last4=Davé | first5=Alfonso F. | last5=Davila | first6=Jennifer L. | last6=Heldmann | first7=Margarita M. | last7=Marinova | first8=Alberto G. | last8=Fairen | first9=Richard C. | last9=Quinn | first10=Kris A. | last10=Zacny | first11=Gale | last11=Paulsen | first12=Peter H. | last12=Smith | first13=Victor | last13=Parro | first14=Dale T. | last14=Andersen | first15=Michael H. | last15=Hecht | first16=Denis | last16=Lacelle | first17=Wayne H. | last17=Pollard. | display-authors=9 | volume=13 | issue=4 | pages=334–353 | doi=10.1089/ast.2012.0878 | pmid=23560417 | bibcode= 2013AsBio..13..334M }}</ref> Nitrogen in the form of [[Chemistry:Nitrate|nitrate]] could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed by thermal shock from impact or volcanic plume lightning on ancient Mars.<ref name="Stern 2015">{{cite journal|title=Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale crater, Mars |journal=Proceedings of the National Academy of Sciences of the United States of America |date=March 24, 2015 |last=Stern |first=Jennifer C. |doi=10.1073/pnas.1420932112 |volume=112 |issue=14 |pages=4245–4250 |pmid=25831544 |pmc=4394254 |bibcode=2015PNAS..112.4245S |doi-access=free }}</ref>

On March 24, 2015, NASA reported that the [[Astronomy:Sample Analysis at Mars|SAM]] instrument on the ''Curiosity'' rover detected nitrates by heating surface sediments. The nitrogen in nitrate is in a "fixed" state, meaning that it is in an oxidized form that can be used by living organisms. The discovery supports the notion that ancient Mars may have been hospitable for life.<ref name="Stern 2015" /><ref name="NASA-20150324">{{cite web|last1=Neal-Jones |first1=Nancy |last2=Steigerwald |first2=William |last3=Webster |first3=Guy |last4=Brown |first4=Dwayne |title=Curiosity Rover Finds Biologically Useful Nitrogen on Mars |url=http://www.jpl.nasa.gov/news/news.php?feature=4516 |date=March 24, 2015 |work=[[Organization:NASA|NASA]] |access-date=March 25, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150327000753/http://www.jpl.nasa.gov/news/news.php?feature=4516 |archive-date=March 27, 2015 }}</ref><ref>{{cite news|url=https://www.bbc.com/news/science-environment-32048273 |title=Curiosity Mars rover detects 'useful nitrogen' |work=NASA |publisher=BBC News |date=March 25, 2015 |access-date=March 25, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150327172922/http://www.bbc.com/news/science-environment-32048273 |archive-date=March 27, 2015 }}</ref> It is suspected that all nitrate on Mars is a relic, with no modern contribution.<ref name="Nitrogen 2017">[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170002371.pdf Nitrogen 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, and
Paul R. Mahaffy. Lunar and Planetary Science XLVIII (2017).</ref> Nitrate abundance ranges from non-detection to 681 ± 304&nbsp;mg/kg in the samples examined until late 2017.<ref name="Nitrogen 2017"/> 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.<ref>{{cite journal|title=An active nitrogen cycle on Mars sufficient to support a subsurface biosphere |journal=International Journal of Astrobiology |last1=Boxe |first1=C. S. |last2=Hand |first2=K.P. |last3=Nealson |first3=K.H. |last4=Yung |first4=Y.L. |last5=Saiz-Lopez |first5=A. |volume=11 |issue=2 |pages=109–115 |doi=10.1017/S1473550411000401 |bibcode=2012IJAsB..11..109B |year=2012 |s2cid=40894966 |url=https://authors.library.caltech.edu/30213/1/Boxe2012p17592Int_J_Astrobiol.pdf }}</ref>

In contrast, phosphate, one of the chemical nutrients thought to be essential for life, is readily available on Mars.<ref>{{cite journal | doi = 10.1038/ngeo1923 | volume=6 | title=Readily available phosphate from minerals in early aqueous environments on Mars | year=2013 | journal=Nature Geoscience | pages=824–827 | last1 = Adcock | first1 = C. T. | last2 = Hausrath | first2 = E. M. | last3 = Forster | first3 = P. M.| issue=10 | bibcode=2013NatGe...6..824A }}</ref>

===Low pressure===
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).<ref name="Serratia">{{cite journal | title=Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres | journal=Astrobiology | date=February 2013 | first1=Andrew C. | last1=Schuerger | first2=Richard | last2=Ulrich | first3=Bonnie J. | last3=Berry | first4=Wayne L. | last4=Nicholson | volume=13 | issue=2 | pages=115–131 | doi=10.1089/ast.2011.0811 | pmid=23289858 | bibcode=2013AsBio..13..115S | pmc=3582281 }}</ref> In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only ''[[Biology:Serratia liquefaciens|Serratia liquefaciens]]'' strain ATCC 27592 exhibited growth at 7 mbar, 0&nbsp;°C, and CO<sub>2</sub>-enriched anoxic atmospheres.<ref name="Serratia" />

==Liquid water==
{{Main|Astronomy:Water on Mars}}

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.<ref name="NASA-20151015">{{cite web |author=Hays, Linda|display-authors=0|title=Astrobiology Strategy 2015 |url=http://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf |date=October 2015 |work=[[Organization:NASA|NASA]] |url-status=dead |archive-url=https://web.archive.org/web/20161222190939/https://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf |archive-date=December 22, 2016 |access-date=September 21, 2017 }}</ref> Liquid water cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.<ref>{{cite journal |doi=10.1029/2004JE002261 |title=Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions |date=2005 |last1=Heldmann |first1=Jennifer L. |first2=Owen B. |last2=Toon |first3=Wayne H. |last3=Pollard |first4=Michael T. |last4=Mellon |first5=John |last5=Pitlick |first6=Christopher P. |last6=McKay |first7=Dale T. |last7=Andersen |journal=Journal of Geophysical Research |volume=110 |issue=E5 |pages=E05004|bibcode=2005JGRE..110.5004H |hdl=2060/20050169988 |s2cid=1578727 |hdl-access=free }}</ref><ref>{{cite journal |bibcode=2006GeoRL..3311201K |title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement |last1=Kostama |first1=V.-P. |last2=Kreslavsky |first2=M. A. |last3=Head |first3=J. W. |volume=33 |date=2006 |page=11201 |journal=Geophysical Research Letters |doi=10.1029/2006GL025946 |issue=11|citeseerx=10.1.1.553.1127 |s2cid=17229252 }}</ref> Liquid water does not appear at the surface itself,<ref>{{cite journal |bibcode=2006IJMSE...2...83H |title=Transient liquid water near an artificial heat source on Mars |last1=Hecht |first1=Michael H. |last2=Vasavada |first2=Ashwin R. |volume=2 |date=2006 |pages=83–96 |journal=International Journal of Mars Science and Exploration |doi=10.1555/mars.2006.0006}}</ref> but it could form in minuscule amounts around dust particles in snow heated by the Sun.<ref name="Shiga2009">{{cite web|first=David |last=Shiga |url=https://www.newscientist.com/article/mg20427373.700 |date=December 7, 2009 |title=Watery niche may foster life on Mars |work=New Scientist |url-status=live |archive-url=https://web.archive.org/web/20131007014344/http://www.newscientist.com/article/mg20427373.700 |archive-date=October 7, 2013 }}</ref><ref name="news.softpedia">{{cite web|first=Tudor |last=Vieru |url=http://news.softpedia.com/news/Greenhouse-Effect-on-Mars-May-Be-Allowing-for-Life-129065.shtml |title=Greenhouse Effect on Mars May Be Allowing for Life |publisher=Softpedia |date=December 7, 2009 |url-status=live |archive-url=https://web.archive.org/web/20130731075219/http://news.softpedia.com/news/Greenhouse-Effect-on-Mars-May-Be-Allowing-for-Life-129065.shtml |archive-date=July 31, 2013 }}</ref>{{unreliable source?|date=June 2013}} Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.<ref>{{cite web |first=Michael T. |last=Mellon |url=https://science.nasa.gov/media/medialibrary/2011/06/29/Mellon_Water_PPS_May2011_-_TAGGED.pdf |title=Subsurface Ice at Mars: A review of ice and water in the equatorial regions |publisher=University of Colorado |date=May 10, 2011 |work=Planetary Protection Subcommittee Meeting |url-status=dead |archive-url=https://web.archive.org/web/20140228064342/https://science.nasa.gov/media/medialibrary/2011/06/29/Mellon_Water_PPS_May2011_-_TAGGED.pdf |archive-date=February 28, 2014 }}</ref><ref>{{cite web|first=Robert Roy |last=Britt |url=http://www.space.com/812-ice-packs-methane-mars-suggest-present-life.html |title=Ice Packs and Methane on Mars Suggest Present Life Possible |work=space.com |date=February 22, 2005 |url-status=live |archive-url=https://web.archive.org/web/20130503084616/http://www.space.com/812-ice-packs-methane-mars-suggest-present-life.html |archive-date=May 3, 2013 }}</ref><ref>{{cite journal |bibcode=1997JGR...10219357M |title=The persistence of equatorial ground ice on Mars |last1=Mellon |first1=Michael T. |last2=Jakosky |first2=Bruce M. |last3=Postawko |first3=Susan E. |volume=102 |issue=E8 |date=1997 |pages=19357–69 |journal=Journal of Geophysical Research |doi=10.1029/97JE01346|doi-access=free }}</ref><ref>{{cite journal |bibcode=2012LPICo1675.8001A |title=A Conceptual Model of Equatorial Ice Sheets on Mars |last=Arfstrom |first=J. D. |volume=1675 |date=2012 |page=8001 |journal=Comparative Climatology of Terrestrial Planets}}</ref>

{{multiple image|direction=horizontal|align=center|total_width=800|header=[[Astronomy:Mars|Mars]] - [[Astronomy:Utopia Planitia|Utopia Planitia]]<br />[[Astronomy:Scalloped topography|Scalloped terrain]] led to the discovery of a large amount of [[Astronomy:Water on Mars|underground ice]]<br />enough water to fill Lake Superior (November 22, 2016)<ref name="NASA-20161122" /><ref name="Register-2016" /><ref name="NASA-20161122jpl" /> |header_align=center
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Water on Mars exists almost exclusively as water ice, located in the [[Astronomy:Martian polar ice caps|Martian polar ice caps]] and under the shallow Martian surface even at more temperate latitudes.<ref name="mars.jpl.nasa.gov">{{cite web|url=http://mars.jpl.nasa.gov/odyssey/newsroom/pressreleases/20020528a.html |title=Mars Odyssey: Newsroom |publisher=Mars.jpl.nasa.gov |date=May 28, 2002 |url-status=live |archive-url=https://web.archive.org/web/20110606235941/http://mars.jpl.nasa.gov/odyssey/newsroom/pressreleases/20020528a.html |archive-date=June 6, 2011 }}</ref><ref name="Feildman, T. 2004">{{cite journal |doi=10.1029/2003JE002160 |title=Global distribution of near-surface hydrogen on Mars |date=2004 |last=Feldman |first=W. C. |journal=Journal of Geophysical Research |volume=109|issue=E9 |bibcode=2004JGRE..109.9006F |doi-access=free }}</ref> A small amount of water vapor is present in the [[Astronomy:Atmosphere of Mars|atmosphere]].<ref name="ucar">{{cite web |url=http://www.windows.ucar.edu/tour/link=/mars/exploring/MGS_water_clouds.html |title=Mars Global Surveyor Measures Water Clouds |access-date=March 7, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090812084157/http://www.windows.ucar.edu/tour/link%3D/mars/exploring/MGS_water_clouds.html |archive-date=August 12, 2009 }}</ref> There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages {{Convert|600|Pa|sp=us}}—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, ({{Convert|210|K|C}}) leading to immediate freezing. Despite this, about 3.8&nbsp;billion years ago,<ref name="Baker">{{cite journal |bibcode=1991Natur.352..589B |title=Ancient oceans, ice sheets and the hydrological cycle on Mars |last1=Baker |first1=V. R. |last2=Strom |first2=R. G. |last3=Gulick |first3=V. C. |last4=Kargel |first4=J. S. |last5=Komatsu |first5=G. |volume=352 |date=1991 |pages=589–594 |journal=Nature |doi=10.1038/352589a0 |last6=Kale |first6=V. S. |issue=6336|s2cid=4321529 }}</ref> there was a denser [[Astronomy:Atmosphere of Mars|atmosphere]], higher temperature, and vast amounts of liquid water flowed on the surface,<ref name="flashback">{{cite web|url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago |publisher=SPACE.com |date=June 22, 2000 |url-status=live |archive-url=https://web.archive.org/web/20101222210332/http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |archive-date=December 22, 2010 }}</ref><ref>{{cite web |url=https://science.nasa.gov/headlines/y2001/ast05jan_1.htm |work=Science@NASA |title=The Case of the Missing Mars Water |access-date=March 7, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090327234049/https://science.nasa.gov/headlines/y2001/ast05jan_1.htm |archive-date=March 27, 2009 }}</ref><ref name="Clay clues">{{cite news|title=Mars Rover Opportunity Examines Clay Clues in Rock |date=May 17, 2013 |publisher=Jet Propulsion Laboratory |url=http://www.jpl.nasa.gov/news/news.php?release=2013-167 |work=NASA |url-status=live |archive-url=https://web.archive.org/web/20130611181547/http://www.jpl.nasa.gov/news/news.php?release=2013-167 |archive-date=June 11, 2013 }}</ref><ref>{{cite news|title=NASA Rover Helps Reveal Possible Secrets of Martian Life |date=November 29, 2005 |url=http://www.nasa.gov/home/hqnews/2005/nov/HQ_05415_rover_secrets_prt.htm |work=NASA |url-status=live |archive-url=https://web.archive.org/web/20131122193015/http://www.nasa.gov/home/hqnews/2005/nov/HQ_05415_rover_secrets_prt.htm |archive-date=November 22, 2013 }}</ref> including large oceans.<ref>"Mapping Mars: Science, Imagination and the Birth of a World". Oliver Morton, 2002. {{ISBN|0-312-24551-3}}{{page needed|date=June 2013}}</ref><ref name="Sea">{{cite web|url=http://www.psrd.hawaii.edu/July03/MartianSea.html |title=PSRD: Ancient Floodwaters and Seas on Mars |publisher=Psrd.hawaii.edu |date=July 16, 2003 |url-status=live |archive-url=https://web.archive.org/web/20110104093144/http://www.psrd.hawaii.edu/July03/MartianSea.html |archive-date=January 4, 2011 }}</ref><ref>{{cite web| url=http://www.spaceref.com/news/viewpr.html?pid=26947 |title=Gamma-Ray Evidence Suggests Ancient Mars Had Oceans |publisher=SpaceRef |date=November 17, 2008 }}</ref><ref name="2003JGRE..108.5042C">{{cite journal |bibcode=2003JGRE..108.5042C |title=Oceans on Mars: An assessment of the observational evidence and possible fate |last1=Carr |first1=Michael H. |last2=Head |first2=James W. |volume=108 |issue=E5 |date=2003 |page=5042 |journal=Journal of Geophysical Research: Planets |doi=10.1029/2002JE001963|doi-access=free }}</ref><ref name="SFN-20130125">{{cite web|last=Harwood |first=William |title=Opportunity rover moves into 10th year of Mars operations |url=http://www.spaceflightnow.com/news/n1301/25opportunity/ |date=January 25, 2013 |publisher=Space Flight Now |url-status=live |archive-url=https://web.archive.org/web/20131224095028/http://www.spaceflightnow.com/news/n1301/25opportunity/ |archive-date=December 24, 2013 }}</ref> 
[[File:History of Water on Mars.jpg|thumb|left|A series of artist's conceptions of past water coverage on Mars]]
[[File:Mars-SubglacialWater-SouthPoleRegion-20180725.jpg|thumb|right|150px|Mars [[Astronomy:Planum Australe|SouthPole]]<br />Site of Subglacial Water<br />(July 25, 2018)]]
It has been estimated that the primordial oceans on Mars would have covered between 36%<ref name="ReferenceA">{{cite journal | last1=Di Achille | first1=Gaetano | last2=Hynek | first2=Brian M. | title=Ancient ocean on Mars supported by global distribution of deltas and valleys | journal=Nature Geoscience | volume=3 | pages=459–63 | date=2010 | doi=10.1038/ngeo891 |bibcode=2010NatGe...3..459D | issue=7}}
*{{cite press release |date=June 14, 2010 |title=Ancient ocean may have covered third of Mars |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2010/06/100613181245.htm}}</ref> and 75% of the planet.<ref name="Smith">{{cite journal |bibcode=1999Sci...286...94S |title=The gravity field of Mars: Results from Mars Global Surveyor |last1=Smith |first1=D. E. |last2=Sjogren |first2=W. L. |last3=Tyler |first3=G. L. |last4=Balmino |first4=G. |last5=Lemoine |first5=F. G. |last6=Konopliv |first6=A. S. |volume=286 |date=1999 |pages=94–7 |journal=Science |doi=10.1126/science.286.5437.94 |pmid=10506567 |issue=5437}}</ref> On November 22, 2016, NASA reported finding a large amount of [[Astronomy:Water on Mars|underground ice]] in the [[Astronomy:Utopia Planitia|Utopia Planitia]] region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.<ref name="NASA-20161122">{{cite web|author=Staff |title=Scalloped Terrain Led to Finding of Buried Ice on Mars |url=http://photojournal.jpl.nasa.gov/catalog/PIA21136 |date=November 22, 2016 |work=[[Organization:NASA|NASA]] |access-date=November 23, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161124094205/http://photojournal.jpl.nasa.gov/catalog/PIA21136 |archive-date=November 24, 2016 }}</ref><ref name="Register-2016">{{cite web|url=https://www.theregister.co.uk/2016/11/22/nasa_finds_ice_under_martian_surface/ |title=Lake of frozen water the size of New Mexico found on Mars – NASA |publisher=The Register |date=November 22, 2016 |access-date=November 23, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161123120850/http://www.theregister.co.uk/2016/11/22/nasa_finds_ice_under_martian_surface/ |archive-date=November 23, 2016 }}</ref><ref name="NASA-20161122jpl">{{cite web|url=http://www.jpl.nasa.gov/news/news.php?release=2016-299 |title=Mars Ice Deposit Holds as Much Water as Lake Superior |publisher=NASA |date=November 22, 2016 |access-date=November 23, 2016 |url-status=live |archive-url=https://web.archive.org/web/20161123145052/http://www.jpl.nasa.gov/news/news.php?release=2016-299 |archive-date=November 23, 2016 }}</ref>
<!---[[File:Warm Season Flows on Slope in Newton Crater (animated).gif|thumb|left|Warm-season flows on slope in [[Astronomy:Newton (Martian crater)|Newton Crater]] ]]--->
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 [[Physics:Water activity|water activity]], a<sub>w</sub> ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.<ref>{{cite journal |bibcode=2008Sci...320.1204T |title=Water Activity and the Challenge for Life on Early Mars |last1=Tosca |first1=Nicholas J. |last2=Knoll |first2=Andrew H. |last3=McLennan |first3=Scott M. |volume=320 |date=2008 |pages=1204–7 |journal=Science |doi=10.1126/science.1155432 |pmid=18511686 |issue=5880|s2cid=27253871 }}</ref> [[Biology:Haloarchaea|Haloarchaea]], however, are able to live in hypersaline solutions, up to the saturation point.<ref>{{cite journal|title=Extreme Halophiles Are Models for Astrobiology |journal=Microbe |date=2006 |first=Shiladitya |last=DasSarma |volume=1 |issue=3 |pages=120–6 |url=http://forms.asm.org/microbe/index.asp?bid=41227 |url-status=dead |archive-url=https://web.archive.org/web/20110722193334/http://forms.asm.org/microbe/index.asp?bid=41227 |archive-date=July 22, 2011 }}</ref>

In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.<ref name="underground">{{cite journal |bibcode=2000Sci...288.2330M |title=Evidence for Recent Groundwater Seepage and Surface Runoff on Mars |last1=Malin |first1=Michael C. |last2=Edgett |first2=Kenneth S. |volume=288 |date=2000 |pages=2330–5 |journal=Science |doi=10.1126/science.288.5475.2330 |pmid=10875910 |issue=5475}}</ref><ref>{{cite conference|url=http://www.planets.ucla.edu/wp-content/form-data/mars-abstracts-2013/37-Martinez_2013_UCLA_Mars_Habitability.pdf |title=Present Day Liquid Water On Mars: Theoretical Expectations, Observational Evidence And Preferred Locations |first1=G. M. |last1=Martínez |first2=N. O. |last2=Renno |first3=H. M. |last3=Elliott |first4=E. |last4=Fischer |date=2013 |conference=The Present-day Mars Habitability Conference |location=Los Angeles |url-status=live |archive-url=https://web.archive.org/web/20140225212529/http://www.planets.ucla.edu/wp-content/form-data/mars-abstracts-2013/37-Martinez_2013_UCLA_Mars_Habitability.pdf |archive-date=February 25, 2014 }}</ref> Additional similar images were published in 2006, taken by the [[Astronomy:Mars Global Surveyor|Mars 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.<ref>{{cite journal | doi=10.1016/j.icarus.2009.09.009 | last1=Kolb | first1=K. | last2=Pelletier | first2=Jon D. | last3=McEwen | first3=Alfred S. | date=2010 | title=Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water | journal=Icarus | volume=205 | issue=1 | pages=113–137 | bibcode = 2010Icar..205..113K }}</ref><ref name="moon">{{cite web |url=http://uanews.org/cgi-bin/WebObjects/UANews.woa/1/wa/SRStoryDetails?ArticleID=12376 |publisher=University of Arizona |title=Press Release |date=March 16, 2006 |url-status=usurped |archive-url=https://web.archive.org/web/20060721113049/http://uanews.org/cgi-bin/WebObjects/UANews.woa/1/wa/SRStoryDetails?ArticleID=12376 |archive-date=July 21, 2006 }}</ref><ref>{{cite journal | title=Mars Orbiter's Swan Song: The Red Planet Is A-Changin' |journal=Science |date=December 8, 2006 |first=Richard| last=Kerr| volume=314|issue=5805 |pages=1528–1529 |doi=10.1126/science.314.5805.1528| pmid=17158298|s2cid=46381976 |doi-access=free }}</ref> Others suggest it may be liquid [[Earth:Brine|brine]] near the surface,<ref name="voanews">{{cite web|url=http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html |title=NASA Finds Possible Signs of Flowing Water on Mars |date=August 3, 2011 |publisher=voanews.com |url-status=live |archive-url=https://web.archive.org/web/20110917071451/http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html |archive-date=September 17, 2011 }}</ref><ref name="Ames">{{cite web | author= Ames Research Center | url=http://www.spaceref.com/news/viewpr.html?pid=28377 | title=NASA Scientists Find Evidence for Liquid Water on a Frozen Early Mars | publisher=SpaceRef | date=June 6, 2009 }}</ref><ref>{{cite web|url=http://www.space.com/scienceastronomy/mars-phoenix-water-salt-data-100831.html |title=Dead Spacecraft on Mars Lives on in New Study |publisher=SPACE.com |date=June 10, 2008 |url-status=live |archive-url=https://web.archive.org/web/20101124070859/http://www.space.com/scienceastronomy/mars-phoenix-water-salt-data-100831.html |archive-date=November 24, 2010 }}</ref> but the exact source of the water and the mechanism behind its motion are not understood.<ref name="hirise">{{cite journal |bibcode=2011Sci...333..740M |title=Seasonal Flows on Warm Martian Slopes |last1=McEwen |first1=Alfred S. |last2=Ojha |first2=Lujendra |last3=Dundas |first3=Colin M. |last4=Mattson |first4=Sarah S. |last5=Byrne |first5=Shane |last6=Wray |first6=James J. |last7=Cull |first7=Selby C. |last8=Murchie |first8=Scott L. |last9=Thomas |first9=Nicolas |last10=Gulick |first10=V. C. |display-authors=8 |volume=333 |date=2011 |pages=740–3 |journal=Science |doi=10.1126/science.1204816 |pmid=21817049 |issue=6043 |s2cid=10460581 }}</ref>

In July 2018, scientists reported the discovery of a subglacial lake on Mars, {{convert|1.5|km|mi|abbr=on}} below the [[Astronomy:Planum Australe|southern polar ice cap]], and extending sideways about {{convert|20|km|mi|abbr=on}}, the first known stable body of water on the planet.<ref name="SCI-20180725">{{cite journal |author=Orosei, R. |display-authors=0 |title=Radar evidence of subglacial liquid water on Mars |date=July 25, 2018 |journal=Science |volume=361 |issue=6401 |pages=490–493 |doi=10.1126/science.aar7268 |pmid=30045881 |arxiv=2004.04587 |bibcode=2018Sci...361..490O |hdl=11573/1148029 |doi-access=free }}</ref><ref name="NYT-20180725">{{cite news |last1=Chang |first1=Kenneth |last2=Overbye |first2=Dennis |title=A Watery Lake Is Detected on Mars, Raising the Potential for Alien Life - The discovery suggests that watery conditions beneath the icy southern polar cap may have provided one of the critical building blocks for life on the red planet. |url=https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |date=July 25, 2018 |work={{wipe|The New York Times}} |access-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725205154/https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |archive-date=July 25, 2018 |url-status=live }}</ref><ref>{{cite web |title=Huge reservoir of liquid water detected under the surface of Mars |url=https://www.eurekalert.org/pub_releases/2018-07/aaft-hro072318.php |work=EurekAlert |date=July 25, 2018 |access-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725163215/https://www.eurekalert.org/pub_releases/2018-07/aaft-hro072318.php |archive-date=July 25, 2018 |url-status=live }}</ref><ref>{{Cite news |title=Liquid water 'lake' revealed on Mars |url=https://www.bbc.co.uk/news/science-environment-44952710 |work=BBC News |date=July 25, 2018 |access-date=July 25, 2018 |last1=Halton |first1=Mary |archive-url=https://web.archive.org/web/20180725141308/https://www.bbc.co.uk/news/science-environment-44952710 |archive-date=July 25, 2018 |url-status=live }}</ref> The lake was discovered using the [[Astronomy:MARSIS|MARSIS]] radar on board the ''[[Astronomy:Mars Express|Mars Express]]'' orbiter, and the profiles were collected between May 2012 and December 2015.<ref name="Suppl material">[https://www.science.org/doi/10.1126/science.aar7268 Supplementary Materials] for: {{cite journal | doi = 10.1126/science.aar7268 | pmid=30045881 | volume=361 | title=Radar evidence of subglacial liquid water on Mars | year=2018 | journal=Science | pages=490–493 | last1 = Orosei | first1 = R | last2 = Lauro | first2 = SE | last3 = Pettinelli | first3 = E | last4 = Cicchetti | first4 = A | last5 = Coradini | first5 = M | last6 = Cosciotti | first6 = B | last7 = Di Paolo | first7 = F | last8 = Flamini | first8 = E | last9 = Mattei | first9 = E | last10 = Pajola | first10 = M | last11 = Soldovieri | first11 = F | last12 = Cartacci | first12 = M | last13 = Cassenti | first13 = F | last14 = Frigeri | first14 = A | last15 = Giuppi | first15 = S | last16 = Martufi | first16 = R | last17 = Masdea | first17 = A | last18 = Mitri | first18 = G | last19 = Nenna | first19 = C | last20 = Noschese | first20 = R | last21 = Restano | first21 = M | last22 = Seu | first22 = R | issue=6401 | arxiv=2004.04587 | bibcode = 2018Sci...361..490O| doi-access = free }}</ref> 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.<ref name="SCI-20180725"/>

===Silica===
[[File:Spirit Mars Silica April 20 2007.jpg|thumb|right|The silica-rich patch discovered by ''Spirit'' rover ]]

In May 2007, the [[Engineering:Spirit (rover)|''Spirit'' rover]] disturbed a patch of ground with its inoperative wheel, uncovering an area 90% rich in [[Chemistry:Silica|silica]].<ref>{{cite press release|title=Mars Rover Spirit Unearths Surprise Evidence of Wetter Past |date=May 21, 2007 |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |url=http://www.nasa.gov/mission_pages/mer/mer-20070521.html |url-status=live |archive-url=https://web.archive.org/web/20070524070213/http://www.nasa.gov/mission_pages/mer/mer-20070521.html |archive-date=May 24, 2007 }}</ref> The feature is reminiscent of the effect of [[Earth:Hot spring|hot spring]] water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.<ref>{{Cite journal |title=Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile |url=https://www.nature.com/articles/ncomms13554 |journal=Nature Communications |volume=7}}</ref>

Based on Earth analogs, [[Earth:Hydrothermal vent|hydrothermal systems]] on Mars would be highly attractive for their potential for preserving [[Chemistry:Organic compound|organic]] and [[Chemistry:Inorganic compound|inorganic]] [[Astronomy:Biosignature|biosignature]]s.<ref name="Leveille">{{cite journal |bibcode=2010AGUFM.P12A..07L |title=Mineralized iron oxidizing bacteria from hydrothermal vents: Targeting biosignatures on Mars |last=Leveille |first=R. J. |volume=12 |pages=P12A–07 |date=2010 |journal=AGU Fall Meeting Abstracts}}</ref><ref>{{cite journal |bibcode=1993Icar..101..129W |title=Preservation of Biological Information in Thermal Spring Deposits: Developing a Strategy for the Search for Fossil Life on Mars |last1=Walter |first1=M. R. |last2=Des Marais |first2=David J. |volume=101 |date=1993 |pages=129–43 |journal=Icarus |doi=10.1006/icar.1993.1011 |pmid=11536937 |issue=1}}</ref><ref>{{cite journal |bibcode=2000Icar..147...49A |title=Microscopic Physical Biomarkers in Carbonate Hot Springs: Implications in the Search for Life on Mars |last1=Allen |first1=Carlton C. |last2=Albert |first2=Fred G. |last3=Chafetz |first3=Henry S. |last4=Combie |first4=Joan |last5=Graham |first5=Catherine R. |last6=Kieft |first6=Thomas L. |last7=Kivett |first7=Steven J. |last8=McKay |first8=David S. |last9=Steele |first9=Andrew |last10=Taunton |first10=A. E. |last11=Taylor |first11=M. R. |last12=Thomas-Keprta |first12=K. L. |last13=Westall |first13=F |display-authors=8 |volume=147 |date=2000 |pages=49–67 |journal=Icarus |doi=10.1006/icar.2000.6435 |pmid=11543582 |issue=1 }}</ref> For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.<ref>{{cite journal |bibcode=1999JGR...104.8489W |title=A Mössbauer investigation of iron-rich terrestrial hydrothermal vent systems: Lessons for Mars exploration |last1=Wade |first1=Manson L. |last2=Agresti |first2=David G. |last3=Wdowiak |first3=Thomas J. |last4=Armendarez |first4=Lawrence P. |last5=Farmer |first5=Jack D. |volume=104 |date=1999 |pages=8489–507 |journal=Journal of Geophysical Research |doi=10.1029/1998JE900049 |pmid=11542933 |issue=E4|doi-access=free }}</ref><ref>{{cite journal |bibcode=1995LPI....26....7A |title=A Mossbauer Investigation of Hot Springs Iron Deposits |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |last5=Farmer |first5=J. D. |volume=26 |date=1995 |page=7 |journal=Abstracts of the Lunar and Planetary Science Conference}}</ref><ref>{{cite journal |bibcode=1997LPICo.916....1A |title=Mössbauer Spectroscopy of Thermal Springs Iron Deposits as Martian Analogs |last1=Agresti |first1=D. G. |last2=Wdowiak |first2=T. J. |last3=Wade |first3=M. L. |last4=Armendarez |first4=L. P. |volume=916 |date=1997 |page=1 |journal=Early Mars: Geologic and Hydrologic Evolution}}</ref>

==Possible biosignatures==
In May 2017, evidence of the [[Biology:Earliest known life forms|earliest known life]] on land on Earth may have been found in 3.48-billion-year-old [[Earth:Geyserite|geyserite]] and other related mineral deposits (often found around [[Earth:Hot spring|hot spring]]s and [[Earth:Geyser|geyser]]s) uncovered in the [[Earth:Pilbara Craton|Pilbara Craton]] of Western Australia.<ref name="PO-20170509">{{cite news|author=Staff |title=Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks |url=https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html |date=May 9, 2017 |work=Phys.org |access-date=May 13, 2017 |url-status=live |archive-url=https://web.archive.org/web/20170510013721/https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html |archive-date=May 10, 2017 }}</ref><ref name="NC-20170509">{{cite journal|last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits |date=May 9, 2017 |journal=Nature Communications |doi=10.1038/ncomms15263 |pmid=28486437 |pmc=5436104 |volume=8 |page=15263 |bibcode = 2017NatCo...815263D }}</ref> These findings may be helpful in deciding where best to search for [[Biology:Abiogenesis|early signs of life]] on the planet Mars.<ref name="PO-20170509" /><ref name="NC-20170509" />

===Methane===
{{Main|Astronomy:Methane on Mars}}

[[Chemistry:Methane|Methane]] (CH<sub>4</sub>) 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 several parts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASA [[Organization:Goddard Space Flight Center|Goddard Space Flight Center]] in 2003.<ref name="methane33">{{cite journal|author1=Mumma, M. J.|author2=Novak, R. E.|author3=DiSanti, M. A.|author4=Bonev, B. P.|year=2003|title=A Sensitive Search for Methane on Mars|journal=Bulletin of the American Astronomical Society|volume=35|pages=937|bibcode=2003DPS....35.1418M}}</ref><ref>{{cite web|url=http://www.skyandtelescope.com/astronomy-news/mars-methane-boosts-chances-for-life/|title=Mars Methane Boosts Chances for Life|author=Naeye, Robert|date=September 28, 2004|work=Sky & Telescope|access-date=December 20, 2014}}</ref> 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.<ref>{{cite journal | doi = 10.1126/science.359.6371.16 | volume=359 | title=Mars methane rises and falls with the seasons | year=2018 | journal=Science | pages=16–17 | last1 = Hand | first1 = Eric| issue=6371 | pmid=29301992 | bibcode=2018Sci...359...16H }}</ref> On June 7, 2018, NASA announced it has detected a seasonal variation of methane levels on Mars.<ref name="NASA-20180607"/><ref name="NASA-20180607vid">{{cite web |author=NASA |title=Ancient Organics Discovered on Mars - video (03:17) |url=https://www.youtube.com/watch?v=a0gsz8EHiNc |date=June 7, 2018 |work=[[Organization:NASA|NASA]] |access-date=June 7, 2018 |archive-url=https://web.archive.org/web/20180607220111/https://www.youtube.com/watch?v=a0gsz8EHiNc |archive-date=June 7, 2018 |url-status=live }}</ref><ref name="SPC-20180607"/><ref name="NYT-20180607" /><ref name=Voosen_Science>{{cite journal |title=NASA Curiosity rover hits organic pay dirt on Mars |last=Voosen |first=Paul |journal=Science |year=2018 |volume=260 |issue=6393 |pages=1054–55 |doi=10.1126/science.360.6393.1054 |pmid=29880665 |bibcode=2018Sci...360.1054V |s2cid=47015070 }}</ref><ref name="SCI-20180608a">{{cite journal |last=ten Kate |first=Inge Loes |title=Organic molecules on Mars |date=June 8, 2018 |journal=Science |volume=360 |issue=6393 |pages=1068–1069 |doi=10.1126/science.aat2662 |pmid=29880670 |bibcode=2018Sci...360.1068T |s2cid=46952468 }}</ref><ref name="SCI-20180608b">{{cite journal |author=Webster, Christopher R. |display-authors=0 |title=Background levels of methane in Mars' atmosphere show strong seasonal variations |date=June 8, 2018 |journal=Science |volume=360 |issue=6393 |pages=1093–1096 |doi=10.1126/science.aaq0131 |pmid=29880682 |bibcode=2018Sci...360.1093W |doi-access=free }}</ref><ref name="SCI-20180608c" />

The [[Astronomy:ExoMars Trace Gas Orbiter|ExoMars Trace Gas Orbiter]] (TGO), launched in March 2016, began on April 21, 2018, to map the concentration and sources of methane in the atmosphere,<ref name="space20180223">{{cite news |url=https://www.space.com/39796-methane-sniffing-mars-orbiter-aerobraking-dives.html |title=Methane-Sniffing Orbiter Finishes 'Aerobraking' Dives Through Mars' Atmosphere |work=Space.com |first=Mike |last=Wall |date=February 23, 2018 |access-date=February 24, 2018 |archive-url=https://web.archive.org/web/20180612142110/https://www.space.com/39796-methane-sniffing-mars-orbiter-aerobraking-dives.html |archive-date=June 12, 2018 |url-status=live }}</ref><ref>{{cite conference |title=ExoMars Trace Gas Orbiter provides atmospheric data during Aerobraking into its final orbit |conference=49th Annual Division for Planetary Sciences Meeting. October 15–20, 2017. Provo, Utah. |first1=Hakan |last1=Svedhem |first2=Jorge L. |last2=Vago |first3=Sean |last3=Bruinsma |first4=Ingo |last4=Müller-Wodarg |display-authors=0 |date=2017 |bibcode=2017DPS....4941801S |id=418.01}}</ref> as well as its decomposition products such as [[Physics:Formaldehyde|formaldehyde]] and [[Chemistry:Methanol|methanol]]. As of May 2019, the Trace Gas Orbiter showed that the concentration of methane is under detectable level (< 0.05 ppbv).<ref name=":37">{{Cite journal| last1=Vago| first1=Jorge L.|last2=Svedhem |first2=Håkan|last3=Zelenyi| first3=Lev|last4=Etiope| first4=Giuseppe|last5=Wilson|first5=Colin F.| last6=López-Moreno| first6=Jose-Juan| last7=Bellucci| first7=Giancarlo| last8=Patel| first8=Manish R.|last9=Neefs| first9=Eddy|date=April 2019| title=No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations | journal=Nature| volume=568|issue=7753 |pages=517–520 |doi=10.1038/s41586-019-1096-4|issn=1476-4687| bibcode=2019Natur.568..517K| url=http://oro.open.ac.uk/60547/2/2019%20Korablev%20TGO%20methane%20Nature_accepted.pdf | pmid=30971829| s2cid=106411228}}</ref><ref>{{Cite web| url=http://www.esa.int/Our_Activities/Human_and_Robotic_Exploration/Exploration/ExoMars/First_results_from_the_ExoMars_Trace_Gas_Orbiter| title=First results from the ExoMars Trace Gas Orbiter |last=esa| website=European Space Agency| access-date=June 12, 2019}}</ref>

[[File:PIA22328-MarsCuriosityRover-Methane-SeasonalCycle-20180607.jpg|thumb|300x300px|<div style="text-align: center;">''Curiosity'' detected a cyclical seasonal variation in atmospheric methane.</div>|alt=]]

The principal candidates for the origin of Mars's methane include non-biological processes such as [[Chemistry:Properties of water|water]]-rock reactions, [[Physics:Radiolysis|radiolysis]] of water, and [[Chemistry:Pyrite|pyrite]] formation, all of which produce [[Chemistry:Hydrogen|H<sub>2</sub>]] that could then generate methane and other hydrocarbons via [[Chemistry:Fischer–Tropsch process|Fischer–Tropsch synthesis]] with [[Chemistry:Carbon monoxide|CO]] and CO<sub>2</sub>.<ref name="asc.2010">{{cite conference|last=Mumma|first=Michael|display-authors=0|year=2010|title=Astrobiology Science Conference 2010|location=Greenbelt, MD|publisher=Goddard Space Flight Center|access-date=July 24, 2010|contribution=The Astrobiology of Mars: Methane and Other Candinate Biomarker Gases, and Related Interdisciplinary Studies on Earth and Mars|work=Astrophysics Data System|contribution-url=http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5590.pdf}}</ref> It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral [[Chemistry:Olivine|olivine]], which is known to be common on Mars.<ref name="olivine">{{cite journal|last1=Oze|first1=C.|last2=Sharma|first2=M.|year=2005|title=Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars|url=http://www.agu.org/journals/gl/gl0510/2005GL022691/|journal=Geophys. Res. Lett.|volume=32|issue=10|pages=L10203|bibcode=2005GeoRL..3210203O|doi=10.1029/2005GL022691|s2cid=28981740|doi-access=free}}</ref> Although geologic sources of methane such as [[Earth:Serpentinization|serpentinization]] are possible, the lack of current volcanism, hydrothermal activity or [[Earth:Hotspot (geology)|hotspots]]<ref name="thermal signature">{{cite news|title=Hunting for young lava flows |date=June 1, 2011 |publisher=Red Planet |url=http://redplanet.asu.edu/?p=501 |work=Geophysical Research Letters |url-status=live |archive-url=https://web.archive.org/web/20131004234201/http://redplanet.asu.edu/?p=501 |archive-date=October 4, 2013 }}</ref> are not favorable for geologic methane.

Living [[Biology:Microorganism|microorganism]]s, such as [[Biology:Methanogen|methanogen]]s, are another possible source, but no evidence for the presence of such organisms has been found on Mars,<ref name="PNAS-20120607">{{cite journal|last1=Oze|first1=Christopher|last2=Jones|first2=Camille|last3=Goldsmith|first3=Jonas I.|last4=Rosenbauer|first4=Robert J.|date=June 7, 2012|title=Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces|journal=PNAS|volume=109|issue=25|pages=9750–9754|bibcode=2012PNAS..109.9750O|doi=10.1073/pnas.1205223109|pmc=3382529|pmid=22679287|doi-access=free}}</ref><ref name="Space-20120625"/><ref>{{Cite journal|last1=Krasnopolsky|first1=Vladimir A.|last2=Maillard|first2=Jean Pierre|last3=Owen|first3=Tobias C.|date=December 2004|title=Detection of methane in the martian atmosphere: evidence for life?|journal=Icarus|volume=172|issue=2|pages=537–547|doi=10.1016/j.icarus.2004.07.004|bibcode=2004Icar..172..537K}}</ref> until June 2019 as methane was detected by the ''Curiosity'' rover.<ref>{{cite news|url=https://www.nytimes.com/2019/06/22/science/nasa-mars-rover-life.html|title=NASA Rover on Mars Detects Puff of Gas That Hints at Possibility of Life|work=The New York Times|date=June 22, 2019}}</ref> Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source and carbon dioxide (CO<sub>2</sub>) as their carbon source, so they could exist in subsurface environments on Mars.<ref name="Mickol">{{cite news|url=https://www.sciencedaily.com/releases/2015/06/150602125843.htm |title=Earth organisms survive under low-pressure Martian conditions |work=University of Arkansas |date=June 2, 2015 |access-date=June 4, 2015 |url-status=live |archive-url=https://web.archive.org/web/20150604065906/https://www.sciencedaily.com/releases/2015/06/150602125843.htm |archive-date=June 4, 2015 }}</ref> 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.<ref name="Steigerwald">{{cite news | first=Bill | last=Steigerwald | title=Martian Methane Reveals the Red Planet is not a Dead Planet | date=January 15, 2009 | publisher=NASA | url=http://www.nasa.gov/mission_pages/mars/news/marsmethane.html | work=NASA's Goddard Space Flight Center | url-status=live | archive-date=January 16, 2009 | archive-url=https://web.archive.org/web/20090116151746/http://www.nasa.gov/mission_pages/mars/news/marsmethane.html |quote=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 }}</ref>

Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and ''in vitro'' experiments testing the growth of [[Biology:Methanogen|methanogen]]ic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0wt% [[Chemistry:Perchlorate|perchlorate]] salt.<ref name="Kral">{{cite journal |bibcode=2009M&PSA..72.5136K |title=Can Methanogens Grow in a Perchlorate Environment on Mars? |last1=Kral |first1=T. A. |last2=Goodhart |first2=T. |last3=Howe |first3=K. L. |last4=Gavin |first4=P. |volume=72 |date=2009 |page=5136 |journal=72nd Annual Meeting of the Meteoritical Society}}</ref>

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.<ref name="Howe">{{cite journal |bibcode=2009LPI....40.1287H |title=Methane Production by Methanogens in Perchlorate-supplemented Media |last1=Howe |first1=K. L. |last2=Gavin |first2=P. |last3=Goodhart |first3=T. |last4=Kral |first4=T. A. |volume=40 |date=2009 |page=1287 |journal=40th Lunar and Planetary Science Conference}}</ref><ref name="Levin 2009">{{cite book |bibcode=2009SPIE.7441E..0DL |chapter=Methane and life on Mars |last1=Levin |first1=Gilbert V. |last2=Straat |first2=Patricia Ann |title=Instruments and Methods for Astrobiology and Planetary Missions XII |volume=7441 |date=2009 |pages=12–27 |doi=10.1117/12.829183 |editor1-last=Hoover |editor1-first=Richard B |editor2-last=Levin |editor2-first=Gilbert V |editor3-last=Rozanov |editor3-first=Alexei Y |editor4-last=Retherford |editor4-first=Kurt D |isbn=978-0-8194-7731-6|s2cid=73595154 }}</ref>

[[File:Martian Methane Map.jpg|thumb|right|<div style="text-align: center;">Distribution of methane in the atmosphere of Mars in the Northern Hemisphere during summer</div>]]

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 were ''[[Biology:Methanothermobacter|Methanothermobacter]] wolfeii'', ''[[Biology:Methanosarcina|Methanosarcina]] barkeri'', ''[[Biology:Methanobacterium|Methanobacterium]] formicicum'', and ''[[Biology:Methanococcus maripaludis|Methanococcus maripaludis]]''.<ref name="Mickol" /> In June 2012, scientists reported that measuring the ratio of [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Methane|methane]] levels on Mars may help determine the likelihood of life on Mars.<ref name="PNAS-20120607"/><ref name="Space-20120625">{{cite web|author=Staff |title=Mars Life Could Leave Traces in Red Planet's Air: Study |url=http://www.space.com/16284-mars-life-atmosphere-hydrogen-methane.html |date=June 25, 2012 |publisher=Space.com |url-status=live |archive-url=https://web.archive.org/web/20120630004450/http://www.space.com/16284-mars-life-atmosphere-hydrogen-methane.html |archive-date=June 30, 2012 }}</ref> According to the scientists, "low H<sub>2</sub>/CH<sub>4</sub> ratios (less than approximately 40)" would "indicate that life is likely present and active".<ref name="PNAS-20120607" /> The observed ratios in the lower Martian atmosphere were "approximately 10 times" higher "suggesting that biological processes may not be responsible for the observed CH<sub>4</sub>".<ref name="PNAS-20120607" /> The scientists suggested measuring the H<sub>2</sub> and CH<sub>4</sub> flux at the Martian surface for a more accurate assessment. Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.<ref name="Nature-20120627">{{cite journal |last1=Brogi |first1=Matteo |last2=Snellen |first2=Ignas A. G. |last3=de Krok |first3=Remco J. |last4=Albrecht |first4=Simon |last5=Birkby |first5=Jayne |last6=de Mooij |first6=Ernest J. W. |title=The signature of orbital motion from the dayside of the planet τ Boötis b |date=June 28, 2012 |journal=Nature |volume=486 |pages=502–504 |doi=10.1038/nature11161 |arxiv=1206.6109 |bibcode=2012Natur.486..502B |issue=7404 |pmid=22739313|s2cid=4368217 }}</ref><ref name="Wired-20120627">{{Cite magazine|last=Mann |first=Adam |title=New View of Exoplanets Will Aid Search for E.T. |magazine=Wired |url=https://www.wired.com/wiredscience/2012/06/tau-bootis-b/ |date=June 27, 2012 |url-status=live |archive-url=https://web.archive.org/web/20120829202703/http://www.wired.com/wiredscience/2012/06/tau-bootis-b/ |archive-date=August 29, 2012 }}</ref>

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.<ref>{{cite news | first=Bill | last=Steigerwald | title=Martian Methane Reveals the Red Planet is not a Dead Planet | date=January 15, 2009 | publisher=NASA | url=http://www.nasa.gov/mission_pages/mars/news/marsmethane.html | work=NASA's Goddard Space Flight Center | archive-url=https://web.archive.org/web/20090117141425/http://www.nasa.gov/mission_pages/mars/news/marsmethane.html | archive-date=January 17, 2009 | url-status=live}}</ref>

===Formaldehyde===
In February 2005, it was announced that the [[Physics:Planetary Fourier Spectrometer|Planetary Fourier Spectrometer]] (PFS) on the [[Organization:European Space Agency|European Space Agency]]'s Mars Express Orbiter had detected traces of [[Physics:Formaldehyde|formaldehyde]] in the [[Astronomy:Atmosphere of Mars|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 harboring colonies of microbial life.<ref>{{cite journal |doi=10.1038/news050221-15 |first=Mark |last=Peplow |title=Formaldehyde claim inflames martian debate |journal=Nature |date=February 25, 2005 |s2cid=128986558 }}</ref><ref>{{cite news|first=Jenny |last=Hogan |title=A whiff of life on the Red Planet |date=February 16, 2005 |url=https://www.newscientist.com/article.ns?id=dn7014 |work=New Scientist |url-status=live |archive-url=https://web.archive.org/web/20080422072900/http://www.newscientist.com/article.ns?id=dn7014 |archive-date=April 22, 2008 }}</ref> NASA scientists consider the preliminary findings well worth a follow-up but have also rejected the claims of life.<ref name="PFS">{{cite journal |doi=10.1038/news050905-10 |title=Martian methane probe in trouble |date=September 7, 2005 |last=Peplow |first=Mark |journal=Nature}}</ref><ref name="NASA Releasease : 05-052">{{cite news |title=NASA Statement on False Claim of Evidence of Life on Mars |date=February 18, 2005 |publisher=[[Organization:NASA|NASA]] |url=http://www1.nasa.gov/home/hqnews/2005/feb/HQ_05052_mars_claim.html |work=NASA News |url-status=dead |archive-url=https://web.archive.org/web/20080922024230/http://www.nasa.gov/home/hqnews/2005/feb/HQ_05052_mars_claim.html |archive-date=September 22, 2008 }}</ref>

===Viking lander biological experiments===

The 1970s [[Engineering:Viking program|Viking program]] placed two identical landers on the surface of Mars tasked to look for biosignatures of microbial life on the surface. Of the four experiments performed by each Viking lander, only the 'Labeled Release' (LR) experiment gave a positive result for [[Biology:Metabolism|metabolism]], while the other three did not detect [[Chemistry:Organic compound|organic compound]]s. 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.<ref name="chambers" /> No Mars lander mission has found meaningful traces of biomolecules or biosignatures. The claim of extant microbial life on Mars is based on old data collected by the Viking landers, currently reinterpreted as sufficient evidence of life, mainly by [[Biography:Gilbert Levin|Gilbert Levin]],<ref name="Levin" /><ref>{{Cite web|url=https://blogs.scientificamerican.com/observations/im-convinced-we-found-evidence-of-life-on-mars-in-the-1970s/|title=I'm Convinced We Found Evidence of Life on Mars in the 1970s|last=Levin|first=Gilbert V.|date=October 10, 2019|website=Scientific American Blog Network|language=en|access-date=January 14, 2020}}</ref> Joseph D. Miller,<ref>{{cite press release|title=Mars Viking Robots 'Found Life' |date=April 12, 2012 |last=Klotz |first=Irene |publisher=Discovery Communications, LLC |url=http://news.discovery.com/space/alien-life-exoplanets/mars-life-viking-landers-discovery-120412.htm |url-status=live |archive-url=https://web.archive.org/web/20130126193200/http://news.discovery.com/space/alien-life-exoplanets/mars-life-viking-landers-discovery-120412.htm |archive-date=January 26, 2013 }}</ref> Navarro,<ref name="Contreras2008_1">{{cite book | first1=Mario | last1=Crocco | first2=N- C. | last2=Contreras | title=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" | date=2008 | publisher=Ediciones Análisis, Buenos Aires–Rosario–Bahía Blanca | isbn=978-987-29362-0-4 | page=70}}</ref> Giorgio Bianciardi and Patricia Ann Straat.

Assessments published in December 2010 by Rafael Navarro-Gonzáles<ref name="reanalysis">{{cite journal|url=http://www.agu.org/pubs/crossref/2010/2010JE003599.shtml |title=Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars |journal=Journal of Geophysical Research: Planets |volume=115 |issue=E12010 |pages=E12010 |date=December 15, 2010 |first1=Rafael |last1=Navarro-Gonzáles |first2=Edgar |last2=Vargas |first3=José |last3=de la Rosa |first4=Alejandro C. |last4=Raga |first5=Christopher P. |last5=McKay |doi=10.1029/2010JE003599 |access-date=January 7, 2011 |bibcode=2010JGRE..11512010N |url-status=live |archive-url=https://web.archive.org/web/20110109102058/http://www.agu.org/pubs/crossref/2010/2010JE003599.shtml |archive-date=January 9, 2011 |doi-access=free }}</ref><ref>{{cite journal |title=Correction to "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars" |doi=10.1029/2011JE003854 | bibcode=2011JGRE..116.8011N | volume=116 |issue=E8 |pages=E08011 |journal=Journal of Geophysical Research|year=2011 |last1=Navarro-González |first1=Rafael |last2=Vargas |first2=Edgar |last3=de la Rosa |first3=José |last4=Raga |first4=Alejandro C. |last5=McKay |first5=Christopher P. |doi-access=free }}</ref><ref>{{cite news |title=Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars |year=2010 |bibcode=2010JGRE..11512010N |doi = 10.1029/2010JE003599 |last1=Navarro-González |first1=Rafael |last2=Vargas |first2=Edgar |last3=de la Rosa |first3=José |last4=Raga |first4=Alejandro C. |last5=McKay |first5=Christopher P. |journal=Journal of Geophysical Research |volume=115 }}</ref><ref name="Navarro">{{cite journal |bibcode=2006PNAS..10316089N |title=The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results |last1=Navarro-González |first1=Rafael |last2=Navarro |first2= Karina F. |last3=de la Rosa |first3=José |last4=Iñiguez |first4=Enrique |last5=Molina |first5=Paola |last6=Miranda |first6=Luis D. |last7=Morales |first7=Pedro |last8=Cienfuegos |first8=Edith |last9=Coll |first9=Patrice |last10=Raulin |first10=F |last11=Amils |first11=R |last12=McKay |first12=C. P. |display-authors=9 |volume=103 |date=2006 |pages=16089–94 |journal=Proceedings of the National Academy of Sciences |doi=10.1073/pnas.0604210103 |issue=44 |jstor=30052117 |pmid=17060639 |pmc=1621051 |doi-access=free }}</ref> indicate that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2. The study determined that [[Chemistry:Perchlorate|perchlorate]]—discovered in 2008 by Phoenix lander<ref>{{cite news|url=https://www.latimes.com/news/printedition/asection/la-sci-phoenix6-2008aug06,0,4986721.story |title=Perchlorate found in Martian soil |date=August 6, 2008 |work=Los Angeles Times |last=Johnson |first=John |url-status=live |archive-url=https://web.archive.org/web/20090318042116/http://www.latimes.com/news/printedition/asection/la-sci-phoenix6-2008aug06%2C0%2C4986721.story |archive-date=March 18, 2009 }}</ref><ref name="SD_2008-08"/>—can destroy organic compounds when heated, and produce [[Chemistry:Chloromethane|chloromethane]] and [[Chemistry:Dichloromethane|dichloromethane]] 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.<ref>{{cite news|title=Did Viking Mars Landers Find Life's Building Blocks? Missing Piece Inspires New Look at Puzzle |date=September 5, 2010 |url=https://www.sciencedaily.com/releases/2010/09/100904081050.htm |work=ScienceDaily |access-date=September 23, 2010 |url-status=live |archive-url=https://web.archive.org/web/20100908214707/https://www.sciencedaily.com/releases/2010/09/100904081050.htm |archive-date=September 8, 2010 }}</ref><ref>{{cite journal |title=Comment on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars |first=Rafael |last=Navarro-González |display-authors=0 |doi=10.1029/2011JE003869 |bibcode=2011JGRE..11612001B |volume=116 |issue=E12 |pages=E12001 |journal=Journal of Geophysical Research|year=2011 |doi-access=free }}</ref>

The Labeled Release evidence was not generally accepted initially, and, to this day lacks the consensus of the scientific community.<ref name="Dead or Alive 2014">{{cite conference|last1=Levin |first1=Gilbert V. |last2=Straat |first2=Patricia Ann |title=MARS: Dead or Alive? |url=http://www.gillevin.com/Mars/Levin-Straat_Mars_Society_Paper_8-8-14.pdf |conference=Mars Society Convention |url-status=live |archive-url=https://web.archive.org/web/20140819090841/http://www.gillevin.com/Mars/Levin-Straat_Mars_Society_Paper_8-8-14.pdf |archive-date=August 19, 2014 }}</ref>

===Meteorites===
As of 2018, there are 224 known [[Astronomy:Martian meteorite|Martian meteorite]]s (some of which were found in several fragments).<ref>[http://www.imca.cc/mars/martian-meteorites-list.htm An up-to-date List of Martian Meteorites] {{Webarchive|url=https://web.archive.org/web/20180724154648/http://www.imca.cc/mars/martian-meteorites-list.htm |date=July 24, 2018 }}. Dr. Tony Irving of the University of Washington. International Meteorite Collectors Association (IMCA Inc).</ref> These are valuable because they are the only physical samples of Mars available to Earth-bound laboratories. Some researchers have argued that microscopic morphological features found in ALH84001 are biomorphs, however this interpretation has been highly controversial and is not supported by the majority of researchers in the field.<ref name="meteoritos-Bio">{{cite conference |url=http://mars.jpl.nasa.gov/mgs/sci/fifthconf99/6142.pdf |title=Evidence for ancient Martian life |type=Abstract |book-title=The Fifth International Conference on Mars, July 19–24, 1999, Pasadena, California, a Lunar and Planetary Science Conference |page=6142 |first1=E. K. Jr. |last1=Gibson |first2=F. |last2=Westall |first3=D. S. |last3=McKay |first4=K. |last4=Thomas-Keprta |first5=S. |last5=Wentworth |first6=C. S. |last6=Romanek |publisher=NASA |url-status=live |archive-url=https://web.archive.org/web/20150319180405/http://mars.jpl.nasa.gov/mgs/sci/fifthconf99/6142.pdf |archive-date=March 19, 2015 |bibcode=1999ficm.conf.6142G |year=1999 }}</ref>

Seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:<ref name="meteoritos-Bio" />
# 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.<ref name="meteoritos-Bio" />

====ALH84001====
[[File:ALH84001 structures.jpg|right|thumb|An electron microscope reveals bacteria-like structures in meteorite fragment ALH84001]]

In 1996, the Martian meteorite ALH84001, a specimen that is much older than the majority of Martian meteorites that have been recovered so far, received considerable attention when a group of NASA scientists led by [[Biography:David S. McKay|David S. McKay]] reported microscopic features and geochemical anomalies that they considered to be best explained by the rock having hosted Martian bacteria in the distant past. Some of these features resembled terrestrial bacteria, aside from their being much smaller than any known form of life. Much controversy arose over this claim, and ultimately all of the evidence McKay's team cited as evidence of life 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.<ref name="disbelief">{{cite news | title=After 10 years, few believe life on Mars | url=http://www.space.com/scienceastronomy/ap_060806_mars_rock.html | last=Crenson | first=Matt | agency=Associated Press | work=Space.com | date=August 6, 2006 | url-status=dead | archive-date=August 9, 2006 | archive-url=https://web.archive.org/web/20060809161936/http://www.space.com/scienceastronomy/ap_060806_mars_rock.html }}</ref><ref>{{cite journal |bibcode=1996Sci...273..924M |title=Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001 |last1=McKay |first1=David S. |last2=Gibson |first2=Everett K. |last3=Thomas-Keprta |first3=Kathie L. |last4=Vali |first4=Hojatollah |last5=Romanek |first5=Christopher S. |last6=Clemett |first6=Simon J. |last7=Chillier |first7=Xavier D. F. |last8=Maechling |first8=Claude R. |last9=Zare |first9=Richard N. |volume=273 |date=1996 |pages=924–30 |journal=Science |doi=10.1126/science.273.5277.924 |pmid=8688069 |issue=5277 |s2cid=40690489 }}</ref>
 
[[File:Nakhla meteorite.jpg|thumb|left|Nakhla meteorite]]

====Nakhla====
The Nakhla meteorite fell on Earth on June 28, 1911, on the locality of Nakhla, [[Place:Alexandria|Alexandria]], Egypt.<ref name="NASA">{{cite web|url=http://www2.jpl.nasa.gov/snc/nakhla.html |title=The Nakhla Meteorite |access-date=August 17, 2008 |last=Baalke |first=Ron |date=1995 |work=Jet Propulsion Lab |publisher=NASA |url-status=live |archive-url=https://web.archive.org/web/20080914182645/http://www2.jpl.nasa.gov/snc/nakhla.html |archive-date=September 14, 2008 }}</ref><ref>{{cite web|url=http://www.nhm.ac.uk/nature-online/virtual-wonders/vrmeteorite5.html |title=Rotating image of a Nakhla meteorite fragment |date=2008 |location=London |publisher=Natural History Museum |url-status=live |archive-url=https://web.archive.org/web/20060716033716/http://www.nhm.ac.uk/nature-online/virtual-wonders/vrmeteorite5.html |archive-date=July 16, 2006 }}</ref>

In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects<ref>{{cite news|first=Paul |last=Rincon |title=Space rock re-opens Mars debate |date=February 8, 2006 |url=http://news.bbc.co.uk/2/hi/science/nature/4688938.stm |work=BBC News |url-status=live |archive-url=https://web.archive.org/web/20060222021257/http://news.bbc.co.uk/2/hi/science/nature/4688938.stm |archive-date=February 22, 2006 }}</ref> of the size and shape consistent with Earthly [[Biology:Fossil|fossil]]ized nanobacteria.
Analysis with [[Gas chromatography|gas chromatography]] and [[Physics:Mass spectrometry|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 compounds in Nakhla "may not be recent terrestrial contamination".<ref name="meteoritos-Bio" /><ref>{{cite web|url=http://www-curator.jsc.nasa.gov/antmet/mmc/Nakhla.pdf |title=Mars Meteorite Compendium |last=Meyer |first=C. |date=2004 |publisher=NASA |url-status=live |archive-url=https://web.archive.org/web/20080923212609/http://www-curator.jsc.nasa.gov/antmet/mmc/Nakhla.pdf |archive-date=September 23, 2008 }}</ref>

This caused additional interest in this meteorite, so in 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic [[Chemistry:Carbon|carbon]] content was observed. When the results and evidence were published in 2006, some independent researchers claimed that the carbon deposits are of biologic origin. It was remarked that since carbon is the fourth most abundant element in the [[Astronomy:Universe|Universe]], finding it in curious patterns is not indicative or suggestive of biological origin.<ref>{{cite news|first=David |last=Whitehouse |title=Life on Mars – new claims |date=August 27, 1999 |url=http://news.bbc.co.uk/2/hi/science/nature/289214.stm |work=BBC News |url-status=live |archive-url=https://web.archive.org/web/20080502113931/http://news.bbc.co.uk/2/hi/science/nature/289214.stm |archive-date=May 2, 2008 }}</ref><ref>Compilation of scientific research references on the Nakhla meteorite: {{cite web |url=http://curator.jsc.nasa.gov/antmet/marsmets/nakhla/references.cfm |title=Nakhla References |access-date=August 21, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080904223727/http://curator.jsc.nasa.gov/antmet/marsmets/nakhla/references.cfm |archive-date=September 4, 2008 }}</ref>

====Shergotty====
The [[Unsolved:Shergotty meteorite|Shergotty meteorite]], a {{convert|4|kg}} Martian meteorite, fell on Earth on Shergotty, India on August 25, 1865, and was retrieved by witnesses almost immediately.<ref name="nasa_shergotty">{{cite web|url=http://www2.jpl.nasa.gov/snc/shergotty.html |title=Shergoti Meteorite |publisher=JPL, NASA |url-status=live |archive-url=https://web.archive.org/web/20110118011546/http://www2.jpl.nasa.gov/snc/shergotty.html |archive-date=January 18, 2011 }}</ref> It is composed mostly of [[Chemistry:Pyroxene|pyroxene]] and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest remnants of a biofilm and its associated microbial communities.<ref name="meteoritos-Bio" />

====Yamato 000593====
[[Unsolved:Yamato 000593|Yamato 000593]] is the second largest [[Astronomy:Meteorite|meteorite]] from [[Astronomy:Mars|Mars]] found on Earth. Studies suggest the [[Astronomy:Martian meteorite|Martian meteorite]] was formed about 1.3&nbsp;billion years ago from a lava flow on [[Astronomy:Mars|Mars]]. An [[Physics:Impact (mechanics)|impact]] occurred on Mars about 12&nbsp;million years ago and ejected the meteorite from the Martian surface into [[Astronomy:Outer space|space]]. The meteorite landed on Earth in [[Earth:Antarctica|Antarctica]] about 50,000 years ago. The [[Physics:Mass|mass]] of the meteorite is {{convert|13.7|kg|lbs|abbr=on}} and it has been found to contain evidence of past [[Astronomy:Water on Mars|water]] movement.<ref name="NASA-20140227" /><ref name="AJ-20140219" /><ref name="SP-20140228" /> At a microscopic level, [[Astronomy:Martian spherules|spheres]] are found in the meteorite that are rich in [[Chemistry:Carbon|carbon]] compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by [[Chemistry:Biotic material|biotic activity]] according to NASA scientists.<ref name="NASA-20140227">{{cite web|last=Webster |first=Guy |title=NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-065&1 |date=February 27, 2014 |work=[[Organization:NASA|NASA]] |url-status=live |archive-url=https://web.archive.org/web/20140301170153/http://www.jpl.nasa.gov/news/news.php?release=2014-065&1 |archive-date=March 1, 2014 }}</ref><ref name="AJ-20140219">{{cite journal |last1=White |first1=Lauren M. |last2=Gibson |first2=Everett K. |last3=Thomnas-Keprta |first3=Kathie L. |last4=Clemett |first4=Simon J. |last5=McKay |first5=David |title=Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593 |date=February 19, 2014 |journal=Astrobiology |volume=14 |number=2 |pages=170–181 |doi=10.1089/ast.2011.0733 |bibcode=2014AsBio..14..170W |pmid=24552234 |pmc=3929347}}</ref><ref name="SP-20140228">{{cite web|last=Gannon |first=Megan |title=Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life |url=http://www.space.com/24834-strange-mars-meteorite-life-evidence-debate.html |date=February 28, 2014 |work=Space.com |url-status=live |archive-url=https://web.archive.org/web/20140301170314/http://www.space.com/24834-strange-mars-meteorite-life-evidence-debate.html |archive-date=March 1, 2014 }}</ref>

===Ichnofossil-like structures===
Organism–substrate interactions and their products are important biosignatures on Earth as they represent direct evidence of biological behaviour.<ref>{{Cite book|last=Seilacher, Adolf.|title=Trace fossil analysis|date=2007|publisher=Springer|isbn=978-3-540-47226-1|location=Berlin|oclc=191467085}}</ref> 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.<ref>{{Cite journal|last1=Mcloughlin|first1=N.|last2=Staudigel|first2=H.|last3=Furnes|first3=H.|last4=Eickmann|first4=B.|last5=Ivarsson|first5=M.|date=2010|title=Mechanisms of microtunneling in rock substrates: distinguishing endolithic biosignatures from abiotic microtunnels|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1472-4669.2010.00243.x|journal=Geobiology|language=en|volume=8|issue=4|pages=245–255|doi=10.1111/j.1472-4669.2010.00243.x|pmid=20491948|bibcode=2010Gbio....8..245M |s2cid=46368300|issn=1472-4669}}</ref><ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Van Kranendonk|first4=Martin J.|last5=Chivas|first5=Allan R.|date=September 2016|title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures|url=https://www.nature.com/articles/nature19355|journal=Nature|language=en|volume=537|issue=7621|pages=535–538|doi=10.1038/nature19355|pmid=27580034|bibcode=2016Natur.537..535N|s2cid=205250494|issn=1476-4687}}</ref><ref>{{Cite journal|last1=Ohmoto|first1=Hiroshi|last2=Runnegar|first2=Bruce|last3=Kump|first3=Lee R.|last4=Fogel|first4=Marilyn L.|last5=Kamber|first5=Balz|last6=Anbar|first6=Ariel D.|last7=Knauth|first7=Paul L.|last8=Lowe|first8=Donald R.|last9=Sumner|first9=Dawn Y.|last10=Watanabe|first10=Yumiko|date=October 1, 2008|title=Biosignatures in Ancient Rocks: A Summary of Discussions at a Field Workshop on Biosignatures in Ancient Rocks|url=https://www.liebertpub.com/doi/10.1089/ast.2008.0257|journal=Astrobiology|volume=8|issue=5|pages=883–907|doi=10.1089/ast.2008.0257|pmid=19025466|bibcode=2008AsBio...8..883O|issn=1531-1074}}</ref><ref>{{Cite journal|last=Jensen|first=Sören|date=February 1, 2003|title=The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives|url=https://academic.oup.com/icb/article/43/1/219/604558|journal=Integrative and Comparative Biology|language=en|volume=43|issue=1|pages=219–228|doi=10.1093/icb/43.1.219|pmid=21680425|issn=1540-7063|doi-access=free}}</ref><ref>{{Cite journal|last1=Albani|first1=Abderrazak El|last2=Mangano|first2=M. Gabriela|last3=Buatois|first3=Luis A.|last4=Bengtson|first4=Stefan|last5=Riboulleau|first5=Armelle|last6=Bekker|first6=Andrey|last7=Konhauser|first7=Kurt|last8=Lyons|first8=Timothy|last9=Rollion-Bard|first9=Claire|last10=Bankole|first10=Olabode|last11=Baghekema|first11=Stellina Gwenaelle Lekele|date=February 26, 2019|title=Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=9|pages=3431–3436|doi=10.1073/pnas.1815721116|issn=0027-8424|pmc=6397584|pmid=30808737|bibcode=2019PNAS..116.3431E |doi-access=free}}</ref><ref name="Baucon 141–180">{{Cite journal|last1=Baucon|first1=Andrea|last2=Neto de Carvalho|first2=Carlos|last3=Barbieri|first3=Roberto|last4=Bernardini|first4=Federico|last5=Cavalazzi|first5=Barbara|last6=Celani|first6=Antonio|last7=Felletti|first7=Fabrizio|last8=Ferretti|first8=Annalisa|last9=Schönlaub|first9=Hans Peter|last10=Todaro|first10=Antonio|last11=Tuniz|first11=Claudio|date=August 1, 2017|title=Organism-substrate interactions and astrobiology: Potential, models and methods|url=http://www.sciencedirect.com/science/article/pii/S0012825216303646|journal=Earth-Science Reviews|language=en|volume=171|pages=141–180|doi=10.1016/j.earscirev.2017.05.009|bibcode=2017ESRv..171..141B|issn=0012-8252}}</ref> 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 rover ''Curiosity'' 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.<ref>{{Cite journal|last1=Baucon|first1=Andrea|last2=Neto De Carvalho|first2=Carlos|last3=Felletti|first3=Fabrizio|last4=Cabella|first4=Roberto|date=2020|title=Ichnofossils, Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera Rubin Ridge, Mars|journal=Geosciences|language=en|volume=10|issue=2|pages=39|doi=10.3390/geosciences10020039|bibcode=2020Geosc..10...39B|doi-access=free|hdl=2434/717600|hdl-access=free}}</ref> 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.<ref>{{Cite journal|last1=Fisk|first1=M.r.|last2=Popa|first2=R.|last3=Mason|first3=O.u.|last4=Storrie-Lombardi|first4=M.c.|last5=Vicenzi|first5=E.p.|date=February 1, 2006|title=Iron-Magnesium Silicate Bioweathering on Earth (and Mars?)|url=https://www.liebertpub.com/doi/10.1089/ast.2006.6.48|journal=Astrobiology|volume=6|issue=1|pages=48–68|doi=10.1089/ast.2006.6.48|pmid=16551226|bibcode=2006AsBio...6...48F|issn=1531-1074}}</ref><ref>{{Cite journal|last1=McKay|first1=D. S.|last2=Gibson|first2=E. K.|last3=Thomas-Keprta|first3=K. L.|last4=Vali|first4=H.|last5=Romanek|first5=C. S.|last6=Clemett|first6=S. J.|last7=Chillier|first7=X. D. F.|last8=Maechling|first8=C. R.|last9=Zare|first9=R. N.|date=August 16, 1996|title=Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001|url=https://www.science.org/doi/10.1126/science.273.5277.924|journal=Science|language=en|volume=273|issue=5277|pages=924–930|doi=10.1126/science.273.5277.924|pmid=8688069|bibcode=1996Sci...273..924M|s2cid=40690489|issn=0036-8075}}</ref><ref name="Baucon 141–180"/> Further studies are needed to confirm biogenicity.

==Geysers==
{{Main|Astronomy:Geysers on Mars}}
{{multiple image
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| caption1 = Artist's concept showing sand-laden jets erupt from geysers on Mars.
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| caption2 = Close up of dark dune spots, probably created by cold geyser-like eruptions.
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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 CO<sub>2</sub> – and probably water – increase pressure in their interior producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.<ref name="2006-100">{{cite news|title=NASA Findings Suggest Jets Bursting From Martian Ice Cap |date=August 16, 2006 |publisher=NASA |url=http://www.jpl.nasa.gov/news/news.cfm?release=2006-100 |work=Jet Propulsion Laboratory |url-status=live |archive-url=https://web.archive.org/web/20091010164741/http://www.jpl.nasa.gov/news/news.cfm?release=2006-100 |archive-date=October 10, 2009 }}</ref><ref name="Kieffer2000">{{cite journal |bibcode=2000mpse.conf...93K |title=Annual Punctuated CO2 Slab-Ice and Jets on Mars |last=Kieffer |first=H. H. |date=2000 |page=93 |journal=International Conference on Mars Polar Science and Exploration|issue=1057 }}</ref><ref name="Portyankina">{{cite journal |bibcode=2006LPICo1323.8040P |title=Simulations of Geyser-type Eruptions in Cryptic Region of Martian South Polar Cap |last1=Portyankina |first1=G. |last2=Markiewicz |first2=W. J. |last3=Garcia-Comas |first3=M. |last4=Keller |first4=H. U. |last5=Bibring |first5=J.-P. |last6=Neukum |first6=G. |volume=1323 |date=2006 |page=8040 |journal=Fourth International Conference on Mars Polar Science and Exploration}}</ref><ref name="Hugh2006">{{cite journal |bibcode=2006Natur.442..793K |title=CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap |last1=Kieffer |first1=Hugh H. |last2=Christensen |first2=Philip R. |last3=Titus |first3=Timothy N. |volume=442 |date=2006 |pages=793–6 |journal=Nature |doi=10.1038/nature04945 |pmid=16915284 |issue=7104|s2cid=4418194 }}</ref> 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.<ref name="Ness 2002">{{cite journal |title=Spider-Ravine Models and Plant-like Features on Mars – Possible Geophysical and Biogeophysical Modes of Origin |journal=Journal of the British Interplanetary Society (JBIS) |date=2002 |first=Peter K. |last=Ness |author2=Greg M. Orme |volume=55 |pages=85–108 |url=http://spsr.utsi.edu/articles/ness.pdf |access-date=September 3, 2009 |archive-url=https://web.archive.org/web/20120220130910/http://spsr.utsi.edu/articles/ness.pdf |archive-date=February 20, 2012 |url-status=dead }}</ref>

A team of Hungarian scientists propose that the geysers' most visible features, dark dune spots and spider channels, may be colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap, and as the [[Astronomy:Sunlight|sunlight]] returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesize and heat their immediate surroundings. A pocket of liquid water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through grey. When the layer has completely melted, the microorganisms rapidly desiccate and turn black, surrounded by a grey aureole.<ref name="Andras">{{cite journal |bibcode=2001LPI....32.1543H |title=Probable Evidences of Recent Biological Activity on Mars: Appearance and Growing of Dark Dune Spots in the South Polar Region |last1=Horváth |first1=A. |last2=Gánti |first2=T. |last3=Gesztesi |first3=A. |last4=Bérczi |first4=Sz. |last5=Szathmáry |first5=E. |volume=32 |date=2001 |page=1543 |journal=32nd Annual Lunar and Planetary Science Conference}}</ref><ref name="Pócs et al. 2004">{{cite journal |bibcode=2004ESASP.545..265P |title=Possible crypto-biotic-crust on Mars? |last1=Pócs |first1=T. |last2=Horváth |first2=A. |last3=Gánti |first3=T. |last4=Bérczi |first4=Sz. |last5=Szathemáry |first5=E. |volume=545 |date=2004 |pages=265–6 |journal=Proceedings of the Third European Workshop on Exo-Astrobiology}}</ref><ref>{{cite journal |doi=10.1023/A:1025705828948 |pmid=14604189 |date=2003 |last1=Gánti |first1=Tibor |last2=Horváth |first2=András |last3=Bérczi |first3=Szaniszló |last4=Gesztesi |first4=Albert |last5=Szathmáry |first5=Eörs |journal=Origins of Life and Evolution of the Biosphere |volume=33 |issue=4/5 |pages=515–57 |title=Dark Dune Spots: Possible Biomarkers on Mars? |bibcode=2003OLEB...33..515G|s2cid=23727267 }}</ref> 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.<ref name="Planetary">{{cite journal |bibcode=2002LPI....33.1108H |title=Morphological Analysis of the Dark Dune Spots on Mars: New Aspects in Biological Interpretation |last1=Horváth |first1=A. |last2=Gánti |first2=T. |last3=Bérczi |first3=Sz. |last4=Gesztesi |first4=A. |last5=Szathmáry |first5=E. |volume=33 |date=2002 |page=1108 |journal=33rd Annual Lunar and Planetary Science Conference}}</ref><ref>{{cite web|url=http://www.monochrom.at/dark-dune-spots/ |title=Dark Dune Spots – Could it be that it's alive? |access-date=September 4, 2009 |last=András Sik |first=Ákos Kereszturi |publisher=Monochrom |url-status=live |archive-url=https://web.archive.org/web/20090903012601/http://www.monochrom.at/dark-dune-spots/ |archive-date=September 3, 2009 }} (Audio interview, MP3 6 min.)</ref> Since their discovery, fiction writer [[Biography:Arthur C. Clarke|Arthur C. Clarke]] promoted these formations as deserving of study from an [[Astronomy:Astrobiology|astrobiological]] perspective.<ref name="Orme">{{cite journal|title=Martian Spiders |journal=Marsbugs |date=June 9, 2003 |first1=Greg M. |last1=Orme |first2=Peter K. |last2=Ness |volume=10 |issue=23 |pages=5–7 |url=http://www.lyon.edu/projects/marsbugs/2003/20030609.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070927032609/http://www.lyon.edu/projects/marsbugs/2003/20030609.pdf |archive-date=September 27, 2007 }}</ref>

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.<ref name="Manrubia">{{Cite book |bibcode=2004ESASP.545...77M |title=Comparative analysis of geological features and seasonal processes in 'Inca City' and 'Pityusa Patera' regions on Mars |last1=Manrubia |first1=S. C. |last2=Prieto Ballesteros |first2=O. |last3=González Kessler |first3=C. |last4=Fernández Remolar |first4=D. |last5=Córdoba-Jabonero |first5=C. |last6=Selsis |first6=F. |last7=Bérczi |first7=S. |last8=Gánti |first8=T. |last9=Horváth |first9=A. |volume=545 |date=2004 |pages=77–80 |journal=Proceedings of the Third European Workshop on Exo-Astrobiology |isbn=978-92-9092-856-0 }}</ref> A British team also considers the possibility that [[Chemistry:Organic matter|organic matter]], microbes, or even simple plants might co-exist with these inorganic formations, especially if the mechanism includes liquid water and a [[Earth:Geothermal energy|geothermal]] energy source.<ref name="Ness 2002"/> They also remark that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis.<ref name="Ness 2002"/> It has been proposed to develop the [[Astronomy:Mars Geyser Hopper|Mars Geyser Hopper]] lander to study the geysers up close.<ref name="aiaa_50">{{cite conference|first1=Geoffrey |last1=Landis |first2=Steven |last2=Oleson |first3=Melissa |last3=McGuire |date=2012 |title=Design Study for a Mars Geyser Hopper |conference=50th AIAA Aerospace Sciences Meeting |location=Nashville |url=https://ntrs.nasa.gov/search.jsp?R=20120004036 |doi=10.2514/6.2012-631 |url-status=live |archive-url=https://web.archive.org/web/20160603031840/https://ntrs.nasa.gov/search.jsp?R=20120004036 |archive-date=June 3, 2016 |hdl=2060/20120004036 |hdl-access=free }}</ref>

==Forward contamination==
{{Further|Astronomy:Planetary protection|Astronomy:Interplanetary contamination}}

[[Astronomy:Planetary protection|Planetary protection]] of Mars aims to prevent biological contamination of the planet.<ref name="strategy">{{cite book | author1=Committee on an Astrobiology Strategy for the Exploration of Mars | author2=National Research Council | date=2007 | chapter=Planetary Protection for Mars Missions | chapter-url=http://www.nap.edu/openbook.php?record_id=11937&page=95 | pages=95–98 | title=An Astrobiology Strategy for the Exploration of Mars | publisher=The National Academies Press | isbn=978-0-309-10851-5 }}</ref> A major goal is to preserve the planetary record of natural processes by preventing human-caused microbial introductions, also called forward contamination. There is abundant evidence as to what can happen when organisms from regions on Earth that have been isolated from one another for significant periods of time are introduced into each other's environment. Species that are constrained in one environment can thrive – often out of control – in another environment much to the detriment of the original species that were present. In some ways, this problem could be compounded if life forms from one planet were introduced into the totally alien ecology of another world.<ref name="Cowing_201304">{{cite web | url=http://astrobiology.com/2013/04/planetary-protection-a-work-in-progress.html | title=Planetary Protection: A Work in Progress | first=Keith | last=Cowing | date=April 11, 2013 | work=Astrobiology | access-date=June 2, 2013 | archive-url=https://archive.today/20130616040403/http://astrobiology.com/2013/04/planetary-protection-a-work-in-progress.html | archive-date=June 16, 2013 | url-status=live | df=mdy-all }}</ref>

The prime concern of hardware contaminating Mars derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (extremophiles) despite best efforts.<ref name="Beaty" /><ref name="Debus">{{cite journal | bibcode=2005AdSpR..35.1648D | title=Estimation and assessment of Mars contamination | last=Debus | first=A. | volume=35 | date=2005 | pages=1648–53 | journal=Advances in Space Research | doi=10.1016/j.asr.2005.04.084 | pmid=16175730 | issue=9}}</ref> 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 of [[Biology:Radioresistance|radiation-resistant microorganisms]] including the species ''[[Biology:Deinococcus radiodurans|Deinococcus radiodurans]]'' and genera ''[[Biology:Brevundimonas|Brevundimonas]]'', ''[[Biology:Rhodococcus|Rhodococcus]]'', and ''[[Biology:Pseudomonas|Pseudomonas]]'' under simulated Martian conditions.<ref name="Planetary protection – radiodurans">{{cite journal |bibcode=2010AsBio..10..717D |title=Low-Temperature Ionizing Radiation Resistance of Deinococcus radiodurans and Antarctic Dry Valley Bacteria |last1=Dartnell |first1=Lewis R. |last2=Hunter |first2=Stephanie J. |last3=Lovell |first3=Keith V. |last4=Coates |first4=Andrew J. |last5=Ward |first5=John M. |volume=10 |date=2010 |pages=717–32 |journal=Astrobiology |doi=10.1089/ast.2009.0439 |pmid=20950171 |issue=7}}</ref> Results from one of these experimental irradiation experiments, combined with previous radiation modeling, indicate that ''[[Biology:Brevundimonas|Brevundimonas]]'' sp. MV.7 emplaced only 30&nbsp;cm deep in Martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10<sup>6</sup> population reduction.<ref name="Planetary protection – radiodurans" /> The diurnal Mars-like cycles in temperature and relative humidity affected the viability of ''Deinococcus radiodurans'' cells quite severely.<ref name="simulation">{{cite journal |bibcode=2007AdSpR..40.1672D |title=Simulation of the environmental climate conditions on martian surface and its effect on ''Deinococcus radiodurans'' | last1=de la Vega | first1=U. Pogoda | last2=Rettberg | first2=P. | last3=Reitz | first3=G. |volume=40 |date=2007 |pages=1672–7 |journal=Advances in Space Research |doi=10.1016/j.asr.2007.05.022 |issue=11}}</ref> In other simulations, ''Deinococcus radiodurans'' also failed to grow under low atmospheric pressure, under 0&nbsp;°C, or in the absence of oxygen.<ref name="serratia">{{cite journal | title=Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres | journal=Astrobiology | date=February 2013 | first1=Andrew C. | last1=Schuerger | first2=Richard | last2=Ulrich | first3=Bonnie J. | last3=Berry | first4=Wayne L. | last4=Nicholson. | volume=13 | issue=2 | pages=115–131 | doi=10.1089/ast.2011.0811 | bibcode=2013AsBio..13..115S | pmid=23289858 | pmc=3582281}}</ref>

==Survival under simulated Martian conditions==
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 "[[Astronomy:Mars jar|Mars jar]]s" or "Mars simulation chambers", were first described and used in U.S. Air Force research in the 1950s by Hubertus Strughold, and popularized in civilian research by [[Biography:Joshua Lederberg|Joshua Lederberg]] and [[Biography:Carl Sagan|Carl Sagan]].<ref name=":03">{{Cite news|last=Scoles|first=Sarah|date=July 24, 2020|title=The Doctor From Nazi Germany and the Roots of the Hunt for Life on Mars|language=en-US|work=The New York Times|url=https://www.nytimes.com/2020/07/24/science/mars-jars-strughold.html|access-date=July 24, 2020|issn=0362-4331}}</ref>

On April 26, 2012, scientists reported that an [[Earth:Extremophile|extremophile]] [[Biology:Lichen|lichen]] survived and showed remarkable results on the [[Social:Adaptive capacity|adaptation capacity]] of [[Earth:Photosynthesis|photosynthetic activity]] within the [[Simulation|simulation]] time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the [[Organization:German Aerospace Center|German Aerospace Center]] (DLR).<ref name="lab_study">{{cite journal |bibcode=2010AsBio..10..215D |title=Survival Potential and Photosynthetic Activity of Lichens Under Mars-Like Conditions: A Laboratory Study |last1=de Vera |first1=Jean-Pierre |last2=Möhlmann |first2=Diedrich |last3=Butina |first3=Frederike |last4=Lorek |first4=Andreas |last5=Wernecke |first5=Roland |last6=Ott |first6=Sieglinde |volume=10 |date=2010 |pages=215–27 |journal=Astrobiology |doi=10.1089/ast.2009.0362 |pmid=20402583 |issue=2}}</ref><ref name="EGU-20120426">{{cite journal |bibcode=2012EGUGA..14.2113D |title=The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars |last1=de Vera |first1=J.-P. P. |last2=Schulze-Makuch |first2=D. |last3=Khan |first3=A. |last4=Lorek |first4=A. |last5=Koncz |first5=A. |last6=Möhlmann |first6=D. |last7=Spohn |first7=T. |volume=14 |date=2012 |page=2113 |journal=EGU General Assembly 2012}}</ref><ref name="dlrMarsStudy">{{cite web|url=http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-3409/ |title=Surviving the conditions on Mars |publisher=DLR |date=April 26, 2012 |url-status=live |archive-url=https://web.archive.org/web/20121113081036/http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-3409/ |archive-date=November 13, 2012 }}</ref><ref name="fungal">{{cite journal |doi=10.1016/j.funeco.2012.01.008 |title=Lichens as survivors in space and on Mars |date=2012 |last=de Vera |first=Jean-Pierre |journal=Fungal Ecology |volume=5 |issue=4 |pages=472–9}}</ref><ref name="nordita_eana2012">{{cite journal | first1=R. | last1=de la Torre Noetzel | first2=F.J. | last2=Sanchez Inigo | first3=E. | last3=Rabbow | first4=G. | last4=Horneck | first5=J. P. | last5=de Vera | first6=L.G. | last6=Sancho | title=Lichens Survive in Space: Results from the 2005 LICHENS Experiment | doi=10.1089/ast.2006.0046 | bibcode=2007AsBio...7..443S | volume=7 | issue=3 | journal=Astrobiology | pages=443–454 | pmid=17630840 | date=June 2007}}</ref><ref name="lichen">{{cite journal |bibcode=2012P&SS...72..102S |title=The resistance of the lichen ''Circinaria gyrosa'' (nom. Provis.) towards simulated Mars conditions—a model test for the survival capacity of an eukaryotic extremophile |last1=Sánchez |first1=F. J. |last2=Mateo-Martí |first2=E. |last3=Raggio |first3=J. |last4=Meeßen |first4=J. |last5=Martínez-Frías |first5=J. |last6=Sancho |first6=L. G. |last7=Ott |first7=S. |last8=de la Torre |first8=R. |volume=72 |issue=1 |date=2012 |pages=102–10 |journal=Planetary and Space Science |doi=10.1016/j.pss.2012.08.005}}</ref> The ability to survive in an environment is not the same as the ability to thrive, reproduce, and evolve in that same environment, necessitating further study.<ref name="dust-up" /><ref name="Beaty" />

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, and others, all at the same time and in combination.<ref>{{cite journal | doi = 10.1089/ast.2017.1772 | volume=18 | title=Is Searching for Martian Life a Priority for the Mars Community? | year=2018 | journal=Astrobiology | pages=101–107 | last1 = Fairén | first1 = Alberto G. | last2 = Parro | first2 = Victor | last3 = Schulze-Makuch | first3 = Dirk | last4 = Whyte | first4 = Lyle| issue=2 | pmid=29359967 | pmc=5820680 | bibcode=2018AsBio..18..101F }}</ref> Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.<ref name="dust-up"/>

===Water salinity and temperature===
Astrobiologists funded by NASA are researching the limits of microbial life in solutions with high salt concentrations at low temperature.<ref name="Schneegurt-Chen 2018">[http://adsabs.harvard.edu/abs/2018cosp...42E3027S Bacterial growth and survival under the extreme chemical and physical conditions of Mars and the icy worlds.] Schneegurt, Mark; Chen, Fei; Clark, Benton; Wilks, Jonathan; Zayed, Hadi; Joad, Md; Mahdi, Ammar; Zbeeb, Hassan. 42nd COSPAR Scientific Assembly. Held July 14–22, 2018, in Pasadena, California, USA, Abstract id. F3.1-14-18.</ref> 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 of ''Phoenix'' 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.<ref name="Schneegurt-Chen 2018"/><ref name="Kaspin-Powell Halophiles">[https://www.space.com/42881-mars-chlorate-saltwater-search-for-life.html Chlorate-rich Soil May Help Us Find Liquid Water on Mars.] {{Webarchive|url=https://web.archive.org/web/20190109062302/https://www.space.com/42881-mars-chlorate-saltwater-search-for-life.html |date=January 9, 2019 }} Lisa Kaspin-Powell, ''Astrobiology Magazine''. January 3, 2019. Published by ''Space.com''.</ref><ref name="Toner-Catling Sep 2018">{{Cite journal |doi = 10.1016/j.epsl.2018.06.011|title = Chlorate brines on Mars: Implications for the occurrence of liquid water and deliquescence|journal = Earth and Planetary Science Letters|volume = 497|pages = 161–168|year = 2018|last1 = Toner|first1 = J.D.|last2 = Catling|first2 = D.C.|bibcode = 2018E&PSL.497..161T|s2cid = 134197775}}</ref> Earth bacteria capable of growth and reproduction in the presence of highly salted solutions, called [[Biology:Halophile|halophile]] or "salt-lover", were tested for survival using salts commonly found on Mars and at decreasing temperatures.<ref name="Schneegurt-Chen 2018"/> The species tested include ''[[Biology:Halomonas|Halomonas]]'', ''[[Biology:Marinococcus|Marinococcus]]'', ''Nesterenkonia'', and ''[[Biology:Virgibacillus|Virgibacillus]]''.<ref name="Schneegurt-Chen 2018"/> Laboratory simulations show that whenever multiple Martian environmental factors are combined, the survival rates plummet quickly,<ref name="dust-up"/> however, halophile bacteria were grown in a lab in water solutions containing more than 25% of salts common on Mars, and starting in 2019{{update inline|date=November 2022}}, the experiments will incorporate exposure to low temperature, salts, and high pressure.<ref name="Schneegurt-Chen 2018"/>

===Mars-like regions on Earth===
On 21 February 2023, scientists reported the findings of a "dark microbiome" of unfamiliar [[Biology:Microorganism|microorganism]]s in the [[Earth:Atacama Desert|Atacama Desert]] in {{wipe|Chile}}, a [[Astronomy:Mars|Mars-like region]] of planet [[Astronomy:Earth|Earth]].<ref name="WP-20230221">{{cite news |last=Achenbach |first=Joel |title=Strange DNA found in the desert offers lessons in the hunt for Mars life |url=https://www.washingtonpost.com/science/2023/02/21/mars-life-atacama-microbiome/ |date=21 February 2023 |newspaper={{wipe|The Washington Post}} |accessdate=21 February 2023 }}</ref><ref name="NC-20230221">{{cite journal |author=Azua-Bustos, Armando |display-authors=et al. |title=Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits |date=21 February 2023 |journal=Nature Communications |volume=14 |issue=808 |page=808 |doi=10.1038/s41467-023-36172-1 |pmid=36810853 |pmc=9944251 |bibcode=2023NatCo..14..808A }}</ref>

==Missions==

===Mars-2===
{{Main|Astronomy:Mars program}}

[[Astronomy:Mars program|Mars-1]] was the first spacecraft launched to Mars in 1962,<ref>{{cite web|url=http://jtgnew.sjrdesign.net/exploration_space_planetary_mars.html |title="Journey Through the Galaxy" Mars Program: Mars ~ 1960–1974 |access-date=January 26, 2014 |last=Robbins |first=Stuart |date=2008 |publisher=SJR Design |url-status=live |archive-url=https://web.archive.org/web/20140204004556/http://jtgnew.sjrdesign.net/exploration_space_planetary_mars.html |archive-date=February 4, 2014 }}</ref> but communication was lost while en route to Mars. With [[Astronomy:Mars 2|Mars-2]] and [[Astronomy:Mars 3|Mars-3]] in 1971–1972, information was obtained on the nature of the surface rocks and altitude profiles of the surface density of the soil, its thermal conductivity, and thermal anomalies detected on the surface of Mars. The program found that its northern polar cap has a temperature below {{cvt|−110|C}} and that the water vapor content in the atmosphere of Mars is five thousand times less than on Earth. No signs of life were found.<ref>{{cite web|url=http://burro.astr.cwru.edu/stu/advanced/20th_soviet_mars.html |title=Mars (1960–1974): Mars 1 |access-date=January 26, 2014 |last=Mihos |first=Chris |date=January 11, 2006 |work=Department of Astronomy, Case Western Reserve University. |url-status=dead |archive-url=https://web.archive.org/web/20131013211415/http://burro.astr.cwru.edu/stu/advanced/20th_soviet_mars.html |archive-date=October 13, 2013 }}</ref>

Signs of life of the Mars space program AMS from orbit were not found. The descent vehicle Mars-2 crashed on landing, the descent vehicle [[Astronomy:Mars 3|Mars-3]] launched 1.5 minutes after landing in the [[Astronomy:Ptolemaeus (Martian crater)|Ptolemaeus crater]], but worked only 14.5 seconds/<ref>{{Cite web |title=Russia's Mars 3 lander maybe found by Russian amateurs |url=https://www.planetary.org/articles/0412-how-we-searched-for-mars-3 |access-date=2023-06-06 |website=The Planetary Society |language=en}}</ref>

===Mariner 4===
{{Main|Astronomy:Mariner 4}}
{{multiple image
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| image1 = Mars m04 11e.jpg
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| caption1 = Mariner Crater, as seen by Mariner 4 in 1965. Pictures like this suggested that Mars is too dry for any kind of life.
| image2 = Streamlined Islands in Maja Valles.jpg
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| caption2 = Streamlined Islands seen by Viking orbiter showed that large floods occurred on Mars. The image is located in [[Astronomy:Lunae Palus quadrangle|Lunae Palus quadrangle]].
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}}

[[Astronomy:Mariner 4|Mariner 4]] probe performed the first successful [[Engineering:Planetary flyby|flyby]] of the planet Mars, returning the first pictures of the Martian surface in 1965. The photographs showed an arid Mars without rivers, oceans, or any signs of life. Further, it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4&nbsp;billion years. The probe also found that Mars has no [[Astronomy:Magnetosphere|global magnetic field]] that would protect the planet from potentially life-threatening cosmic rays. The probe was able to calculate the [[Physics:Atmospheric pressure|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.<ref name="chambers" /> 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.<ref name="chambers" /><ref name="Momsen1">{{cite web| url=http://home.earthlink.net/~nbrass1/mariner/miv.htm
|title=Mariner IV - First Flyby of Mars: Some personal experiences |last=Momsen |first=Bill |date=2006 |page=1 |access-date =February 11, 2009 |url-status=dead |archive-url =https://web.archive.org/web/20020620141059/http://home.earthlink.net/~nbrass1/mariner/miv.htm |archive-date=June 20, 2002 }}</ref><ref name="Momsen2">
{{cite web |url=http://home.earthlink.net/~nbrass1/mariner/miv-2.htm |title=Mariner IV - First Flyby of Mars: Some personal experiences |last=Momsen |first=Bill |date=2006 |page=2 |access-date=February 11, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081230212629/http://home.earthlink.net/~nbrass1/mariner/miv-2.htm |archive-date=December 30, 2008 }}</ref>

===''Viking'' orbiters===
{{Main|Engineering:Viking program}}

Liquid water is necessary for known life and [[Biology:Metabolism|metabolism]], so if water was present on Mars, the chances of it having supported life may have been determinant. The ''Viking'' orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams.<ref name="cratering">{{cite book | last1=Strom | first1=R. G. | first2=Steven K. | last2=Croft | first3=Nadine G. | last3=Barlow | title=The Martian Impact Cratering Record | publisher=University of Arizona Press | isbn=978-0-8165-1257-7 | date=1992 | bibcode=1992mars.book..383S | url-access=registration | url=https://archive.org/details/mars0000unse }}{{page needed|date=June 2013}}</ref><ref name="raeburn_1998">{{cite journal |last=Raeburn | first=P.| date=1998 | title=Uncovering the Secrets of the Red Planet Mars | journal=National Geographic Society }}{{page needed|date=June 2013}}</ref><ref name="moore_atlas">{{cite book | last=Moore| first=P.| display-authors=0| date=1990| title=The Atlas of the Solar System| publisher=Mitchell Beazley Publishers| location=New York}}{{page needed|date=June 2013}}</ref>

===Viking biological experiments===

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.<ref name="BC">{{cite web|url=http://biocab.org/Astrobiology.html |title=Astrobiology |date=September 26, 2006 |publisher=Biology Cabinet |url-status=live |archive-url=https://web.archive.org/web/20101212184044/http://biocab.org/Astrobiology.html |archive-date=December 12, 2010 }}</ref> 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|date=July 2013}} showing increased <sup>14</sup>CO<sub>2</sub> production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled <sup>14</sup>CO<sub>2</sub> was evolved in the Labeled Release experiment, and that the [[Gas chromatography–mass spectrometry|GCMS]] detected no organic molecules. There are vastly different interpretations of what those results imply: A 2011 [[Astronomy:Astrobiology|astrobiology]] textbook notes that the GCMS was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that the ''Viking'' missions failed to detect life in the Martian soil."<ref name="PlaxcoGross2011_1">{{cite book|first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA282 |date=2011 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=282–283 |url-status=live |archive-url=https://web.archive.org/web/20140920155308/http://books.google.com/books?id=x83omgI5pGQC&pg=PA282 |archive-date=September 20, 2014 }}</ref>

Norman Horowitz was the head of the [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] bioscience section for the [[Astronomy:Mariner program|Mariner]] and [[Engineering:Viking program|Viking]] 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.<ref name = Horowitz1986>Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}</ref> 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, [[Biography:Gilbert Levin|Gilbert Levin]], believes his results are a definitive diagnostic for life on Mars.<ref name="chambers" /> Levin's interpretation is disputed by many scientists.<ref name="cnn">{{cite web|first=Richard |last=Stenger |url=http://edition.cnn.com/2000/TECH/space/11/07/mars.sample/ |title=Mars sample return plan carries microbial risk, group warns |publisher=CNN |date=November 7, 2000 |url-status=live |archive-url=https://web.archive.org/web/20131007051214/http://edition.cnn.com/2000/TECH/space/11/07/mars.sample/ |archive-date=October 7, 2013 }}</ref> A 2006 [[Astronomy:Astrobiology|astrobiology]] textbook noted that "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas."<ref name="Plaxco2006">{{cite book|first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://archive.org/details/astrobiologybrie0000plax |url-access=registration |date=2006 |publisher=JHU Press |isbn=978-0-8018-8366-8 |page=[https://archive.org/details/astrobiologybrie0000plax/page/223 223] }}</ref> Other scientists argue that [[Chemistry:Superoxide|superoxide]]s in the soil could have produced this effect without life being present.<ref name="PlaxcoGross2011_2">{{cite book|first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA285 |date=2011 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=285–286 |edition=2nd |url-status=live |archive-url=https://web.archive.org/web/20170401060538/https://books.google.com/books?id=x83omgI5pGQC&pg=PA285 |archive-date=April 1, 2017 }}</ref> An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph and mass spectrometer, designed to identify natural organic matter, did not detect organic molecules.<ref name="Levin">{{cite journal |bibcode=2007arXiv0705.3176L |title=Analysis of evidence of Mars life |last=Levin |first=Gilbert V. |journal=Electroneurobiología |volume=15 |issue=2 |date=2007 |pages=39–47 |arxiv=0705.3176}}</ref> More recently, high levels of [[Chemistry:Organic compound|organic chemicals]], particularly [[Chemistry:Chlorobenzene|chlorobenzene]], were [[Organization:Timeline of Mars Science Laboratory#Detection of organics|detected]] in powder drilled from one of the rocks, named "[[Astronomy:List of rocks on Mars|Cumberland]]", analyzed by the [[Engineering:Curiosity (rover)|''Curiosity'' rover]].<ref name="NASA-20141216-GW">{{cite web|last1=Webster |first1=Guy |last2=Neal-Jones |first2=Nancy |last3=Brown |first3=Dwayne |title=NASA Rover Finds Active and Ancient Organic Chemistry on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-432 |date=December 16, 2014 |work=[[Organization:NASA|NASA]] |access-date=December 16, 2014 |url-status=live |archive-url=https://web.archive.org/web/20141217031232/http://www.jpl.nasa.gov/news/news.php?release=2014-432 |archive-date=December 17, 2014 }}</ref><ref name="NYT-20141216-KC">{{cite news|last=Chang |first=Kenneth |title='A Great Moment': Rover Finds Clue That Mars May Harbor Life |url=https://www.nytimes.com/2014/12/17/science/a-new-clue-in-the-search-for-life-on-mars.html |date=December 16, 2014 |work={{wipe|New York Times}} |access-date=December 16, 2014 |url-status=live |archive-url=https://web.archive.org/web/20141216220037/http://www.nytimes.com/2014/12/17/science/a-new-clue-in-the-search-for-life-on-mars.html |archive-date=December 16, 2014 }}</ref> The results of the Viking mission concerning life are considered by the general expert community as inconclusive.<ref name="chambers" /><ref name="PlaxcoGross2011_2" /><ref name="sci_99">{{cite journal |bibcode=1976Sci...194...99K |title=The Viking Biological Investigation: Preliminary Results |last1=Klein |first1=Harold P. |last2=Horowitz |first2=Norman H. |last3=Levin |first3=Gilbert V. |last4=Oyama |first4=Vance I. |last5=Lederberg |first5=Joshua |last6=Rich |first6=Alexander |last7=Hubbard |first7=Jerry S. |last8=Hobby |first8=George L. |last9=Straat |first9=Patricia A. |volume=194 |date=1976 |pages=99–105 |journal=Science |doi=10.1126/science.194.4260.99 |pmid=17793090 |issue=4260 |s2cid=24957458 }}</ref>

In 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, D.C., US), [[Biography:Gilbert Levin|Gilbert Levin]]'s investigation was assessed once more.<ref name="Levin" /> Levin still maintains that his original data were correct, as the positive and negative control experiments were in order.<ref name="Bianciardi-2012">{{cite journal |bibcode=2012IJASS..13...14B |title=Complexity Analysis of the Viking Labeled Release Experiments |last1=Bianciardi |first1=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. |volume=13 |issue=1 |date=2012 |pages=14–26 |journal=International Journal of Aeronautical and Space Sciences |doi=10.5139/IJASS.2012.13.1.14|doi-access=free }}</ref> Moreover, Levin's team, on April 12, 2012, reported a statistical speculation, based on old data—reinterpreted mathematically through [[Cluster analysis|cluster analysis]]—of the Labeled Release experiments, that may suggest evidence of "extant microbial life on Mars".<ref name="Bianciardi-2012" /><ref name="natgeo_201204">{{cite web|url=http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/ |title=Life on Mars Found by NASA's Viking Mission? |date= April 15, 2012|url-status=dead |archive-url=https://web.archive.org/web/20130704205919/http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/ |archive-date=July 4, 2013 }}</ref> 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.<ref name="Discovery-20120412">{{cite web|last=Klotz |first=Irene |title=Mars Viking Robots 'Found Life' |url=http://news.discovery.com/space/mars-life-viking-landers-discovery-120412.html |date=April 12, 2012 |publisher=DiscoveryNews |url-status=live |archive-url=https://web.archive.org/web/20120414195922/http://news.discovery.com/space/mars-life-viking-landers-discovery-120412.html |archive-date=April 14, 2012 }}</ref>

A research team from the National Autonomous University of Mexico headed by Rafael Navarro-González concluded that the GCMS equipment (TV-GC-MS) used by the [[Engineering:Viking program|Viking program]] to search for organic molecules, may not be sensitive enough to detect low levels of organics.<ref name="Navarro"/> Klaus Biemann, the principal investigator of the GCMS experiment on ''Viking'' wrote a rebuttal.<ref name="nas_104">{{cite journal | title=On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter | last=Biemann | first=Klaus | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=104 | issue=25 | pages=10310–10313 | date=2007 | doi=10.1073/pnas.0703732104 | pmid=17548829 | pmc=1965509 | bibcode=2007PNAS..10410310B | doi-access=free }}</ref> 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.<ref name="Navarro"/>

After the discovery of [[Chemistry:Perchlorate|perchlorate]]s on Mars by the [[Engineering:Phoenix (spacecraft)|Phoenix lander]], practically the same team of Navarro-González published a paper arguing that the Viking GCMS results were compromised by the presence of perchlorates.<ref name="Webster">{{cite web|last1=Webster |first1=Guy |last2=Hoover |first2=Rachel |last3=Marlaire |first3=Ruth |last4=Frias |first4=Gabriela |date=September 3, 2010 |url=http://www.jpl.nasa.gov/news/news.cfm?release=2010-286 |title=Missing Piece Inspires New Look at Mars Puzzle |publisher=Jet Propulsion Laboratory, NASA |access-date=October 24, 2010 |url-status=live |archive-url=https://web.archive.org/web/20101103012112/http://www.jpl.nasa.gov/news/news.cfm?release=2010-286 |archive-date=November 3, 2010 }}</ref> 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."<ref name="PlaxcoGross2011_1_2nd_ed">{{cite book|first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA282 |date=2011 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=282–283 |edition=2nd |url-status=live |archive-url=https://web.archive.org/web/20140920155308/http://books.google.com/books?id=x83omgI5pGQC&pg=PA282 |archive-date=September 20, 2014 }}</ref> Biemann has written a commentary critical of this Navarro-González paper as well,<ref name="doi_3869">{{Cite journal | doi = 10.1029/2011JE003869| title = Comment on 'Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars' by Rafael Navarro-González et al| journal = Journal of Geophysical Research| volume = 116| pages = E12001| year = 2011| last1 = Biemann | first1 = K. | last2 = Bada | first2 = J. L. | issue = E12| bibcode=2011JGRE..11612001B| doi-access = free}}</ref> to which the latter have replied;<ref name="doi_3880">{{Cite journal | doi = 10.1029/2011JE003880| title = Reply to comment by Biemann and Bada on 'Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars'| journal = Journal of Geophysical Research| volume = 116| issue = E12| pages = E12002| year = 2011| last1 = Navarro-González | first1 = R. | last2 = McKay | first2 = C. P. | bibcode=2011JGRE..11612002N| doi-access = free}}</ref> the exchange was published in December 2011.

===''Phoenix'' lander, 2008===
{{Main|Phoenix (spacecraft)}}[[File:Pia09344.jpg|thumb|left|An artist's concept of the ''Phoenix'' spacecraft]]

The ''[[Engineering:Phoenix (spacecraft)|Phoenix]]'' mission landed a robotic spacecraft in the polar region of Mars on May 25, 2008, and it operated until November 10, 2008. One of the mission's two primary objectives was to search for a "habitable zone" in the Martian regolith where microbial life could exist, the other main goal being to study the geological history of water on Mars. The lander has a 2.5 meter robotic arm that was capable of digging shallow trenches in the regolith. There was an electrochemistry experiment which analysed the [[Physics:Ion|ion]]s in the regolith and the amount and type of [[Biology:Antioxidant|antioxidant]]s on Mars. The [[Engineering:Viking program|Viking program]] data indicate that oxidants on Mars may vary with latitude, noting that ''[[Astronomy:Viking 2|Viking 2]]'' saw fewer oxidants than ''[[Astronomy:Viking 1|Viking 1]]'' in its more northerly position. ''Phoenix'' landed further north still.<ref name="MarsDaily">{{cite web | url=http://www.marsdaily.com/reports/Piecing_Together_Life_Potential_999.html | title=Piecing Together Life's Potential | work=Mars Daily | access-date=March 10, 2007 | archive-url=https://web.archive.org/web/20140805200518/http://www.marsdaily.com/reports/Piecing_Together_Life_Potential_999.html | archive-date=August 5, 2014 | url-status=live | df=mdy-all }}</ref>
''Phoenix''{{'}}s preliminary data revealed that Mars soil contains [[Chemistry:Perchlorate|perchlorate]], and thus may not be as life-friendly as thought earlier.<ref name="perchlorate">{{cite news|title=NASA Spacecraft Confirms Perchlorate on Mars |date=August 5, 2008 |url=http://www.nasa.gov/mission_pages/phoenix/multimedia/audioclips-20080805.html |work=NASA |url-status=live |archive-url=https://web.archive.org/web/20090303002135/http://www.nasa.gov/mission_pages/phoenix/multimedia/audioclips-20080805.html |archive-date=March 3, 2009 }}</ref><ref name="la_4986721">{{cite web|url=https://www.latimes.com/news/printedition/asection/la-sci-phoenix6-2008aug06,0,4986721.story |title=Perchlorate found in Martian soil |date=August 6, 2008 |work=Los Angeles Times |last=Johnson |first=John |url-status=live |archive-url=https://web.archive.org/web/20090318042116/http://www.latimes.com/news/printedition/asection/la-sci-phoenix6-2008aug06%2C0%2C4986721.story |archive-date=March 18, 2009 }}</ref><ref name="SD_2008-08">{{cite web|url=https://www.sciencedaily.com/releases/2008/08/080805192122.htm |publisher=Science Daily |date=August 6, 2008 |title=Martian Life Or Not? NASA's ''Phoenix'' Team Analyzes Results |url-status=live |archive-url=https://web.archive.org/web/20160305014043/https://www.sciencedaily.com/releases/2008/08/080805192122.htm |archive-date=March 5, 2016 }}</ref> The [[Chemistry:PH|pH]] and salinity level were viewed as benign from the standpoint of biology. The analysers also indicated the presence of bound water and CO<sub>2</sub>.<ref name="phoenix sol 30">{{cite web|last=Lakdawalla |first=Emily |title=''Phoenix'' sol 30 update: Alkaline soil, not very salty, "nothing extreme" about it! |work=The Planetary Society weblog |publisher=Planetary Society |date=June 26, 2008 |url=http://www.planetary.org/blog/article/00001526/ |url-status=live |archive-url=https://web.archive.org/web/20080630074727/http://www.planetary.org/blog/article/00001526/ |archive-date=June 30, 2008 }}</ref> A recent analysis of Martian meteorite EETA79001 found 0.6 ppm ClO<sub>4</sub><sup>−</sup>, 1.4 ppm ClO<sub>3</sub><sup>−</sup>, and 16 ppm NO<sub>3</sub><sup>−</sup>, most likely of Martian origin. The ClO<sub>3</sub><sup>−</sup> suggests presence of other highly oxidizing oxychlorines such as ClO<sub>2</sub><sup>−</sup> or ClO, produced both by UV oxidation of Cl and X-ray radiolysis of ClO<sub>4</sub><sup>−</sup>. Thus only highly refractory and/or well-protected (sub-surface) organics are likely to survive.<ref>{{cite journal | last1 = Kounaves | first1 = S. P. | display-authors = etal | year = 2014| title = Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics | journal = Icarus | volume = 2014 | issue = 229| pages = 206–213 | doi = 10.1016/j.icarus.2013.11.012 | bibcode = 2014Icar..229..206K }}</ref> In addition, recent analysis of the ''Phoenix'' WCL showed that the Ca(ClO<sub>4</sub>)<sub>2</sub> in the ''Phoenix'' soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO<sub>4</sub>)<sub>2</sub> in contact with liquid water would have formed only CaSO<sub>4</sub>. This suggests a severely arid environment, with minimal or no liquid water interaction.<ref>{{cite journal | last1 = Kounaves | first1 = S. P. | display-authors = etal | year = 2014 | title = Identification of the perchlorate parent salts at the Phoenix Mars landing site and implications | journal = Icarus | volume = 232 | pages = 226–231 | doi = 10.1016/j.icarus.2014.01.016 |bibcode = 2014Icar..232..226K }}</ref>

===Mars Science Laboratory===

[[File:PIA16239 High-Resolution Self-Portrait by Curiosity Rover Arm Camera.jpg|thumb|upright|right|''Curiosity'' rover self-portrait]]

The [[Organization:Mars Science Laboratory|Mars Science Laboratory]] mission is a [[Organization:NASA|NASA]] project that launched on November 26, 2011, the [[Engineering:Curiosity (rover)|''Curiosity'' rover]], a nuclear-powered robotic vehicle, bearing instruments designed to assess past and present [[Astronomy:Planetary habitability|habitability]] conditions on Mars.<ref name="nasa_msl_launch">{{cite web|date=November 26, 2011 |url=http://www.nasa.gov/mission_pages/msl/launch/ |title=Mars Science Laboratory Launch |url-status=live |archive-url=https://web.archive.org/web/20120704195743/http://www.nasa.gov/mission_pages/msl/launch/ |archive-date=July 4, 2012 }}</ref><ref name="NYT-MSL">{{cite news | agency=Associated Press | title=NASA Launches Super-Size Rover to Mars: 'Go, Go!'| url=https://www.nytimes.com/aponline/2011/11/26/science/AP-US-SCI-Mars-Rover.html | work=New York Times | date=November 26, 2011 }}</ref> The ''Curiosity'' rover landed on Mars on [[Astronomy:Aeolis Palus|Aeolis Palus]] in [[Astronomy:Gale (crater)|Gale Crater]], near Aeolis Mons (a.k.a. Mount Sharp),<ref name="IAU-20120516">{{cite web |author=USGS |title=Three New Names Approved for Features on Mars |url=https://astrogeology.usgs.gov/HotTopics/index.php?/archives/447-Three-New-Names-Approved-for-Features-on-Mars.html |date=May 16, 2012 |publisher=USGS |url-status=dead |archive-url=https://web.archive.org/web/20120728141903/http://astrogeology.usgs.gov/HotTopics/index.php?%2Farchives%2F447-Three-New-Names-Approved-for-Features-on-Mars.html |archive-date=July 28, 2012 |access-date=May 3, 2019 }}</ref><ref name="NASA-20120327">{{cite web|author=NASA Staff |title='Mount Sharp' on Mars Compared to Three Big Mountains on Earth |url=http://www.nasa.gov/mission_pages/msl/multimedia/pia15292-Fig2.html |date=March 27, 2012 |publisher=[[Organization:NASA|NASA]] |url-status=live |archive-url=https://web.archive.org/web/20120331212659/http://www.nasa.gov/mission_pages/msl/multimedia/pia15292-Fig2.html |archive-date=March 31, 2012 }}</ref><ref name="NASA-20120328">{{cite web|last=Agle |first=D. C. |title='Mount Sharp' On Mars Links Geology's Past and Future |url=http://www.nasa.gov/mission_pages/msl/news/msl20120328.html |date=March 28, 2012 |publisher=[[Organization:NASA|NASA]] |url-status=live |archive-url=https://web.archive.org/web/20120331022443/http://www.nasa.gov/mission_pages/msl/news/msl20120328.html |archive-date=March 31, 2012 }}</ref><ref name="Space-20120329">{{cite web|author=Staff |title=NASA's New Mars Rover Will Explore Towering 'Mount Sharp' |url=http://www.space.com/15097-mars-mountain-sharp-curiosity-rover.html |date=March 29, 2012 |publisher=Space.com |url-status=live |archive-url=https://web.archive.org/web/20120330033934/http://www.space.com/15097-mars-mountain-sharp-curiosity-rover.html |archive-date=March 30, 2012 }}</ref> on August 6, 2012.<ref name="Gale Crater">{{cite web|last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA's Next Mars Rover To Land At Gale Crater |date=July 22, 2011 |publisher=NASA JPL |url=http://www.jpl.nasa.gov/news/news.cfm?release=2011-222#1 |url-status=live |archive-url=https://web.archive.org/web/20110726081814/http://www.jpl.nasa.gov/news/news.cfm?release=2011-222 |archive-date=July 26, 2011 }}</ref><ref name="Gale Crater2">{{cite web|last=Chow |first=Dennis |title=NASA's Next Mars Rover to Land at Huge Gale Crater |url=http://www.space.com/12394-nasa-mars-rover-landing-site-unveiled.html |date=July 22, 2011 |publisher=Space.com |url-status=live |archive-url=https://web.archive.org/web/20110723190058/http://www.space.com/12394-nasa-mars-rover-landing-site-unveiled.html |archive-date=July 23, 2011 }}</ref><ref name="Gale Crater3">{{cite news|last=Amos |first=Jonathan |title=Mars rover aims for deep crater |date=July 22, 2011 |url=https://www.bbc.co.uk/news/science-environment-14249524 |work=BBC News |url-status=live |archive-url=https://web.archive.org/web/20110722170810/http://www.bbc.co.uk/news/science-environment-14249524 |archive-date=July 22, 2011 }}</ref>

On December 16, 2014, NASA reported the ''Curiosity'' rover detected a "tenfold spike", likely localized, in the amount of [[Chemistry:Methane|methane]] in the [[Astronomy:Atmosphere of Mars|Martian atmosphere]]. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere". Before and after that, readings averaged around one-tenth that level.<ref name="NASA-20141216-GW"/><ref name="NYT-20141216-KC"/> In addition, low levels of [[Chemistry:Chlorobenzene|chlorobenzene]] ({{chem|C|6|H|5|Cl}}), were detected in powder drilled from one of the rocks, named "[[Astronomy:List of rocks on Mars|Cumberland]]", analyzed by the ''Curiosity'' rover.<ref name="NASA-20141216-GW" /><ref name="NYT-20141216-KC" />

<gallery mode=packed heights=200px>
File:PIA19087-MarsCuriosityRover-GaleCrater-MethaneChart-20141216.png|[[Chemistry:Methane|Methane]] measurements in the [[Astronomy:Atmosphere of Mars|atmosphere of Mars]]<br />by the [[Engineering:Curiosity (rover)|''Curiosity'' rover]] (August 2012 to September 2014).
File:PIA19088-MarsCuriosityRover-MethaneSource-20141216.png|Methane (CH<sub>4</sub>) on Mars – potential sources and sinks.
File:PIA19090-MarsCuriosityRover-CumberlandRockAnalysis-20141216.png|Comparison of [[Chemistry:Organic compound|organic compound]]s in [[Astronomy:List of rocks on Mars|Martian rocks]] – [[Chemistry:Chlorobenzene|chlorobenzene]] levels were much higher in the "[[Astronomy:List of rocks on Mars|Cumberland]]" rock sample.
File:PIA19089-MarsCuriosityRover-CumberlandRockAnalysis-Organics-20141216.png|Detection of [[Chemistry:Organic compound|organic compound]]s in the "[[Astronomy:List of rocks on Mars|Cumberland]]" rock sample.
File:PIA17599-MarsCuriosityRover-CumberlandRock-Spectra-20121209.jpg|Sample analysis at Mars (SAM) of [[Astronomy:List of rocks on Mars#Curiosity|"Cumberland" rock]].<ref>{{cite web|url=http://mars.nasa.gov/multimedia/images/?ImageID=5769 |title=Volatiles Released by Heating Sample Powder from Martian Rock "Cumberland" {{!}} Mars Image |website=mars.nasa.gov |access-date=February 23, 2017 |url-status=live |archive-url=https://web.archive.org/web/20170224212015/http://mars.nasa.gov/multimedia/images/?ImageID=5769 |archive-date=February 24, 2017 }}</ref>
</gallery>

===Mars 2020===
{{Main|Astronomy:Mars 2020}}

The ''Mars 2020'' rover is a [[Astronomy:Mars|Mars]] [[Astronomy:Rover (space exploration)|planetary rover mission]] by [[Organization:NASA|NASA]], launched on July 30, 2020. It is intended to investigate an [[Astronomy:Astrobiology|astrobiologically]] relevant ancient environment on Mars, investigate its surface [[Astronomy:Geology of Mars|geological processes]] and history, including the assessment of its past [[Astronomy:Planetary habitability|habitability]] and potential for preservation of [[Astronomy:Biosignature|biosignature]]s within accessible geological materials.<ref name="Cowing_2020">{{cite web | url=http://spaceref.com/mars/science-definition-team-for-the-2020-mars-rover.html | archive-url=https://archive.today/20130203233556/http://spaceref.com/mars/science-definition-team-for-the-2020-mars-rover.html | url-status=dead | archive-date=February 3, 2013 | title=Science Definition Team for the 2020 Mars Rover | first=Keith | last=Cowing | date=December 21, 2012 | work=NASA | publisher=Science Ref }}</ref>

===Future astrobiology missions===
* [[Astronomy:ExoMars|ExoMars]] is a European-led multi-spacecraft programme currently under development by the European Space Agency (ESA) and the Russian Federal Space Agency for launch in 2016 and 2020.<ref name="ESA signed">{{cite news|title=ExoMars: ESA and Roscosmos set for Mars missions |date=March 14, 2013 |url=http://www.esa.int/Our_Activities/Space_Science/ExoMars_ESA_and_Roscosmos_set_for_Mars_missions |work=European Space Agency (ESA) |url-status=live |archive-url=https://web.archive.org/web/20130316135513/http://www.esa.int/Our_Activities/Space_Science/ExoMars_ESA_and_Roscosmos_set_for_Mars_missions |archive-date=March 16, 2013 }}</ref> Its primary scientific mission will be to search for possible [[Astronomy:Biosignature|biosignature]]s on Mars, past or present. A rover with a {{convert|2|m|ft|abbr=on}} core drill will be used to sample various depths beneath the surface where liquid water may be found and where microorganisms or organic biosignatures might survive cosmic radiation.<ref name="Wall">{{cite news|last=Wall |first=Mike |title=Q & A with Mars Life-Seeker Chris Carr |date=March 25, 2011 |url=http://www.space.com/11232-mars-life-evolution-carr-interview.html |work=Space.com |url-status=live |archive-url=https://web.archive.org/web/20130603003111/http://www.space.com/11232-mars-life-evolution-carr-interview.html |archive-date=June 3, 2013 }}</ref>
* [[Astronomy:Mars sample-return mission|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."<ref>[https://mepag.jpl.nasa.gov/reports/OCP_2014_final_report_docx.pdf Planning Considerations Related to the Organic Contamination of Martian Samples and Implications for the Mars 2020 Rover.] By the 2014 Organic Contamination Panel. NASA. September 24, 2014.</ref>

==Human colonization of Mars==
{{Main|Astronomy:Colonization of Mars}}

Some of the main reasons for colonizing Mars include economic interests, long-term scientific research best carried out by humans as opposed to robotic probes, and sheer curiosity. Surface conditions and the presence of [[Astronomy:Water on Mars|water on Mars]] make it arguably the most [[Astronomy:Planetary habitability|hospitable of the planets]] in the [[Astronomy:Solar System|Solar System]], other than Earth. Human colonization of Mars would require ''in situ'' resource utilization ([[Astronomy:In situ resource utilization|ISRU]]); A NASA report states that "applicable frontier technologies include robotics, machine intelligence, nanotechnology, synthetic biology, [[3D printing|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."<ref>{{cite journal|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005963.pdf|title=Frontier In-Situ Resource Utilization for Enabling Sustained Human Presence on Mars|date=April 2016|website=NASA|access-date=October 3, 2017|archive-url=https://web.archive.org/web/20170502223955/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160005963.pdf|archive-date=May 2, 2017|url-status=live|last1=Moses|first1=Robert W.|last2=Bushnell|first2=Dennis M.}}</ref><ref name="spaceref">{{cite web|url=http://www.spaceref.com/news/viewsr.html?pid=12418|title=House Science Committee Hearing Charter: Lunar Science & Resources: Future Options|work=spaceref.com|date=April 2004 |access-date=June 12, 2015|archive-url=https://archive.today/20120703160557/http://www.spaceref.com/news/viewsr.html?pid=12418|archive-date=July 3, 2012|url-status=live}}</ref><ref name="race">{{Cite news |title=Space Race Rekindled? Russia Shoots for Moon, Mars |work=ABC News |date=September 2, 2007 |url=https://www.abcnews.go.com/GMA/story?id=3550741&page=1 |access-date=September 2, 2007 |archive-url=https://web.archive.org/web/20170922134733/http://abcnews.go.com/GMA/story?id=3550741&page=1 |archive-date=September 22, 2017 |url-status=live }}</ref>

==Interactive Mars map==
{{Mars map}}

{{clear}}

==See also==
{{div col|colwidth=30em}}
* Areography (geography of Mars)
* {{annotated link|Biology:Astrobotany}}
* [[Astronomy:Carbonates on Mars|Carbonates on Mars]]
* {{annotated link|Biology:Chemical gardening}}
* [[Astronomy:Chloride-bearing deposits on Mars|Chloride-bearing deposits on Mars]]
* {{annotated link|Astronomy:Circumstellar habitable zone}}
* [[Chemistry:Composition of Mars|Composition of Mars]]
* [[Astronomy:Elysium Planitia|Elysium Planitia]]
* {{annotated link|Astronomy:Extraterrestrial life}}
* [[Astronomy:Fretted terrain|Fretted terrain]]
* [[Astronomy:Geology of Mars|Geology of Mars]]
* [[Astronomy:Glaciers on Mars|Glaciers on Mars]]
* [[Physics:Gravity of Mars|Gravity of Mars]]
* [[Astronomy:Groundwater on Mars|Groundwater on Mars]]
* [[Astronomy:Hecates Tholus|Hecates Tholus]]
* {{annotated link|Unsolved:Hypothetical types of biochemistry}}
* [[Astronomy:Lakes on Mars|Lakes on Mars]]
* [[Astronomy:List of quadrangles on Mars|List of quadrangles on Mars]]
* [[Astronomy:List of rocks on Mars|List of rocks on Mars]]
* [[Astronomy:Magnetic field of Mars|Magnetic field of Mars]]
* [[Astronomy:Mars Geyser Hopper|Mars Geyser Hopper]]
* {{annotated link|Astronomy:Mars habitability analogue environments on Earth}}
* {{Section link|Mars in fiction|Life on Mars}}
* Martian craters
* [[Astronomy:Martian dichotomy|Martian dichotomy]]
* Martian geyser
* Martian gullies
* [[Astronomy:Martian soil|Martian soil]]
* [[Astronomy:Mineralogy of Mars|Mineralogy of Mars]]
* [[Astronomy:Ore resources on Mars|Ore resources on Mars]]
* Scientific information from the Mars Exploration Rover mission
* [[Astronomy:Seasonal flows on warm Martian slopes|Seasonal flows on warm Martian slopes]]
* {{annotated link|Astronomy:Terraforming of Mars}}
* [[Earth:Vallis (planetary geology)|Vallis]]
* [[Astronomy:Water on Mars|Water on Mars]]
{{div col end}}

==References==
{{Reflist}}

==External links==
* [http://www.space.com/scienceastronomy/mars_microorganisms_040803.html Scientist says that life on Mars is likely today]
* [http://www.space.com/news/science_top10_041216.html Ancient salty sea on Mars wins as the most important scientific achievement of 2004 – Journal Science]
* [http://www2.jpl.nasa.gov/snc/nasa1.html Mars meteor found on Earth provides evidence that suggests microbial life once existed on Mars]
* [https://web.archive.org/web/20071010052259/http://www.sciam.com/article.cfm?chanID=sa006&articleID=00073A97-5745-1359-94FF83414B7F0000 Scientific American Magazine (November 2005 Issue) Did Life Come from Another World?]
* [http://www.monochrom.at/dark-dune-spots/ Audio interview about "Dark Dune Spots"]

{{Geography of Mars}}
{{Mars}}
{{Extraterrestrial life}}
{{Astrobiology}}

[[Category:Astrobiology]]
[[Category:Extraterrestrial life|Mars]]
[[Category:Mars]]
[[Category:Astronomical controversies]]

{{Sourceattribution|Life on Mars}}