Life on Mars#Methane

{{See also|Martian canals}}

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| caption2 = Mars canals illustrated by astronomer Percival Lowell, 1898

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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 Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does—but of nearly double the length owing to its much longer year. These observations led to increasing speculation that the darker albedo features were water and the brighter ones were land, whence followed speculation on whether Mars may be inhabited by some form of life.{{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}}

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

The 1907 book Is Mars Habitable? by British naturalist Alfred Russel Wallace was a reply to, and refutation of, Lowell's Mars and Its Canals. Wallace's book concluded that Mars "is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely uninhabitable."Wallace, Alfred R. (1907). [https://books.google.com/books?id=Gs1HAAAAIAAJ Is Mars Habitable? A Critical Examination of Professor Percival Lowell’s Book 'Mars and its Canals,' with an Alternative Explanation], p. 110, Macmillan. Historian Charles H. Smith refers to Wallace's book as one of the first works in the field of astrobiology.Smith, Charles H. (2018). [https://people.wku.edu/charles.smith/wallace/S730.htm Is Mars Habitable? (S730: 1907)]. The Alfred Russel Wallace Page. Western Kentucky University. Retrieved August 26, 2023.

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.{{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}} The influential observer Eugène Antoniadi used the 83-cm (32.6 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,Dollfus, A. (2010) "The first Pic du Midi photographs of Mars, 1909" [http://adsbit.harvard.edu//full/2010JBAA..120..240D/0000241.000.html] and the notion of canals began to fall out of favor.

{{See also|Colonization of Mars#Conditions for human habitation}}

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.{{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 |author-link6=Jennifer Eigenbrode|display-authors=9 |volume=1719 |issue=1719 |date=2013 |page=2185 |journal=44th Lunar and Planetary Science Conference }} 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 galactic cosmic radiation.{{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}}{{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 }}

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below. Similarly, for each group of parameters, the habitability threshold for each is to be determined. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.{{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 }} There are no full-Mars simulations published yet that include all of the biocidal factors combined. Furthermore, the possibility of Martian life having a far different biochemistry and habitability requirements than the terrestrial biosphere is an open question. A common hypothesis is methanogenic Martian life, and while such organisms exist on Earth too, they are exceptionally rare (while in itself numerous, there are not many environments on Earth life commonly known to humans exists where methanogenic life also can) and cannot survive in the majority of terrestrial environments that contain oxygen.{{Cite journal |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|doi=10.1126/science.ada0209 }}

Recent models have shown that, even with a dense CO₂ atmosphere, early Mars was colder than Earth has ever been.{{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 }}{{cite journal | last = Fairén | first = A. G. | display-authors=etal |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 }}{{cite journal | last = Fairén | first = A. G. | display-authors=etal |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 }}{{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 }} Transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid-late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars. Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid Hesperian onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both. 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.{{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}}

File:PIA19673-Mars-AlgaCrater-ImpactGlassDetected-MRO-20150608.jpg is thought to have deposits of impact glass that may have preserved ancient biosignatures, if present during the impact.{{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 }}]]

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

Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long-lasting hydrothermal system when ice is present in the crust. For example, a 130 km large crater could sustain an active hydrothermal system for up to 2 million years, that is, long enough for microscopic life to emerge, but unlikely to have progressed any further down the evolutionary path.[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.

Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors.{{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=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 }} The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment—perhaps a lake or an ancient streambed—that had neutral acidity and low salinity. On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life.{{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=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 }}{{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 }} The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and solar radiation,{{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 }}{{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 }} together strongly suggest that Mars could have had the environmental factors to support life.{{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 }} The assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively. The exploration of terrestrial analogues provide clues as to how and where best look for signs of life on Mars.{{cite journal | last = Fairén | first = A. G. | display-authors=etal | 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 }}

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.{{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 }}

On June 7, 2018, NASA announced that the Curiosity rover had discovered organic molecules in sedimentary rocks dating to three billion years old.{{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 }}{{cite journal |author=Eigenbrode, Jennifer L. |display-authors=etal |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 }} The detection of organic molecules in rocks indicate that some of the building blocks for life were present.{{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 }}{{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=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 }}

Research into how the conditions for habitability ended is ongoing. On October 7, 2024, NASA announced that the results of the previous three years of sampling onboard Curiosity suggested that based on high carbon-13 and oxygen-18 levels in the regolith, the early Martian atmosphere was less likely than previously thought, to be stable enough to support surface water hospitable to life, with rapid wetting-drying cycles and very high-salinity cryogenic brines providing potential explanations.{{cite journal | last1=Burtt | first1=David G. | last2=Stern | first2=Jennifer C. | last3=Webster | first3=Christopher R. | last4=Hofmann | first4=Amy E. | last5=Franz | first5=Heather B. | last6=Sutter | first6=Brad | last7=Thorpe | first7=Michael T. | last8=Kite | first8=Edwin S. | last9=Eigenbrode | first9=Jennifer L. | last10=Pavlov | first10=Alexander A. | last11=House | first11=Christopher H. | last12=Tutolo | first12=Benjamin M. | last13=Des Marais | first13=David J. | last14=Rampe | first14=Elizabeth B. | last15=McAdam | first15=Amy C. | last16=Malespin | first16=Charles A. | title=Highly enriched carbon and oxygen isotopes in carbonate-derived CO ₂ at Gale crater, Mars | journal=Proceedings of the National Academy of Sciences | volume=121 | issue=42 | date=October 7, 2024 | pages=e2321342121 | issn=0027-8424 | doi=10.1073/pnas.2321342121 | pmid=39374395 | pmc=11494307 | bibcode=2024PNAS..12121342B }}{{cite web |last=Steigerwald|first=William| title=NASA: New Insights into How Mars Became Uninhabitable | website=NASA Science | date=October 7, 2024 | url=https://science.nasa.gov/solar-system/planets/mars/nasa-new-insights-into-how-mars-became-uninhabitable/ | access-date=October 8, 2024}}

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.{{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}} 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 permafrost layer on Mars is only a couple of centimeters below the surface, and salty brines 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.{{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 }}{{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 }}{{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 }}

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.

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.{{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 }} Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years.{{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 }} As a result, the planet has been vulnerable to radiation from space for about 4 billion years.{{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 }}

Recent in-situ data from Curiosity rover indicates that ionizing radiation from galactic cosmic rays (GCR) and solar particle events (SPE) may not be a limiting factor in habitability assessments for present-day surface life on Mars. The level of 76 mGy per year measured by Curiosity is similar to levels inside the ISS.{{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 }}

Curiosity rover measured ionizing radiation levels of 76 mGy per year.{{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 }} 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.{{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 }} 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.{{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 }} 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.{{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 }}{{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 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 }}{{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 }} The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 meters below the planet's surface.

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

Data collected by the Radiation assessment detector (RAD) instrument on board the Curiosity rover revealed that the absorbed dose measured is 76 mGy/year at the surface, and that "ionizing radiation strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic molecules."{{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 }} Regardless of the source of Martian organic compounds (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfiguring with surrounding elements by ionizing charged particle radiation. These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic biosignatures as a function of depth as well as survival times of possible microbial or bacterial life forms left dormant beneath the surface. The report concludes that the in situ "surface measurements—and subsurface estimates—constrain the preservation window for Martian organic matter following exhumation and exposure to ionizing radiation in the top few meters of the Martian surface."

In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled and were associated with an aurora 25 times brighter than any observed earlier, due to a major, and unexpected, solar storm in the middle of the month.{{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 }}

On UV radiation, a 2014 report concludes{{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 |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 }} 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. The penetration depth of UV radiation into soils is in the sub-millimeter to millimeter range and depends on the properties of the soil.{{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 }} A recent study found that photosynthesis could occur within dusty ice exposed in the Martian mid-latitudes because the overlying dusty ice blocks the harmful ultraviolet radiation at Mars’ surface.{{Cite journal |last1=Khuller |first1=Aditya R. |last2=Warren |first2=Stephen G. |last3=Christensen |first3=Philip R. |last4=Clow |first4=Gary D. |date=2024-10-17 |title=Potential for photosynthesis on Mars within snow and ice |journal=Communications Earth & Environment |language=en |volume=5 |issue=1 |page=583 |doi=10.1038/s43247-024-01730-y |issn=2662-4435|doi-access=free |bibcode=2024ComEE...5..583K }}

The Martian regolith is known to contain a maximum of 0.5% (w/v) perchlorate (ClO₄⁻) that is toxic for most living organisms,{{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 }} but since they drastically lower the freezing point of water and a few extremophiles can use it as an energy source (see Perchlorates - Biology) and grow at concentrations of up to 30% (w/v) sodium perchlorate{{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}} by physiologically adapting to increasing perchlorate concentrations,{{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 |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 |bibcode=2022EnvMi..24.5051H }} it has prompted speculation of what their influence would be on habitability.{{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 }}{{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 }}{{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 }}{{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}}

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. In addition, two other compounds of the Martian surface, iron oxides and 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. It was also found that abraded silicates (quartz and basalt) lead to the formation of toxic reactive oxygen species.{{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 }} The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable."[https://time.com/4845251/mars-life-toxins-microbes/ Why Life on Mars May Be Impossible] . Jeffrey Kluger. Time - Science; July 6, 2017. This research demonstrates that the present-day surface is more uninhabitable than previously thought,[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 and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.[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

However, researcher 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, United States.{{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}} She has been studying the biosignatures of these microbes, and is hoping that the Mars Perseverance rover will find matching biosignatures at its Jezero Crater site.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.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.

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".).{{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|bibcode=2014AsBio..14..887R}}{{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 }} They were suspected as involving flowing brines back then.{{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 }}{{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=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 }}{{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=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 }}{{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 }}

The thermodynamic availability of water (water activity) strictly limits microbial propagation on Earth, particularly in hypersaline environments, and there are indications that the brine ionic strength is a barrier to the habitability of Mars. Experiments show that high 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."{{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 }}

After carbon, nitrogen is arguably the most important element needed for life. Thus, measurements of nitrate over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N₂) in the atmosphere at low levels, but this is not adequate to support nitrogen fixation for biological incorporation.{{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 }} Nitrogen in the form of nitrate could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed by thermal shock from impact or volcanic plume lightning on ancient Mars.{{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 }}

On March 24, 2015, NASA reported that the SAM instrument on the Curiosity rover detected nitrates by heating surface sediments. The nitrogen in nitrate is in a "fixed" state, meaning that it is in an oxidized form that can be used by living organisms. The discovery supports the notion that ancient Mars may have been hospitable for life.{{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=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 }}{{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 }} It is suspected that all nitrate on Mars is a relic, with no modern contribution.[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). Nitrate abundance ranges from non-detection to 681 ± 304 mg/kg in the samples examined until late 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.{{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 }}

In contrast, phosphate, one of the chemical nutrients thought to be essential for life, is readily available on Mars.{{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 }}

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).{{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 }} In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only Serratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0 °C, and CO₂-enriched anoxic atmospheres.

{{Main|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.{{cite web |author=Hays, Linda|display-authors=etal|title=Astrobiology Strategy 2015 |url=http://nai.nasa.gov/media/medialibrary/2016/04/NASA_Astrobiology_Strategy_2015_FINAL_041216.pdf |date=October 2015 |work=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 }} Liquid water cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.{{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 }}{{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 }} Liquid water does not appear at the surface itself,{{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}} but it could form in minuscule amounts around dust particles in snow heated by the Sun.{{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 }}{{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 }}{{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.{{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 }}{{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 }}{{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 }}{{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}}

{{multiple image|direction=horizontal|align=center|total_width=800|header=Mars - Utopia Planitia
Scalloped terrain led to the discovery of a large amount of underground ice
enough water to fill Lake Superior (November 22, 2016) |header_align=center

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|caption2=Map of terrain

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Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.{{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 }}{{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 }} A small amount of water vapor is present in the atmosphere.{{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 }} There are no bodies of liquid water on the Martian surface because the water vapor pressure is less than 1 Pa,{{Cite journal |last1=Fischer |first1=E. |last2=Martínez |first2=G. M. |last3=Rennó |first3=N. O. |last4=Tamppari |first4=L. K. |last5=Zent |first5=A. P. |date=November 2019 |title=Relative Humidity on Mars: New Results From the Phoenix TECP Sensor |journal=Journal of Geophysical Research: Planets |language=en |volume=124 |issue=11 |pages=2780–2792 |doi=10.1029/2019JE006080 |issn=2169-9097 |pmc=6988475 |pmid=32025455|bibcode=2019JGRE..124.2780F }} the 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 billion years ago,{{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 }} there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,{{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 }}{{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 }}{{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 }}{{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 }} including large oceans."Mapping Mars: Science, Imagination and the Birth of a World". Oliver Morton, 2002. {{ISBN|0-312-24551-3}}{{page needed|date=June 2013}}{{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 }}{{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 }}{{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 }}{{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 }}

File:History of Water on Mars.jpg

File:Mars-SubglacialWater-SouthPoleRegion-20180725.jpg
Site of Subglacial Water
(July 25, 2018)]]

It has been estimated that the primordial oceans on Mars would have covered between 36%{{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}} and 75% of the planet.{{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}} On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.{{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=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 }}{{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 }}{{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 }}

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

In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.{{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}}{{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 }} Additional similar images were published in 2006, taken by the Mars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images 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.{{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 }}{{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 }}{{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 }} Others suggest it may be liquid brine near the surface,{{cite web|url=https://www.voanews.com/a/nasa-finds-possible-signs-of-flowing-water-on-mars-126807133/143341.html |title=NASA Finds Possible Signs of Flowing Water on Mars |date=August 3, 2011 |publisher=Voice of America |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 }}{{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 }}{{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 }} but the exact source of the water and the mechanism behind its motion are not understood.{{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 }}

In July 2018, scientists reported the discovery of a subglacial lake on Mars, {{convert|1.5|km|mi|abbr=on}} below the southern polar ice cap, and extending sideways about {{convert|20|km|mi|abbr=on}}, the first known stable body of water on the planet.{{cite journal |author=Orosei, R. |display-authors=etal |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 }}{{cite news |last1=Chang |first1=Kenneth |last2=Overbye |first2=Dennis |author-link2=Dennis Overbye |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=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 }}{{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 }}{{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 }} The lake was discovered using the MARSIS radar on board the Mars Express orbiter, and the profiles were collected between May 2012 and December 2015.[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 }} 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. However, subsequent studies disagree on whether any liquid can be present at this depth without anomalous heating from the interior of the planet.{{Cite journal |last1=Sori |first1=Michael M. |last2=Bramson |first2=Ali M. |date=2019-02-16 |title=Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL080985 |journal=Geophysical Research Letters |language=en |volume=46 |issue=3 |pages=1222–1231 |doi=10.1029/2018GL080985 |bibcode=2019GeoRL..46.1222S |hdl=10150/633584 |issn=0094-8276|hdl-access=free }}{{Cite journal |last1=Mattei |first1=Elisabetta |last2=Pettinelli |first2=Elena |last3=Lauro |first3=Sebastian Emanuel |last4=Stillman |first4=David E. |last5=Cosciotti |first5=Barbara |last6=Marinangeli |first6=Lucia |last7=Tangari |first7=Anna Chiara |last8=Soldovieri |first8=Francesco |last9=Orosei |first9=Roberto |last10=Caprarelli |first10=Graziella |date=2022-02-01 |title=Assessing the role of clay and salts on the origin of MARSIS basal bright reflections |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X22000061 |journal=Earth and Planetary Science Letters |volume=579 |pages=117370 |doi=10.1016/j.epsl.2022.117370 |bibcode=2022E&PSL.57917370M |issn=0012-821X}} Instead, some studies propose that other factors may have led to radar signals resembling those containing liquid water, such as clays, or interference between layers of ice and dust.{{Cite journal |last1=Lalich |first1=D. E. |last2=Hayes |first2=A. G. |last3=Poggiali |first3=V. |date=October 2022 |title=Explaining Bright Radar Reflections Below The South Pole of Mars Without Liquid Water |url=https://www.nature.com/articles/s41550-022-01775-z |journal=Nature Astronomy |language=en |volume=6 |issue=10 |pages=1142–1146 |doi=10.1038/s41550-022-01775-z |arxiv=2107.03497 |bibcode=2022NatAs...6.1142L |issn=2397-3366}}{{Cite journal |last1=Bierson |first1=C. J. |last2=Tulaczyk |first2=S. |last3=Courville |first3=S. W. |last4=Putzig |first4=N. E. |date=2021-07-16 |title=Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093880 |journal=Geophysical Research Letters |language=en |volume=48 |issue=13 |doi=10.1029/2021GL093880 |bibcode=2021GeoRL..4893880B |issn=0094-8276}}{{Cite journal |last1=Smith |first1=I. B. |last2=Lalich |first2=D. E. |last3=Rezza |first3=C. |last4=Horgan |first4=B. H. N. |last5=Whitten |first5=J. L. |last6=Nerozzi |first6=S. |last7=Holt |first7=J. W. |date=August 2021 |title=A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093618 |journal=Geophysical Research Letters |language=en |volume=48 |issue=15 |doi=10.1029/2021GL093618 |bibcode=2021GeoRL..4893618S |issn=0094-8276}}

File:Spirit Mars Silica April 20 2007.jpg ]]

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area 90% rich in silica.{{cite press release|title=Mars Rover Spirit Unearths Surprise Evidence of Wetter Past |date=May 21, 2007 |publisher=Jet Propulsion Laboratory |url=http://www.nasa.gov/mission_pages/mer/mer-20070521.html |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 }} The feature is reminiscent of the effect of hot spring water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.{{Cite journal |title=Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile |journal=Nature Communications |volume=7|bibcode=2016NatCo...713554R |last1=Ruff |first1=Steven W. |last2=Farmer |first2=Jack D. |date=2016 |page=13554 |doi=10.1038/ncomms13554 |pmid=27853166 |pmc=5473637 |hdl=2286/R.I.44704 |hdl-access=free }}

Based on Earth analogs, hydrothermal systems on Mars would be highly attractive for their potential for preserving organic and inorganic biosignatures.{{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}}{{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}}{{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 }} For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.{{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 }}{{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}}{{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}}

In May 2017, evidence of the earliest known life on land on Earth may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia.{{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 }}{{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 }} These findings may be helpful in deciding where best to search for early signs of life on the planet Mars.

{{Main|Methane on Mars}}

Methane (CH₄) is chemically unstable in the current oxidizing atmosphere of Mars. It would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases. Therefore, a persistent presence of methane in the atmosphere may imply the existence of a source to continually replenish the gas.

Trace amounts of methane, at the level of several parts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003.{{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}}{{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}} 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.{{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 }} On June 7, 2018, NASA announced it has detected a seasonal variation of methane levels on Mars.{{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=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 }}{{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 }}{{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 |hdl=1874/366378 |s2cid=46952468 |hdl-access=free }}{{cite journal |author=Webster, Christopher R. |display-authors=etal |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 }}

The 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,{{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 }}{{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=etal |date=2017 |bibcode=2017DPS....4941801S |id=418.01}} as well as its decomposition products such as formaldehyde and methanol. As of May 2019, the Trace Gas Orbiter showed that the concentration of methane is under detectable level (< 0.05 ppbv).{{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}}{{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}}

File:PIA22328-MarsCuriosityRover-Methane-SeasonalCycle-20180607.jpg

The principal candidates for the origin of Mars's methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite formation, all of which produce H₂ that could then generate methane and other hydrocarbons via Fischer–Tropsch synthesis with CO and CO₂.{{cite conference|last=Mumma|first=Michael|display-authors=etal|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}} It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.{{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}} Although geologic sources of methane such as serpentinization are possible, the lack of current volcanism, hydrothermal activity or hotspots{{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 }} are not favorable for geologic methane.

Living microorganisms, such as methanogens, are another possible source, but no evidence for the presence of such organisms has been found on Mars,{{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}}{{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}} until June 2019 as methane was detected by the Curiosity rover.{{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}} Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source and carbon dioxide (CO₂) as their carbon source, so they could exist in subsurface environments on Mars.{{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 }} 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.{{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 }}

Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and in vitro experiments testing the growth of methanogenic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0wt% perchlorate salt.{{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}}

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.{{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}}{{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 }}

File:Martian Methane Map.jpg in the atmosphere of Mars in the Northern Hemisphere during summer