Editing Life on Mars (section)

===Present===
Conceivably, if life exists (or existed) on Mars, evidence of life could be found, or is best preserved, in the subsurface, away from present-day harsh surface conditions.<ref name="NASA strategy 2015">{{cite web|url=https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|title=NASA Astrobiology Strategy|year=2015|work=NASA|quote=Subsurface: Conceivably, if life exists (or existed) on Mars, an icy moon, or some other planetary body, evidence of that life could be found, or is best preserved, in the subsurface, away from present-day harsh surface processes.|access-date=November 12, 2017|archive-url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|archive-date=December 22, 2016|url-status=dead}}</ref> Present-day life on Mars, or its biosignatures, could occur kilometers below the surface, or in subsurface geothermal hot spots, or it could occur a few meters below the surface. The [[permafrost]] layer on Mars is only a couple of centimeters below the surface, and salty [[brine]]s can be liquid a few centimeters below that but not far down. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except after a sudden release of underground water.<ref name="Floods 2015">{{cite news |url=http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |archive-url=https://archive.today/20150929035120/http://spaceref.com/mars/regional-not-global-processes-led-to-huge-martian-floods.html |url-status=dead |archive-date=September 29, 2015 |title=Regional, Not Global, Processes Led to Huge Martian Floods |work=Planetary Science Institute |publisher=SpaceRef |date=September 11, 2015 |access-date=September 12, 2015 }}</ref><ref name=Jakosky2001>{{cite journal | last1=Jakosky | first1=B. M. | last2=Phillips | first2=R. J. | year = 2001 |title=Mars' volatile and climate history | journal=Nature |volume=412| issue=6843| pages=237–244| doi=10.1038/35084184 | pmid=11449285| bibcode=2001Natur.412..237J | doi-access=free }}</ref><ref name="Carr">{{cite book |title=The Surface of Mars |publisher=Cambridge Planetary Science Series (No. 6)|isbn=978-0-511-26688-1 |first=Michael H. |last=Carr <!-- United States Geological Survey, Menlo Park -->}}</ref>

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

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

Recent ''in-situ'' data from ''Curiosity'' rover indicates that [[ionizing radiation]] from [[Cosmic ray|galactic cosmic rays]] (GCR) and [[solar particle event]]s (SPE) may not be a limiting factor in habitability assessments for present-day surface life on Mars. The level of 76 mGy per year measured by ''Curiosity'' is similar to levels inside the ISS.<ref>{{cite journal|last1=Joanna Carver and Victoria Jaggard |title=Mars is safe from radiation – but the trip there isn't |journal=New Scientist |date=November 21, 2012 |url=https://www.newscientist.com/article/dn22520-mars-is-safe-from-radiation-but-the-trip-there-isnt/ |url-status=live |archive-url=https://web.archive.org/web/20170212165233/https://www.newscientist.com/article/dn22520-mars-is-safe-from-radiation-but-the-trip-there-isnt/ |archive-date=February 12, 2017 }}</ref> <!-- In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:<ref name="RummelBeaty2014">{{cite journal|last1=Rummel |first1=John D. |last2=Beaty |first2=David W. |last3=Jones |first3=Melissa A. |last4=Bakermans |first4=Corien |last5=Barlow |first5=Nadine G. |last6=Boston |first6=Penelope J. |last7=Chevrier |first7=Vincent F. |last8=Clark |first8=Benton C. |last9=de Vera |first9=Jean-Pierre P. |last10=Gough |first10=Raina V. |last11=Hallsworth |first11=John E. |last12=Head |first12=James W. |last13=Hipkin |first13=Victoria J. |last14=Kieft |first14=Thomas L. |last15=McEwen |first15=Alfred S. |last16=Mellon |first16=Michael T. |last17=Mikucki |first17=Jill A. |last18=Nicholson |first18=Wayne L. |last19=Omelon |first19=Christopher R. |last20=Peterson |first20=Ronald |last21=Roden |first21=Eric E. |last22=Sherwood Lollar |first22=Barbara |last23=Tanaka |first23=Kenneth L. |last24=Viola |first24=Donna |last25=Wray |first25=James J. |title=A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2) |journal=Astrobiology |volume=14 |issue=11 |year=2014 |pages=887–968 |issn=1531-1074 |doi=10.1089/ast.2014.1227 |pmid=25401393 |url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf|page=902 |url-status=live |archive-url=https://web.archive.org/web/20170213000635/https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf |archive-date=February 13, 2017 |bibcode = 2014AsBio..14..887R }}</ref> -->

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

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

In September 2017, NASA reported [[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, [[Coronal mass ejection|solar storm]] in the middle of the month.<ref name="PHYS-20170930">{{cite web |last=Scott |first=Jim |title=Large solar storm sparks global aurora and doubles radiation levels on the martian surface |url=https://phys.org/news/2017-09-large-solar-storm-global-aurora.html |date=September 30, 2017 |work=[[Phys.org]] |access-date=September 30, 2017 |archive-url=https://web.archive.org/web/20170930222447/https://phys.org/news/2017-09-large-solar-storm-global-aurora.html |archive-date=September 30, 2017 |url-status=live }}</ref>

====UV radiation====
On UV radiation, a 2014 report concludes<ref name="RummelBeaty2014">{{cite journal|last1=Rummel |first1=John D. |last2=Beaty |first2=David W. |last3=Jones |first3=Melissa A. |last4=Bakermans |first4=Corien |last5=Barlow |first5=Nadine G. |last6=Boston |first6=Penelope J. |last7=Chevrier |first7=Vincent F. |last8=Clark |first8=Benton C. |last9=de Vera |first9=Jean-Pierre P. |last10=Gough |first10=Raina V. |last11=Hallsworth |first11=John E. |last12=Head |first12=James W. |last13=Hipkin |first13=Victoria J. |last14=Kieft |first14=Thomas L. |last15=McEwen |first15=Alfred S. |last16=Mellon |first16=Michael T. |last17=Mikucki |first17=Jill A. |last18=Nicholson |first18=Wayne L. |last19=Omelon |first19=Christopher R. |last20=Peterson |first20=Ronald |last21=Roden |first21=Eric E. |last22=Sherwood Lollar |first22=Barbara |last23=Tanaka |first23=Kenneth L. |last24=Viola |first24=Donna |last25=Wray |first25=James J. |title=A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2) |journal=Astrobiology |volume=14 |issue=11 |year=2014 |pages=887–968 |issn=1531-1074 |doi=10.1089/ast.2014.1227 |pmid=25401393 |url=https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf<!--|page=902--> |url-status=live |archive-url=https://web.archive.org/web/20170213000635/https://www.researchgate.net/profile/David_Beaty/publication/268444482_A_new_analysis_of_Mars_Special_Regions_findings_of_the_second_MEPAG_Special_Regions_Science_Analysis_Group_SR-SAG2/links/547c9b0b0cf27ed9786229dd.pdf |archive-date=February 13, 2017 |bibcode = 2014AsBio..14..887R }}</ref> that "[T]he Martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms." In addition, laboratory research published in July 2017 demonstrated that UV irradiated perchlorates cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure.<ref name="bacteriocidal"/><ref name="oxides"/> The penetration depth of UV radiation into soils is in the sub-millimeter to millimeter range and depends on the properties of the soil.<ref name="oxides">{{cite journal | doi = 10.1017/S1473550416000331 | volume=16 | title=Shielding biomolecules from effects of radiation by Mars analogue minerals and soils | year=2017 | journal=International Journal of Astrobiology | pages=280–285 | last1 = Ertem | first1 = G. | last2 = Ertem | first2 = M. C. | last3 = McKay | first3 = C. P. | last4 = Hazen | first4 = R. M.| issue=3 | bibcode=2017IJAsB..16..280E | s2cid=125294279 }}</ref> 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.<ref>{{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 }}</ref>

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

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

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

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

The thermodynamic availability of water ([[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."<ref>{{cite journal | doi = 10.1089/ast.2015.1432 | volume=16 | title=Ionic Strength Is a Barrier to the Habitability of Mars | year=2016 | journal=Astrobiology | pages=427–442 | last1 = Fox-Powell | first1 = Mark G. | last2 = Hallsworth | first2 = John E. | last3 = Cousins | first3 = Claire R. | last4 = Cockell | first4 = Charles S.| issue=6 | pmid=27213516 | bibcode=2016AsBio..16..427F | hdl=10023/10912 | s2cid=4314602 | url=http://oro.open.ac.uk/73442/1/Fox-Powell_AST2015-1432_accepted.pdf | hdl-access=free }}</ref>