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[[File:Phoenix landing2.jpg|thumb
</ref>
''Goal I: determine if Mars ever supported life''
* ''Objective A: determine if environments having high potential for prior habitability and preservation of biosignatures contain evidence of past life.''
* '''''Objective B: determine if environments with high potential for current habitability and expression of biosignatures contain evidence of extant life.'''''
[[Life on Mars]] covers the more general topic of habitats for life through the entire history of Mars (with a brief summary for present day habitats).
Regions of the Martian surface where Earth life could potentially survive on Mars are called [[Planetary protection#Special regions|"Special regions"]] in [[Planetary protection]] discussions and require higher levels of sterilization for robotic missions<ref name="RummelBeaty2014" /><ref>[https://scholar.google.co.uk/scholar?hl=en&as_sdt=0%2C5&q=%22Special+region%22+Mars&btnG= Google scholar search for: "Special region Mars"]</ref>. It's possible that native Martian life might be able to survive in conditions that Earth life can't tolerate such as extremes of cold in liquid brines<ref>Schulze-Makuch, D. and Houtkooper, J.M., 2010. [https://meetingorganizer.copernicus.org/EPSC2010/EPSC2010-308.pdf A perchlorate strategy for extreme xerophilic life on Mars]. EPSC Abstracts, 5, pp.EPSC2010-308.</ref>.
Until 2008, many scientists believed that water ''"does not and cannot exist on the surface of Mars today"''<ref name="LevinMarsLifeIdea" />. There are only five regions on present day Mars where liquid fresh water could potentially form, in the Amazonis, Chryse and Elysium Planitia, and the Hellas and Argyre Basins, but even there, in those deep depressions, the water would be close to its boiling point of 10 °C. If any water formed it would soon evaporate<ref name="Hellas">{{cite web|title=Extracts from "Making a Splash on Mars"|url=http://lasp.colorado.edu/home/wp-content/uploads/2013/06/Mars_Articles_20130617.pdf}}</ref>. The equatorial regions are also expected to be ice free, as ice is not long term stable at any depth within ± 30° of the equator, unless trapped by an impervious overlying layer<ref>Schorghofer, N. and Aharonson, O., 2005. [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2004JE002350 Stability and exchange of subsurface ice on Mars]. Journal of Geophysical Research: Planets, 110(E5)</ref>. Although salty water would be liquid at lower temperatures, most scientists had concluded that the conditions on Mars were too extreme for it to form at all. Amongst the few who continued to think Mars could be habitable was Gilbert Levin who was (and still is) of the view that his labeled release experiment on the [[Viking program|Viking landers]] may have found life on Mars in 1976<ref name="LevinMarsLifeIdea" />.
This changed in 2008 with the observations of the [[Phoenix (spacecraft) | Phoenix lander]]. It landed in what is thought to be an ancient ocean bed near the north pole, the first and so far the only spacecraft to land successfully in polar regions. It observed droplet-like features that formed on its landing legs<ref name="phoenix_droplets_2009" />. In December 2013, Nilton Renno<ref name="NiltonRennoFaculty" /> and his team using the Michigan Mars Environmental Chamber<ref name="MicheganMars">Fischer, E., Martinez, G., Elliott, H.M., Borlina, C. and Renno, N.O., 2013, December. [https://www.researchgate.net/publication/283504377_The_Michigan_Mars_Environmental_Chamber_Preliminary_Results_and_Capabilities The Michigan Mars Environmental Chamber: Preliminary Results and Capabilities]. In AGU Fall Meeting Abstracts (Vol. 2013, pp. P41C-1928).
</ref> were able to simulate the conditions at its landing site and the droplets. They formed salty brines within minutes when salt overlaid ice. The team concluded that suitable conditions for brine droplets may be widespread in the polar regions<ref name="salt_ice">[http://www.astrobio.net/news-brief/liquid-water-ice-salt-mars/ Liquid Water from Ice and Salt on Mars], Aaron L. Gronstal -Astrobiology Magazine (NASA), Jul 3, 2014</ref><ref name="salt_ice_paper">Fischer, E., Martínez, G.M., Elliott, H.M. and Rennó, N.O., 2014. [https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL060302 Experimental evidence for the formation of liquid saline water on Mars]. Geophysical research letters, 41(13), pp.4456-4462.</ref>. This is possible because the salts, especially perchlorates, act as an "antifreeze"<ref name="GoughChevrier2014" /> to keep the brines liquid at low temperatures. Nilton Renno talks about their results in this video
<youtube width="540" height="315">iLWv9UGwjdE</youtube>
: "Based on the results of our experiment, we expect this soft ice that can liquify perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. --- This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool ... So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring."
: transcript from 2 minutes into the video onwards
There are many other suggestions of potential surface microhabitats covered in this article, including some in the equatorial regions. If these exist, any extant life that might inhabit them faces additional challenges including:
* Ionizing radiation - this will sterilize any dormant life within 500,000 years of exposure on the surface of Mars<ref name="KminekBada2006" />.
* UV radiation - this is rapidly lethal to microbes, unless shielded<ref name="RummelBeaty2014" />
* Perchlorates - the soil (regolith) and dust contains between 0.5 and 1% of these reactive compounds<ref name="DavilaPerchlorates">Davila, A.F., Willson, D., Coates, J.D. and McKay, C.P., 2013. [https://www.researchgate.net/publication/242525435_Perchlorate_on_Mars_A_chemical_hazard_and_a_resource_for_humans Perchlorate on Mars: a chemical hazard and a resource for humans]. International Journal of Astrobiology, 12(4), pp.321-325.</ref>.
However, these conditions may not make the surface completely uninhabitable to microbial life.
* Ionizing radiation is sterilizing of dormant life, but 500 years of ionizing radiation would kill only 90% of even the most radiation-sensitive bacterium such as E. coli. This is a conclusion fo the NASA's Mars Exploration Program Analysis Group based on the [[Curiosity (rover)| Curiosity rover]]'s RAD measurements<ref name="RummelBeaty2014SpecialRegionsConclusion" />.
* UV is blocked by about 0.3 mm of surface soil<ref name="Mateo-Marti2014" />,a millimeter of dust<ref name="RummelBeaty2014" />, or protective pigments such as [[melanin]], [[parietin]] and [[usnic acid]]<ref name="Ustvedt" />.
* Perchlorates, though harmful to some forms of life, are metabolized by others<ref name="Oren">{{cite journal | doi = 10.2166/wst.2009.635 | volume=60 | issue=1 | pages=75–80 | title=Molecular assessment of salt-tolerant, perchlorate- and nitrate-reducing microbial cultures | year=2009 | journal=Water Science & Technology | last1 = Zuo | first1 = G. | last2 = Roberts | first2 = D. J. | last3 = Lehman | first3 = S. G. | last4 = Jackson | first4 = G. W. | last5 = Fox | first5 = G. E. | last6 = Willson | first6 = R. C.|pmid=24150694}}</ref>. Cassie Conley, the NASA planetary protection officer from 2006 to 2018, said of the perchlorates, ''“The environment on Mars potentially is basically one giant dinner plate for Earth organisms,”''<ref>{{cite news|last1=Chang|first1=Kenneth|title=Mars Is Pretty Clean. Her Job at NASA Is to Keep It That Way.|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}}</ref>.
So far, there are no confirmed habitats for Earth life on or beneath the surface of Mars. However there are several [[#Planned and proposed missions to search for present day life on Mars | planned and proposed spacecraft missions]] to search for these potential habitats There are many [[#Instruments designed to search for present day life on Mars "in situ" | instruments designed by astrobiologists to search directly for this life on Mars]]. The [[Urey instrument]]<ref>{{cite journal |title=Development and evaluation of a microdevice for amino acid biomarker detection and analysis on Mars |journal=Proceedings of the National Academy of Sciences |first1=Alison M. |last1=Skelley |first2=James R. |last2=Scherer |first3=Andrew D. |last3=Aubrey |first4=William H. |last4=Grover |first5=Robin H. C. |last5=Ivester |first6=Pascale |last6=Ehrenfreund |first7=Frank J. |last7=Grunthaner |first8=Jeffrey L. |last8=Bada |first9=Richard A. |last9=Mathies |display-authors=5 |volume=102 |issue=4 |pages=1041–1046 |date=January 2005 |doi=10.1073/pnas.0406798102 |pmc=545824 |pmid=15657130 |bibcode=2005PNAS..102.1041S}}</ref><ref>{{cite journal |title=The Urey Instrument: An Advanced In Situ Organic and Oxidant Detector for Mars Exploration |journal=[[Astrobiology (journal)|Astrobiology]] |first1=Andrew D. |last1=Aubrey |first2=John H. |last2=Chalmers |first3=Jeffrey L. |last3=Bada |first4=Frank J. |last4=Grunthaner |first5=Xenia |last5=Amashukeli |first6=Peter |last6=Willis |first7=Alison M. |last7=Skelley |first8=Richard A. |last8=Mathies |last9=''et al.'' |first9=Richard C. |last10=Zent |first10=Aaron P. |last11=Ehrenfreund |first11=Pascale |last12=Amundson |first12=Ron |last13=Glavin |first13=Daniel P. |last14=Botta |first14=Oliver |last15=Barron |first15=Laurence |last16=Blaney |first16=Diana L. |last17=Clark |first17=Benton C. |last18=Coleman |first18=Max |last19=Hofmann |first19=Beda A. |last20=Josset |first20=Jean-Luc |last21=Rettberg |first21=Petra |last22=Ride |first22=Sally |last23=Robert |first23=François |last24=Sephton |first24=Mark A. |last25=Yen |first25=Albert |display-authors=5 |volume=8 |issue=3 |pages=583–595 |date=June 2008 |doi=10.1089/ast.2007.0169 |bibcode=2008AsBio...8..583K |pmid=18680409}}</ref> and Life Marker Chip<ref>{{cite conference |title=The life marker chip for the Exomars mission |conference=2011 ICO International Conference on Information Photonics. 18–20 May 2011. Ottawa, Ontario. |first1=A. |last1=Leinse |first2=H. |last2=Leeuwis |first3=A. |last3=Prak |first4=R. G. |last4=Heideman |first5=A. |last5=Borst |pages=1–2 |doi=10.1109/ICO-IP.2011.5953740 |isbn=978-1-61284-315-5}}</ref><ref>{{cite journal |title=In situ biomarkers and the Life Marker Chip |journal=[[Astronomy & Geophysics]] |first=Zita |last=Martins |volume=52 |issue=1 |pages=1.34–1.35 |year=2011 |doi=10.1111/j.1468-4004.2011.52134.x |bibcode=2011A&G....52a..34M}}</ref><ref>{{cite journal |title=Development status of the life marker chip instrument for ExoMars |journal=[[Planetary and Space Science]] |first1=Mark R. |last1=Sims |first2=David C. |last2=Cullen |first3=Catherine S. |last3=Rix |first4=Alan |last4=Buckley |first5=Mariliza |last5=Derveni |first6=Daniel |last6=Evans |first7=Luis Miguel |last7=García-Con |first8=Andrew |last8=Rhodes |last9=''et al.'' |first9=Carla C. |last10=Stefinovic |first10=Marijan |last11=Sephton |first11=Mark A. |last12=Court |first12=Richard W. |last13=Bulloch |first13=Christopher |last14=Kitchingman |first14=Ian |last15=Ali |first15=Zeshan |last16=Pullan |first16=Derek |last17=Holt |first17=John |last18=Blake |first18=Oliver |last19=Sykes |first19=Jonathan |last20=Samara-Ratna |first20=Piyal |last21=Canali |first21=Massimiliano |last22=Borst |first22=Guus |last23=Leeuwis |first23=Henk |last24=Prak |first24=Albert |last25=Norfini |first25=Aleandro |last26=Geraci |first26=Ennio |last27=Tavanti |first27=Marco |last28=Brucato |first28=John |last29=Holm |first29=Nils |display-authors=5 |volume=72 |issue=1 |pages=129–137 |date=November 2012 |doi=10.1016/j.pss.2012.04.007 |bibcode=2012P&SS...72..129S}}</ref> separately got into the manifest for [[ExoMars]] but were later [[ExoMars#De-scoped instruments|de-scoped]]. The first and only dedicated astrobiology missions to Mars were the two [[Viking program|Viking landers]], ''[[Viking 1]]'' and ''[[Viking 2]]'' in 1976.
This article focuses on the few places on Mars where microhabitats or deep subsurface habitats may be possible. Most features and processes on Mars are not thought to be associated with life.
== Do these habitats exist? ==
The sites with the best evidence for brines to date include:
* The Phoenix leg droplets mentioned in the lede - the researchers are optimistic that some may be habitable - see [[#Phoenix observations]]
* A possible lake discovered by radar 1.5 kilometers below the ice of the Southern polar plain of Mars [[Planum Australe]].<ref name=SupPolarIceLake/> - thought to be cold and salty, not known if it is habitable - see [[#Ice covered lakes in polar regions]].
* Dark streaks that form seasonally on steep slopes - the hydrated salts suggests brines are involved in their formation - if these brines exist, it's not known if they are habitable - see [[#Warm Seasonal flows (Recurrent Slope Lineae)]]
* [[#Temporary liquid brines forming every night at depths down to 15 cm below the surface of equatorial sand dunes]]<ref name=Rincon/> - thought by the researchers not to be habitable for Earth life
* [[#Flow like features]] - may be multiple causes, but for the particular case of Richardson's crater the two main hypotheses involve liquid water.
If these brines do exist, they could still be outside the range of conditions that life can inhabit. Then there's also the possibility of life that survives without liquid water on Mars:
* [[#Life able to take up water from the high (70-75%) night time humidity of the Mars atmosphere]] - if this habitat exists then large areas of the surface may be habitable, especially at low elevations.
Some scientists continue to regard the surface of Mars as uninhabitable. <ref name="Morais" /><ref name="PlaxcoGross2011_2">{{cite book |first=Kevin W. |last=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA285 |date=2011-08-12 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=285–286 |accessdate=2013-07-16 }}</ref><ref name="Quine2013" />. Others treat it it is an open question whether there are temporary habitats that could be recolonized from below,<ref name="Westall" />, or inhabited continuously on or near the surface<ref name="MorozovaMöhlmann2006" /><ref name="Crisler" /><ref name="Kilmer" /><ref name="Rummel" /><ref name="Davila" /><ref name="Fairen" /><ref name="RummelConley" />. Others say that it is likely that some parts of the Mars surface are already habitable for some lichens and [[cyanobacteria]] ("blue-green algae"), and that they can do this in the absence of liquid water, taking advantage of the night time humidity<ref name="DLRLichenHabitable" /><ref name="ZakharovaMarzban2014" />. Finally, a small minority of astrobiologists say that there is a strong possibility that present day life has already been detected on present day Mars with the Viking Labeled Release experiments<ref name="JosephMiller" /><ref name="Bianciardi-2012" /><ref name="Levin2016" />. This would mean that much of the Martian surface is not only habitable but actually inhabited by some form of life. See [[# Views on the possibility of present day life on or near the surface]].
If these habitats do exist they could be inhabited. Life could have evolved on Mars in the past, as there is evidence that it was far more habitable in the past. There is evidence of an early [[Mars ocean hypothesis| Mars ocean]] covering most of the northern hemisphere<ref>{{cite journal | last1 = DiAchille | first1 = G | last2 = Hynek | first2 = B. | year = 2010 | title = Ancient ocean on Mars supported by global distribution of deltas and valleys. nat | url = | journal = Geosci | volume = 3 | issue = 7| pages = 459–463 | doi = 10.1038/ngeo891 | bibcode=2010NatGe...3..459D}}</ref><ref>{{cite journal | last1 = DiBiasse | first1 = | last2 = Limaye | first2 = A. | last3 = Scheingross | first3 = J. | last4 = Fischer | first4 = W. | last5 = Lamb | first5 = M. | year = 2013 | title = Deltic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars | url = | journal = [[Journal Of Geophysical Research: Planets]] | volume = 118 | issue = | pages = 1285–1302 }}</ref>, and in December 2014, Curiosity scientists presented evidence that [[Lakes on Mars#Gale Crater|Gale Crater]] once contained a huge lake that was filled and evaporated many times.<ref name="NASA-20141208">{{cite web |last=Brown |first=Dwayne |last2=Webster |first2=Guy |title=Release 14-326 - NASA’s Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape |url=http://www.nasa.gov/press/2014/december/nasa-s-curiosity-rover-finds-clues-to-how-water-helped-shape-martian-landscape/ |date=8 December 2014 |work=[[NASA]] |accessdate=8 December 2014}}</ref><ref name="NYT-20141208">{{cite news |last=Kaufmann |first=Marc |title=(Stronger) Signs of Life on Mars |url=https://www.nytimes.com/2014/12/09/science/-stronger-signs-of-life-on-mars.html |date=8 December 2014 |work=[[New York Times]] |accessdate=8 December 2014}}</ref><ref name="sciencedaily2">{{cite web|url=https://www.sciencedaily.com/releases/2014/12/141208122903.htm|archive-url=https://web.archive.org/web/20141213204457/https://www.sciencedaily.com/releases/2014/12/141208122903.htm |archive-date=2014-12-13 |dead-url=yes|title=NASA's Curiosity rover finds clues to how water helped shape Martian landscape -- ScienceDaily|accessdate=4 July 2015}}</ref><ref name="nasa">{{cite web|url=http://www.jpl.nasa.gov/video/details.php?id=1346|title=JPL | Videos | The Making of Mount Sharp|publisher=jpl.nasa.gov|accessdate=4 July 2015}}</ref><ref name="nasa2">{{cite web|url=http://www.jpl.nasa.gov/news/news.php?feature=4398|title=JPL | News | NASA's Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape|publisher=jpl.nasa.gov|accessdate=4 July 2015}}</ref><ref name="usgs">{{cite web|url=https://pubs.er.usgs.gov/publication/70047207|title=Martian fluvial conglomerates at Gale Crater|publisher=pubs.er.usgs.gov|accessdate=4 July 2015}}</ref><ref>{{cite journal |last1=Williams |first1=R. |display-authors=etal |year=2013 |title=Martian fluvial conglomerates at Gale Crater |url=|journal=Science |volume= 340| issue=6136|pages= 1068–1072| doi=10.1126/science.1237317 |pmid=23723230|bibcode=2013Sci...340.1068W}}</ref>. This lake may have been habitable for life<ref>{{cite web |last1=Doyle - |first1=Amanda |title=Ancient Lake On Mars Was Hospitable Enough To Support Life |url=https://www.astrobio.net/news-exclusive/ancient-lake-mars-hospitable-enough-support-life/ |website=NASA Astrobiology Magazine |date=Sep 18, 2017}}</ref>. For more on this see [[Life on Mars]].
The habitats could also exist and be uninhabited, a possibility investigated by Charles Cockell in a series of papers. See [[#Uninhabited habitats|Uninhabited habitats]]
== Conferences on the topic of present day habitats for life on Mars ==
* 2013, February 4-6, conference on the ''Present Day Habitability of Mars'' was held in 2013 in UCLA.<ref>{{cite web|last1=David Paige and Charles Cockell|title=Report to MEPAG on The Present-Day Habitability of Mars Workshop|url=https://mepag.jpl.nasa.gov/meeting/2013-02/present_day_habitability_mepag_report.pdf}}</ref><ref>{{cite web|last1=CASE|first1=ELIZABETH|title=UCLA holds Mars habitability conference|url=http://dailybruin.com/2013/02/07/ucla-holds-mars-habitability-conference/|website=Daily Bruin}}</ref><ref name="PresentDayHabitability">{{cite web|author1=UCLA Institute for Planets and Exoplanets, The UK Center for Astrobiology and the NASA Astrobiology Institute|title=The Present-Day Habitability of Mars 2013 - Includes link to video recordings of the talks which you can stream online|url=http://planets.ucla.edu/meetings/past-meetings/pdhm2013/|website=UCLA Institute for Planets and Exoplanets|date=February 4–6, 2013}}</ref>.
* 2017, April 24-29 conference subsession topics '''''Biosignature Detection on Mars: Where, What, When, Why, and How?''''', '''''"Modern Mars Habitability"''''', and a third one that was about both past and present life, '''''Modern and Ancient Biosignatures and the Search for Life on Mars''''', with a total of 60 presentations each of 15 minutes, in Mesa, Arizona, organized by the NASA Ames Research Center, and LPL, University of Arizona, as part of the Astrobiology Science Conference 2017 <ref name=modernmarshabitability>[https://www.hou.usra.edu/meetings/abscicon2017/program-abstracts/topics/index.shtml#solarSystem Session Topics] - ArbSciCon 2017:
*Theme: Solar System Sites
*Session: Mars
*Subsession: Habitability
*Topic: Modern Mars Habitability
*Summary:
{{quote|Recent discoveries on Mars, including recurring slope lineae, ground ice, and active gully formation, have been interpreted as indications for the transient presence of water. The potential for liquid water on Mars has profound implications for the habitability of the modern Mars environment. This session solicits papers that examine the evidence for habitable environments on Mars, present results about life in analogs to these environments, discuss hypotheses to explain the active processes, evaluate issues for planetary protection, and explore the implications for future explorations of Mars.}}
*Theme: Solar System Sites
*Session: Mars
*Subsession: Biomarkers
*Topic: Biosignature Detection on Mars: Where, What, When, Why, and How?
*Summary:
{{quote|Finding evidence of extant life on Mars would be a watershed event. We have evidence on Mars for many environments that may have been habitable in the past, but the range of possible biogeochemistries those environments allow, the co-evolution of those environments with life, the specific niches that are most likely to host detectable biosignatures, and the path forward to explore those environments are still key unknowns. We invite contributions that (1) explore the succession of physical and environmental processes and their combination on Early Mars, (2) evaluate (ideally quantitatively!) the geo/environmental context of potential sites for biological exploration of Mars, (3) detail the most promising locations, instrument concepts, and strategies for investigating these ecosystems, (4) define the relevant objects, substances or patterns that could serve as definitive biosignatures for martian life, and (5) investigate metabolisms, survival strategies, and energy sources that may be relevant to the search for biosignatures on Mars.}}
*Theme: Solar System Sites
*Session: Mars
*Subsession: Biomarkers
*Topic: Modern and Ancient Biosignatures and the Search for Life on Mars
*Short Title (listed on abstract submission form): Modern and Ancient Biosignatures and the Search for Life on Mars
*Organizers: Andrew Czaja (University of Cincinnati), andrew.czaja@uc.edu, Scott Perl (JPL, USC), scott.m.perl@jpl.nasa.gov , Jeff Havig (University of Cincinnati), jeffhavig@gmail.com, and Andrew Gangidine (University of Cincinnati), agangidine@gmail.com
*Summary:
{{quote|The burden of proof for confirming the existence of life outside of our planet will be unprecedented in scientific history. Finding extraterrestrial microorganisms (whether fossil or extant) would provide the most direct evidence of life. Given planetary protection concerns, we are more likely to sample fossil microorganisms, but the biogenicity of ancient terrestrial microfossils is greatly debated owing to often poor preservation. Thus, other biosignatures are typically required to establish the biogenicity of putative ancient microfossils and other microbial structures. By developing additional novel biosignatures and combining multiple techniques for establishing biogenicity, we can find evidence of life that is more convincing. Such techniques would provide invaluable tools for the search for extraterrestrial life. This session seeks to highlight work being done to develop novel biosignatures or to use established biosignatures to search for new evidence of early life on Earth and/or past or present life on Mars.}}
</ref>.
There were two main topics focused on the search for extant live in possible present day habitats for life,
{{cot|Main topics relevant to search for extant live in possible present day habitats for life on Mars}}
'''''Biosignature Detection on Mars: Where, What, When, Why, and How?'''''
{{quote|Finding evidence of extant life on Mars would be a watershed event. We have evidence on Mars for many environments that may have been habitable in the past, but the range of possible biogeochemistries those environments allow, the co-evolution of those environments with life, the specific niches that are most likely to host detectable biosignatures, and the path forward to explore those environments are still key unknowns. We invite contributions that (1) explore the succession of physical and environmental processes and their combination on Early Mars, (2) evaluate (ideally quantitatively!) the geo/environmental context of potential sites for biological exploration of Mars, (3) detail the most promising locations, instrument concepts, and strategies for investigating these ecosystems, (4) define the relevant objects, substances or patterns that could serve as definitive biosignatures for martian life, and (5) investigate metabolisms, survival strategies, and energy sources that may be relevant to the search for biosignatures on Mars.}}
Monday, April 24, 2017 Poster session with twelve presentations, 8:00–9:00 p.m: [http://www.hou.usra.edu/meetings/abscicon2017/pdf/sess226.pdf]<br>
Tuesday, April 25, 2017 New technologies and techniques: life detection: biosignature detection on Mars: strategies and analog studies to guide Mars 2020 and Exomars<br>
10:15 a.m. (8 speakers)[https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess306.pdf]<br>
Biosignature detection on Mars II: analogue exploration<br>
1:30 p.m. (10 speakers) [https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess355.pdf]<br>
Biosignature detection on Mars III: Habitability studies<br>
4:15 p.m (six speakers)[http://www.hou.usra.edu/meetings/abscicon2017/pdf/sess364.pdf]
'''''"Modern Mars Habitability"'''''
{{quote|Recent discoveries on Mars, including recurring slope lineae, ground ice, and active gully formation, have been interpreted as indications for the transient presence of water. The potential for liquid water on Mars has profound implications for the habitability of the modern Mars environment. This session solicits papers that examine the evidence for habitable environments on Mars, present results about life in analogs to these environments, discuss hypotheses to explain the active processes, evaluate issues for planetary protection, and explore the implications for future explorations of Mars.}}
Poster session Monday, April 24, 9 participants [https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess227.pdf]
Wednesday, April 26, 2017 10:15 a.m. to 12:15 pm: Modern Mars habitability I (8 speakers, each 15 minutes).[https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess406.pdf]<br>
1:30 pm to 3.45 pm Modern Mars habitability II (ten speakers)
[https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess454.pdf]
There were other sessions relevant to the topic, though not particularly focused on extant life including:
'''''Modern and Ancient Biosignatures and the Search for Life on Mars'''''
{{quote|The burden of proof for confirming the existence of life outside of our planet will be unprecedented in scientific history. Finding extraterrestrial microorganisms (whether fossil or extant) would provide the most direct evidence of life. Given planetary protection concerns, we are more likely to sample fossil microorganisms, but the biogenicity of ancient terrestrial microfossils is greatly debated owing to often poor preservation. Thus, other biosignatures are typically required to establish the biogenicity of putative ancient microfossils and other microbial structures. By developing additional novel biosignatures and combining multiple techniques for establishing biogenicity, we can find evidence of life that is more convincing. Such techniques would provide invaluable tools for the search for extraterrestrial life. This session seeks to highlight work being done to develop novel biosignatures or to use established biosignatures to search for new evidence of early life on Earth and/or past or present life on Mars.}}
Modern and ancient biosignatures and the search for life on Mars I 10:15 a.m.
8 speakers. This was about both past and present life detection
Monday, April 24, 2017 7 - 8 pm, Poster session, 13 speakers[https://www.hou.usra.edu/meetings/abscicon2017/pdf/sess215.pdf]<br>
Wednesday April 26, Modern and ancient biosignatures and the search for life on Mars II
1:30 p.m. to 3:45 p.m
10 speakers, about both past and present life detection
[http://www.hou.usra.edu/meetings/abscicon2017/pdf/sess655.pdf] <br>
{{cob}}
* 2019, November 5-8 (originally scheduled for January 29 - February 1): ''Mars Extant Life: What’s Next?''" to discuss the "numerous extant life hypotheses that have been advanced over the years and that have evolved in response to discoveries by on-going Mars missions."<ref>November 5–8, 2019 at the National Cave and Karst Research Institute, 400-1 Cascades Ave., Carlsbad, New Mexico.</ref><ref>[https://www.hou.usra.edu/meetings/lifeonmars2019/ Mars Extant Life: What’s Next? scheduled for January 29–February 1, 2019] at the National Cave and Karst Research Institute, 400-1 Cascades Ave., Carlsbad, New Mexico.</ref>
==Mars surface conditions simulation chambers==
These chambers simulate the Martian day night cycle and other conditions of the Martian surface, with the goal to investigate the present day habitability of Mars. It's especially important to simulate the temperature and pressure variations because, though the amount of water vapour in the Mars atmosphere is low, at night the atmosphere becomes so cold that the relative humidity approaches 100%, which is of significance for any life that may be there. The chambers also have to simulate the Martian sunlight which has much less UV light filtered out than Earth sunlight. This is sterilizing over short timescales to any unprotected life directly exposed to the sunlight.
The Michigan Mars Environmental Chamber<ref name="MicheganMars" /> is run by Nilton Renno<ref name="NiltonRennoFaculty">[http://clasp.engin.umich.edu/people/nrenno/FACULTY Nilton Renno - Faculty page], Mitchigen State University - Honors, Awards and Accomplishments, and Publications, etc</ref> and his team:
{{quote|Introduction: We have developed the Michigan Mars Environmental Chamber (MMEC) to simulate the entire range of Martian surface and shallow subsurface conditions with respect to temperature, pressure, relative humidity, solar radiation and soil wetness. Our goal is to simulate the Martian diurnal cycle for equatorial as well as polar Martian conditions and test the hypothesis that salts known to exist in the Martian regolith can deliquesce and form brine pockets or layers by freeze-thaw cycles. Motivation: Liquid water is one of the necessary ingredients for the development of life as we know it. ... It has been shown that liquid brines are ubiquitous in the Martian polar regions and microbial communities have been seen to survive under similar conditions in Antarctica's Dry Valleys.|sign=|source=}}
The Mars Simulation Facility-Laboratory at the German Aerospace facilities (DLR) in Berlin<ref name="DLRMarsSimuulationFacilityLaboratory">[https://www.dlr.de/pf/en/desktopdefault.aspx/tabid-178/327_read-37506/ The Mars Simulation Facility-Laboratory], German Aerospace Center (DLR), Berlin</ref> is run by Jean-Pierre de Vera<ref name="deVeraProfile" /> used for numerous astrobiological Mars habitability studies<ref name=":0" />. as part of HOME (Habitability of Mars Environments)<ref name="HOME">[https://www.dlr.de/pf/en/desktopdefault.aspx/tabid-10881/19035_read-44438/ HOME: Habitability of Martian Environments: Exploring the Physiological and Environmental Limits of Life] Mars Simulation Facility-Laboratory at the German Aerospace faciliites (DLR) in Berlin run by Jean-Pierre de Vera</ref>:
==Viking observations - did Levin's labeled release experiment find life?==
[[File:Sagan large.jpg|thumb|400 px|Carl Sagan with a model of the Viking Lander in Death Valley California. Viking 1 and II were the first spacecraft to search for present day life on Mars.]]
The [[Viking program#Viking landers|Viking landers]] (operating on Mars from 1976-1982), are the only spacecraft so far to search directly for life on Mars. They landed in the equatorial regions of Mars. With our modern understanding of Mars, this would be a surprising location to find life, as the soil there is thought to be completely ice free to a depth of at least hundred meters, and possibly for a kilometer or more. It is not totally impossible though, as some scientists have suggested ways that life could exist even in such arid conditions, using the night time humidity of the atmosphere, and possibly in some way utilizing the frosts that form frequently in the mornings in equatorial regions.
The Viking results were intriguing, and inconclusive.<ref name="Levin 1976 1322–1329">{{cite journal |doi=10.1126/science.194.4271.1322 |pages=1322–1329 |title=Viking Labeled Release Biology Experiment: Interim Results |date=1976 |last=Levin |first=G. V. |last2=Straat |first2=P. A. |journal=Science |volume=194 |issue=4271 |pmid=17797094 |bibcode=1976Sci...194.1322L }}</ref> There has been much debate since then between a small number of scientists who think that the Viking missions did detect life,<ref name=VikingGasChromatograph/><ref name=JosephMiller/><ref name="Bianciardi-2012"/><ref name=Levin2016>Levin, G.V. and Straat, P.A., 2016. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6445182/ The case for extant life on Mars and its possible detection by the Viking labeled release experiment]. Astrobiology, 16(10), pp.798-810.
{{quote|"'''''It is concluded that extant life is a strong possibility, that abiotic interpretations of the LR data are not conclusive, and that, even setting our conclusion aside, biology should still be considered as an explanation for the LR experiment. Because of possible contamination of Mars by terrestrial microbes after Viking, we note that the LR data are the only data we will ever have on biologically pristine martian samples'''''"}}
</ref> and the majority of scientists who think that it did not.<ref name="PlaxcoGross2011_2"/><ref name=Quine2013/>
The Viking lander had three main biological experiments, but only one of these experiments produced positive results.<ref>[http://www.astrobio.net/news-exclusive/the-viking-files/ The Viking Files, Astrobiology Magazine (NASA) - May 29, 2003]</ref>
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Another paper published in 2012 uses cluster analysis [[cluster analysis]] and suggested once more that they may have detected biological activity.<ref name="Bianciardi-2012">{{cite journal |last=Bianciardi |first=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. |title=Complexity Analysis of the Viking Labeled Release Experiments |url=http://central.oak.go.kr/repository/journal/11315/HGJHC0_2012_v13n1_14.pdf|journal=IJASS |date=March 2012 |volume=13 |issue=1 |pages=14–26 |bibcode=2012IJASS..13...14B |doi=10.5139/IJASS.2012.13.1.14 |accessdate=2012-04-15|quote= These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars }}</ref><ref name="NGS-20120413">{{cite news |last=Than |first=Ker |title=Life on Mars Found by NASA's Viking Mission? |url=http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/ |date=2012-04-13 |work=[[National Geographic (magazine)|National Geographic]] |accessdate=2013-07-16 }}</ref>
On the other hand, a paper published in 2013 by Quinn has refined the chemical explanations suggested for the labeled release observations, using radiation damaged perchlorates. By simulating the radiation environment on Mars, he was able to duplicate radioactive <sup>14</sup>CO<sub>2</sub> emission from the sample.<ref name=Quine2013>[
In short, the findings are intriguing but there is no consensus yet on whether the correct interpretation is biological or chemical. Most scientists still favour the chemical explanation, though a few scientists have recently shown renewed interest in a possible biological explanation.
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===Droplets on the Phoenix legs===
Until 2008, most scientists thought that there was no possibility of liquid water on Mars for any length of time in the current conditions there. However, in 2008 through to 2009, droplets were observed on the landing legs of Phoenix.
[[File:Mars-water-droplets-phoenix-2008-bg.gif|Mars-water-droplets-phoenix-2008-bg]]
Unfortunately, it wasn't equipped to analyse them but the leading theory is that these were droplets of salty water.<ref name=phoenix_droplets_2009/> They were observed to grow,
These may have formed on mixtures of salt and ice that were thrown up onto its legs when it landed. Experiments by Nilton Renno's team in 2014 in Mars simulation chambers show that water can form droplets readily in Mars conditions on the interface between ice and calcium perchlorate salts. The droplets can form within minutes in Mars simulation conditions. This is the easiest way they have found to explain the observations.<ref name=salt_ice/><ref name=salt_ice_paper/>
===Phoenix isotope evidence of liquid water on the Mars surface in the recent geological past===
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* "Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at > 250 K, with <math>a_w</math> [water activity] unknown and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely"The "Special Regions" assessment says of them:<ref name="RummelBeaty2014"/>
: A study of RSLs in Eos Chasma shows that the features are consistent with dust cascades, since they terminate at slopes matching the stopping angle for granular flows of cohesionless dust, and they also ruled out formation of substantial quantities of crust‐forming evaporitic salt deposits, though the hydrated salts and seasonal nature continue to suggest some role for water in their formation (Dundas et al, 2017)<ref name=McEwan>McEwen, A.S., Ojha, L., Dundas, C.M., Mattson, S.S., Byrne, S., Wray, J.J., Cull, S.C., Murchie, S.L., Thomas, N. and Gulick, V.C., 2011. Seasonal flows on warm Martian slopes. Science, 333(6043), pp.740-743.</ref>.
: Difficulties with the dust explanation include the rapid fading away of the streaks at the end of the season, instead of the more usual decades, and a lack of an explanation of how the dust is resupplied year after year. Resupply also remains a major question for the models involving substantial amounts of liquid brines <ref name=Stillman>(Stillman quoted in David, L., 2017, Mars Flows: A Recurring Controversy, Leonard David's "Inside Outer Space" blog (space journalist)</ref>. A study of RSLs in the Valles Marineres finds that they seem to traverse bedrock rather than the regolith of other RSLs, and that if water is involved in their formation, then substantial amounts must be needed to sustain lengthening throughout the season <ref> Stillman, D.E., Michaels, T.I. and Grimm, R.E., 2017. Characteristics of the numerous and widespread recurring slope lineae (RSL) in Valles Marineris, Mars. Icarus, 285, pp.195-210.</ref>
<!-- this para is included from my own OSF preprint https://osf.io/kad38 - Robert Walker --->
==Sun warmed dust grains embedded in ice==
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This process could melt the ice for a few hours per day in the warmest days of summer, and melt a few mms of ice around each grain. For instance, Losiak, et al., modeled dust grains of basalt (2-200 µm in diameter) if exposed to full sunlight on the surface of the ice on the warmest days in summer, on the Northern polar ice cap, and say this about their model, in 2014: "For example, for solar constant 350 W/m2, emissivity 0.80, grain size 2 um, and thermal conductivity 0.4 W/mK melting lasts for ~300 minutes [5 hours] and result in melting of 6 mm of ice."<ref>[http://www.hou.usra.edu/meetings/metsoc2014/pdf/5314.pdf ICE MELTING BY RADIANTLY HEATED DUST GRAINS ON THE MARTIAN NORTHERN POLE] A. Losiak, L. Czechowski and M.A. Velbel, 77th Annual Meteoritical Society Meeting (2014)</ref> They developed this model as a hypothesis to explain presence of extensive deposits of gypsum in the Northern polar ice cap and the dune fields around it, and concluded that, since the atmospheric pressure there is just above the triple point, this mms thin layer of liquid water could persist for a significant period of time there around grains of basalt in the middle of the day in summer.
This process has been observed in Antarctica. On Mars, there could be enough water to create conditions for physical, chemical, and biological processes.<ref>[https://www.newscientist.com/article/mg20427373.700 Watery niche may foster life on Mars]. {{Subscription required|date=August 2011}}</ref><ref>{{cite web|author=Tudor 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=News.softpedia.com |date=2009-12-07 |accessdate=2011-08-20}}</ref><ref>Möhlmann, D., 2010. [https://adsabs.harvard.edu/pdf/2010IJAsB...9...45M The three types of liquid water in the surface of present Mars]. International Journal of Astrobiology, 9(1), pp.45-49.</ref>
==Flow like features==
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Möhlmann uses a solid state greenhouse effect in his model, similarly to the process that forms the geysers, but with translucent ice rather than dry ice as the solid state greenhouse layer.<ref name="LiquidWaterSnowIce">{{cite journal|url=http://www.sciencedirect.com/science/article/pii/S0019103509004539|last1=Möhlmann|first1=Diedrich T.F.|title=Temporary liquid water in upper snow/ice sub-surfaces on Mars?|journal=Icarus|volume=207|issue=1|year=2010|pages=140–148|issn=0019-1035|doi=10.1016/j.icarus.2009.11.013|bibcode=2010Icar..207..140M}}</ref>
<ref>Möhlmann, D.T.F., 2009, June. [https://www.lpi.usra.edu/meetings/hydrous2009/pdf/4001.pdf Liquid Interfacial and Melt-Water in the Upper Sub-Surface of Mars]. In Workshop on Modeling Martian Hydrous Environments (Vol. 1482, p. 48).
</ref>
[[File:JoekullsarlonBlueBlockOfIce.jpg|thumb|JoekullsarlonBlueBlockOfIce| 400px| Blue wall of an Iceberg on [[Jökulsárlón]], Iceland. On the Earth, [[Blue ice (glacial)|Blue ice]] like this forms as a result of air bubbles squeezed out of glacier ice. This has the right optical and thermal properties to act as a solid state greenhouse, trapping a layer of liquid water that forms 0.1 to 1 meters below the surface. In Möhlmann's model, if ice with similar optical and thermal properties forms on Mars, it could form a layer of liquid water centimeters to decimeters thick, which would form 5 - 10 cm below the surface.]]
In his model, first the ice forms a translucent layer - then as summer approaches, the solid state greenhouse effect raises the temperature of a layer below the surface to 0 °C, so melting it. This is a process familiar on the Earth for instance in Antarctica. On Earth, in similar conditions, the surface ice remains frozen, but a layer of liquid water forms from 0.1 to 1 meters below the surface. It forms preferentially in "blue ice".<ref>Nl, K., and T. SAND. [https://web.archive.org/web/20170322112720/http://www.igsoc.org:8080/journal/42/141/igs_journal_vol42_issue141_pg271-278.pdf "Melting, runoff and the formation of frozen lakes in a mixed snow and blue-ice field in Dronning Maud Land, Antarctica."], Journal of Glaciology, T'ol. 42, .\"0.141, 1996</ref>
On Mars, in his model, the melting layer is 5 to 10 cm below the surface. The liquid water layer starts off millimeters thick in their model, and can develop to be centimeters thick as the season progresses. The effect of the warming is cumulative over successive sols. Once formed, the liquid layer can persist overnight. Subsurface liquid water layers like this can form with surface temperatures as low as -56 °C.
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If the ice covers a heat absorbing layer at the right depth, the melted layer can form more rapidly, within a single sol, and can evolve to be tens of centimeters in thickness. In their model this starts as fresh water, insulated from the surface conditions by the overlaying ice layers - and then mixes with any salts to produce salty brines which would then flow beyond the edges to form the extending dark edges of the flow like features.
Later in the year, pressure can build up and cause formation of mini water geysers which may possibly explain the "white collars" that form around the flow like features towards the end of the season - in their model this is the result of liquid water erupting in mini water geysers and then freezing as white pure water ice.
This provides:
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===Interfacial liquid layers model===
Another model for these southern hemisphere features involves ULI water (undercooled liquid water) which forms as a thin layer over surfaces and can be liquid at well below the usual melting point of ice, at {{convert|190|K|C}} <ref>Kereszturi, A., Möhlmann, D., Berczi, S., Ganti, T., Horvath, A., Kuti, A., Pocs, T. and Szathmary, E., 2008, March. [https://www.lpi.usra.edu/meetings/lpsc2008/pdf/1555.pdf Analysis of possible interfacial water driven seepages on Mars]. In Lunar and Planetary Science Conference (Vol. 39, p. 1555)</ref>. In Mohlmann's sandwich model, then the interfacial water layer forms on the surfaces of solar heated grains in the ice, which then flows together down the slope. Calculations of downward flow of water shows that several litres a day of water could be supplied to the seepage flows in this way.<ref name="MartínezRenno2013DarkDuneSpots">{{cite journal|last1=Martínez|first1=G. M.|last2=Renno|first2=N. O.|title=Water and Brines on Mars: Current Evidence and Implications for MSL|journal=Space Science Reviews|volume=175|issue=1–4|year=2013|pages=29–51|issn=0038-6308|doi=10.1007/s11214-012-9956-3|bibcode=2013SSRv..175...29M}}</ref><ref name=Kereszturi2008/>
The idea then is that this ULI water would be the water source for liquid brines which then flow down the surface to form the features. ULI water can also be used by microbes directly - and although the conditions would be cold for Earth microbes, perhaps Martian microbes could use it at lower temperatures. <ref name=MöhlmannULIbiorelevant>Möhlmann, D.T., 2009. [https://www.ncbi.nlm.nih.gov/pubmed/19444970 Are nanometric films of liquid undercooled interfacial water bio-relevant?]. Cryobiology, 58(3), pp.256-261.</ref>
===Northern Hemisphere flow like features===
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For details see the Dark Dune Spots section of Nilton Renno's paper<ref name="MartínezRenno2013DarkDuneSpots"/> which also has images of the two types of feature as they progress through the season.
==
The lichens studied in these experiments have protection from UV light due to special pigments only found in lichens, such as parietin and antioxidants such as b-carotene in epilithic lichens. This gives them enough protection to tolerate the light levels in conditions of partial shade in the simulation chambers and make use of the light to photosynthesize. Indeed, UV protection pigments have been suggested as potential biomarkers to search for on Mars.<ref>"Solar radiation is the primary energy source for surface planetary life, so that pigments are fundamental components of any surface-dwelling organism. They may therefore have evolved in some form on Mars as they did on Earth." {{cite journal | doi = 10.1017/S1473550402001039 | volume=1 | pages=39 | title=Pigmentation as a survival strategy for ancient and modern photosynthetic microbes under high ultraviolet stress on planetary surfaces | year=2002 | journal=International Journal of Astrobiology | last1 = Wynn-Williams | first1 = D.D. | last2 = Edwards | first2 = H.G.M. | last3 = Newton | first3 = E.M. | last4 = Holder | first4 = J.M.| bibcode=2002IJAsB...1...39W }}</ref>
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An experiment on the ISS as part of [[EXPOSE#EXPOSE-E results|Expose-E]] in 2008-2009 showed that one lichen, Xanthoria elegans, retained a viability of 71% for the algae (photobiont) and 84% for the fungus (mycobiont) after 18 months in the ISS, in Mars surface simulation conditions, and the surviving cells returned to 99% photosynthetic capabilities on return to Earth. This was an experiment without the day night temperature cycles of Mars and the lichens were kept in a desiccated state so it didn't test their ability to survive in niche habitats on Mars. This greatly exceeded the post flight viability of any of the other organisms tested in the experiment.<ref name="Brandtde Vera2014">{{cite journal|url=http://elib.dlr.de/90411/1/Annette-Brandt-download.php.pdf|last1=Brandt|first1=Annette|last2=de Vera|first2=Jean-Pierre|last3=Onofri|first3=Silvano|last4=Ott|first4=Sieglinde|title=Viability of the lichen Xanthoria elegans and its symbionts after 18 months of space exposure and simulated Mars conditions on the ISS|journal=International Journal of Astrobiology|year=2014|pages=1–15|issn=1473-5504|doi=10.1017/S1473550414000214|volume=14|issue=3|bibcode=2015IJAsB..14..411B}}</ref>
Another study in 2014 by German aerospace DLR in a Mars simulation chamber used the lichen Pleopsidium chlorophanum. This lives in the most Mars like environmental conditions on Earth, at up to 2000 meters in Antarctica. It is able to cope with high UV, low temperatures and dryness. It is mainly found in cracks, where just a small amount of scattered light reaches it. This is probably adaptive behaviour to protect it from UV light and desiccation. It remains metabolically active in temperatures down to -20 C, and can absorb small amounts of liquid water in an environment with ice and snow.<ref name="DLRLichenHabitable"/>
As measured in Antarctica, the relative humidity in the lichen's niche microhabitat varies from 57 to 79% as the temperature varies from -6 to -8% and externally it varies from 23% to 46% as the external temperature varies from 8 to - 8 C.<ref name="DLRLichenHabitable"/>
In this experiment the temperature varied between +21 °C and -50 °C. Relative humidity is higher in cold air, for the same concentrations of water vapour, and as the temperature varied, the relative humidity varied between 0.1% and 75%. The atmosphere consisted of 5% CO<sub>2</sub>,4%N<sub>2</sub>, and 1% O<sub>2</sub> at 800 Pa or about 0.79% of Earth's sea level atmospheric pressure. This approximates conditions that are encountered in the equatorial and lower lattitude regions of Mars. <ref name="DLRLichenHabitable"/>
When exposed to full UV levels in a 34-day experiment in a Mars simulation chamber at DLR, the fungus component of the lichen Pleopsidium chlorophanum died, and it wasn't clear if the algae component was still photosynthesizing.<ref name="DLRLichenHabitable"/>
However, when partially shaded from the UV light, as
This is remarkable as the fungus is an aerobe, growing in an atmosphere with no appreciable amount of oxygen and 95% CO<sub>2</sub>. It seems that the algae provides it with enough oxygen to survive. The lichen was grown in Sulfatic Mars Regolith Simulant - igneous rock with composition similar to Mars meteorites, consisting of gabbro and olivine, to which quartz and anhydrous iron oxide hematite (the only thermodynamically stable iron oxide under present day Mars conditions) were added. It also contains gypsum and geothite, and was crushed to simulate the martian regolith. This was an ice free environment. They found that photosynthetic activity was strongly correlated with the beginning and the end of the simulated Martian day. Those are times when atmospheric water vapour could condense on the soil and be absorbed by it, and could probably also form cold brines with the salts in the simulated martian regolith. The pressure used for the experiment was 700 - 800 Pa, above the triple point of pure water at 600 Pa and consistent with the conditions measured by Curiosity in Gale crater.<ref name="
The experimenters concluded that it is likely that some lichens and cyanobacteria can adapt to Mars conditions, taking advantage of the night time humidity, and that it is possible that life from early Mars could have adapted to these conditions and still survive today in microniches on the surface.
In another experiment, by Kristina Zakharova et al., two species of microcolonial fungi – Cryomyces antarcticus and Knufia perforans - and a species of black yeasts–Exophiala jeanselmei were found to adapt and recover metabolic activity during exposure to a simulated Mars environment for 7 days. They depended on the temporary saturation of the atmosphere with water vapour like the lichens. The fungi didn't show any signs of stress reactions (such as creating unusual new proteins).
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There Cryomyces antarcticus is an extremophile fungi, one of several from Antarctic dry deserts. Knufia perforans is a fungi from hot arid environments, and Exophiala jeanselmei is a black yeast endolith closely related to human pathogens.
In this experiment, the temperature cycled between 21 °C and -50 and the relative humidity varied up to 70% at the lowest temperatures, with pressure 700 Pascals or about 0.69% of Earth sea level.
The experimenters concluded that these black fungi can survive in a Mars environment.<ref name="ZakharovaMarzban2014">{{cite journal|url=http://www.nature.com/srep/2014/140529/srep05114/full/srep05114.html|last1=Zakharova|first1=Kristina|last2=Marzban|first2=Gorji|last3=de Vera|first3=Jean-Pierre|last4=Lorek|first4=Andreas|last5=Sterflinger|first5=Katja|title=Protein patterns of black fungi under simulated Mars-like conditions|journal=Scientific Reports|volume=4|pages=5114|year=2014|issn=2045-2322|doi=10.1038/srep05114|pmid=24870977|pmc=4037706|quote="The results achieved from our study led to the conclusion that black microcolonial fungi can survive in Mars environment."|bibcode=2014NatSR...4E5114Z}}</ref>
==Deliquescing salts taking up moisture from the Mars atmosphere==
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Though there is little by way of water vapour in the Mars atmosphere, which is also a near vacuum - still it reaches 100% humidity at night due to the low nighttime temperatures. This effect creates the Martian morning frosts, which were observed by Viking in the extremely dry equatorial regions of Mars.
[[File:Ice on Mars Utopia Planitia (PIA00571).jpg|frame|center|600px|alt=Ice on Mars Utopia Planitia |Ice on Mars Utopia Planitia. These frosts formed every morning for about 100 days a year at the Viking location. Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground. But carbon dioxide, which makes up 95 percent of the Martian atmosphere, freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust.<br><br>The ice seen in this picture, is extremely thin, perhaps no more than one-thousandth of an inch thick. These frosts form due to the
The discovery of perchlorates raises the possibility of thin layers of salty brines that could form a short way below the surface by taking moisture from the atmosphere when the atmosphere is cooler. It is now thought that these could occur almost anywhere on Mars if the right mixtures of salts exist on the surface, even possibly in the hyper-arid equatorial regions. In the process of deliquescence, the humidity is taken directly from the atmosphere. It does not require the presence of ice on or near the surface.
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|}
As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO4 releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, using Mars analogue soil, this does not reduce the supercooling and can in some cases permit more supercooling.<ref name="TonerCatling2014">{{cite journal|url=http://faculty.washington.edu/dcatling/Toner2014_SupercoolSalts.pdf|last1=Toner|first1=J.D.|last2=Catling|first2=D.C.|last3=Light|first3=B.|title=The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars|journal=Icarus|volume=233|year=2014|pages=36–47|issn=0019-1035|doi=10.1016/j.icarus.2014.01.018|bibcode=2014Icar..233...36T}}</ref><ref name="GoughChevrier2014">{{cite journal|url=https://web.archive.org/web/20160304075907/http://comp.uark.edu/~vchevrie/sub/papers/Gough%20-%202014%20-%20EPSL%20-%20perchlorate%20chloride%20mixture%20deliquescence.pdf|last1=Gough|first1=R.V.|last2=Chevrier|first2=V.F.|last3=Tolbert|first3=M.A.|title=Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures|journal=Earth and Planetary Science Letters|volume=393|year=2014|pages=73–82|issn=0012-821X|doi=10.1016/j.epsl.2014.02.002|bibcode=2014E&PSL.393...73G}}</ref>
With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.
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The conditions for these liquid layers to form may include regions where there is no ice present on the surface such as the arid equatorial regions of Mars.<ref>{{cite web|last1=Matson|first1=John|title=The New Way to Look for Mars Life: Follow the Salt|url=http://blogs.scientificamerican.com/observations/2013/02/06/the-new-way-to-look-for-mars-life-follow-the-salt/|publisher=Scientific American|date=February 6, 2013}}</ref>
===
Researchers using data from Curiosity in April 2015
At night, the water activity is high enough for life, but it is too cold, and in the day time it is warm enough but too dry. The authors concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms.<ref name=Rincon>{{cite web|last1=Rincon Science editor|first1=Paul|title=Evidence of liquid water found on Mars|url=https://www.bbc.co.uk/news/science-environment-32287609|website=BBC News website|date=April 13, 2015}}</ref><ref name="Martín-TorresZorzano2015">{{cite journal|last1=Martín-Torres|first1=F. Javier|last2=Zorzano|first2=María-Paz|last3=Valentín-Serrano|first3=Patricia|last4=Harri|first4=Ari-Matti|last5=Genzer|first5=Maria|last6=Kemppinen|first6=Osku|last7=Rivera-Valentin|first7=Edgard G.|last8=Jun|first8=Insoo|last9=Wray|first9=James|last10=Bo Madsen|first10=Morten|last11=Goetz|first11=Walter|last12=McEwen|first12=Alfred S.|last13=Hardgrove|first13=Craig|last14=Renno|first14=Nilton|last15=Chevrier|first15=Vincent F.|last16=Mischna|first16=Michael|last17=Navarro-González|first17=Rafael|last18=Martínez-Frías|first18=Jesús|last19=Conrad|first19=Pamela|last20=McConnochie|first20=Tim|last21=Cockell|first21=Charles|last22=Berger|first22=Gilles|last23=R. Vasavada|first23=Ashwin|last24=Sumner|first24=Dawn|last25=Vaniman|first25=David|title=Transient liquid water and water activity at Gale crater on Mars|journal=Nature Geoscience|year=2015|issn=1752-0894|doi=10.1038/ngeo2412|volume=8|issue=5|pages=357–361|bibcode=2015NatGe...8..357M}}</ref>
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The idea behind this proposal is that the constantly moving sand dunes of Mars may be able to create a potential environment for life. Raw materials can be replenished, and the chemical disequilibrium needed for life maintained through churning of the sand by the winds.<ref name=sanddunesbioreactor/>
{{Wide image|PIA15295 Bridges 2-br2.gif|600 px|Advancing Dune in Nili Patera, Mars. Back-and-forth blinking of this two-image animation shows movement of a sand dune on Mars. This discovery shows that entire dunes as thick as 200 feet (61 meters) are moving as coherent units across the Martian landscape. The sand dunes move at about the same flux (volume per time) dunes in Antarctica. This was unexpected because of the thin air and the winds which are weaker than Earth winds. It may be due to "saltation" - ballistic movement of sand grains which travel further in the weaker Mars gravity.<br><br> The lee fronts of the dunes in this region move on average 0.5 meters per years (though the selection may be biased here as they only measured dunes with clear lee edges to measure) and the ripples move on average 0.1 meters per year.<ref name="BridgesAyoub2012">{{cite journal|url=
The sources of carbon would come from space - it is supplied at a steady rate of 5 nanograms per square meter per sol from micrometeorites. At the equator it has a mean lifetime of 300 years - but lasts longer if buried.
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These interfacial layers occur on boundaries between ice and rock due to intermolecular forces that depress the freezing point of the water. The water flows and acts as a solvent. These layers may be used by microbes in arctic permafrost, which have been found to metabolize at temperatures as low as -20 °C. Life may be possible in interfacial layers as thin as three monolayers, and the model by Stephen Jepsen et al. obtained 10<sup>9</sup> cells/g at -20 °C, though the microbes would spend most of their time in survival mode.<ref name="JepsenPriscu2007">{{cite journal|url=http://www.montana.edu/priscu/DOCS/Publications/JepsenEtAl2007LifeOnMars.pdf|last1=Jepsen|first1=Steven M.|last2=Priscu|first2=John C.|last3=Grimm|first3=Robert E.|last4=Bullock|first4=Mark A.|title=The Potential for Lithoautotrophic Life on Mars: Application to Shallow Interfacial Water Environments|journal=Astrobiology|volume=7|issue=2|year=2007|pages=342–354|issn=1531-1074|doi=10.1089/ast.2007.0124|pmid=17480165|bibcode=2007AsBio...7..342J|deadurl=yes|archiveurl=https://web.archive.org/web/20160303225043/http://www.montana.edu/priscu/DOCS/Publications/JepsenEtAl2007LifeOnMars.pdf|archivedate=2016-03-03|df=}}</ref><ref name="PriceSowers2004"/> Models show that interfacial water should form in some regions of Mars, for instance in Richardson crater.<ref name="KereszturiRivera-Valentin2012">{{cite journal|url=http://www.planetary.brown.edu/pdfs/4591.pdf|last1=Kereszturi|first1=Akos|last2=Rivera-Valentin|first2=Edgard G.|title=Locations of thin liquid water layers on present-day Mars|journal=Icarus|volume=221|issue=1|year=2012|pages=289–295|issn=0019-1035|doi=10.1016/j.icarus.2012.08.004|bibcode=2012Icar..221..289K}}</ref>
==Ice covered lakes
Lakes can form at higher latitudes after cometary or meteorite impacts,<ref name="impactlakes" /> or as a result of geothermal heat or volcanic activity. These may remain liquid for centuries, or up to a few thousand years for the largest impacts, with the heat trapped by an insulating layer of ice. Also there are suggestions that Mars may have a deep hydrosphere,<ref name="hydrosphere" /><ref name="perso.utinam.cnrs.fr" /> a liquid layer below its cryosphere, a few kilometers below the surface. Deep rock habitats on Earth are inhabited by life so if this layer exists, it may also be habitable on Mars.<ref name="naturegroundwater" />. In July 2018, a lake was discovered by radar, 20 kilometers across, and 1.5 kilometers below the ice of the Southern polar plain of Mars [[Planum Australe]]. It is not yet known if this lake is habitable to Earth life.<ref name=SupPolarIceLake>{{cite web |last1=Cooper |first1=Keith |title=Liquid water discovered on Mars |website=NASA Astrobiology Magazine |date=Jul 25, 2018}}</ref>
===Lakes through asteroid and comet impcts===
This is a possibility that was highlighted recently with the close flyby of Mars by the comet Siding Spring in 2014 [[C/2013 A1#Predicted effects|C/2013 A1 Siding Spring]]. Before its trajectory was known in detail, there remained a small chance that it could hit Mars. Calculations showed it could create a crater of many km in diameter and perhaps a couple of km deep. If a comet like that was to hit polar regions or higher latitudes of Mars, away from the equator, it would create a temporary lake, which life could survive in.
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==Temporary lakes resulting from volcanic activity==
There is evidence that volcanism formed lakes 210 million years ago on one of the flanks of Arsia Mons, relatively recent in geological terms. This may have consisted of two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, which probably remained liquid for hundreds, or even of the order of thousands of years.<ref>[https://news.brown.edu/articles/2014/05/mars A habitable environment on Martian volcano?], Kevin Stacey, News from Brown University, May 27, 2014, for the paper, see {{cite journal | doi = 10.1016/j.icarus.2014.04.024 | bibcode=2014Icar..237..315S | volume=237 | title=Volcano–ice interactions in the Arsia Mons tropical mountain glacier deposits | year=2014 | journal=Icarus | pages=315–339 | last1 = Scanlon | first1 = Kathleen E. | last2 = Head | first2 = James W. | last3 = Wilson | first3 = Lionel | last4 = Marchant | first4 = David R.}}</ref>
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There is clear evidence that Mars is not yet geologically inactive<ref name='thermal signature'>{{cite news | title = Hunting for young lava flows | date = June 1, 2011 | publisher = Red Planet | url = http://redplanet.asu.edu/?p=501 | work = Geophysical Research Letters |accessdate=4 October 2013}}</ref>
* Small scale volcanic features associated with some of the volcanoes on Mars which must have formed in the very recent geological past<ref name=recentepisodicvolcanicactivity>[http://www.nature.com/nature/journal/v432/n7020/abs/nature03231.html Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera] G. Neukum1, R. Jaumann, H. Hoffmann, E. Hauber, J. W. Head, A. T. Basilevsky, B. A. Ivanov, S. C. Werner, S. van Gasselt, J. B. Murray, T. McCord & The HRSC Co-Investigator Team, Nature 432, 971-979 (23 December 2004) | doi:10.1038/nature03231; Received 3 September 2004; Accepted 30 November 2004</ref>
* The isotopic evidence from Phoenix of release of CO2 in the recent geological past.<ref name=phoenixisotope/>
It seems likely that there are magma plumes at least deep underground, associated with the occasional surface volcanism on the geological timescale of millions of years. And given that there has been activity on Olympus Mons as recently as
But so far no currently active volcanism has been observed, nor have any present day warm areas have ever been found on the surface, in extensive searches.<ref>[http://redplanet.asu.edu/?p=501 Hunting for young lava flows] Red Planet report,
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* '''''Extreme desiccation and scarcity of water''''' - all life on Earth requires liquid water - or else high humidity in the air. So the main focus for the search for present day life on Mars so far starts with this assumption. There may be other possibilities for exotic life that don't use water, for instance a recent suggestion that life may be able to evolve in supercritical liquid CO2 under high pressure - a potential habitat present on both Venus and Mars.<ref>[http://www.space.com/27777-alien-life-supercritical-carbon-dioxide.html Alien Life Could Thrive on 'Supercritical' CO2 Instead of Water]
by Charles Q. Choi, Space.com Contributor November 16, 2014</ref> So probably we shouldn't rule out the possibility of other habitats totally.
* '''''UV light for any life on the surface exposed to full sunlight'''''. Because of the thin atmosphere, this is hardly filtered at all, and is a major challenge for any life exposed to the light. It is easily blocked by about 0.3 mm of surface soil,<ref name="Mateo-Marti2014">{{cite journal|url=http://www.mdpi.com/2078-1547/5/2/213/htm|last1=Mateo-Marti|first1=Eva|title=Planetary Atmosphere and Surfaces Chamber (PASC): A Platform to Address Various Challenges in Astrobiology|journal=Challenges|volume=5|issue=2|year=2014|pages=213–223|issn=2078-1547|doi=10.3390/challe5020213|bibcode=2014Chall...5..213M}}</ref> sheltered by a millimeter of dust or by other organisms,<ref name="RummelBeaty2014"/> or in the shadow of a rock. Mars conditions are likely to favour lifeforms that can tolerate high levels of UV radiation, at least, if they are exposed to direct unfiltered sunlight at any point in their life cycle. This could for instance involve use of protective pigments such as [[melanin]], [[parietin]] and [[usnic acid]] which help protect some lichens from the damaging effects of UV radiation in polar and high alpine regions.<ref name=Ustvedt>{{cite journal | last1 = Gauslaa | first1 = Yngvar | last2 = Margrete Ustvedt | first2 = Elin | year = 2003 | title = Is parietin a UV-B or a blue-light screening pigment in the lichen Xanthoria parietina? | url = | journal = Photochem. Photobiol. Sci. | volume = 2 | issue = 4| pages = 424–432 | doi=10.1039/b212532c}}</ref><ref>{{cite journal | doi = 10.1007/s00442-004-1583-6 | pmid=15138881 | volume=140 | issue=2 | title=The lichens Xanthoria elegans and Cetraria islandica maintain a high protection against UV-B radiation in Arctic habitats | year=2004 | journal=Oecologia | pages=211–216 | last1 = Nybakken | first1 = Line| bibcode=2004Oecol.140..211N }}</ref><ref>{{cite journal|doi=10.1046/j.1469-8137.2003.00708.x | volume=158 | title=UV-induction of sun-screening pigments in lichens | year=2003 | journal=New Phytologist | pages=91–100 | last1 = Asbjorn Solhaug | first1 = Knut}}</ref>
* '''''Low pressures (hypobaria)''''' at 1–14 mbar
* '''''Anoxic CO<sub>2</sub>-enriched atmosphere'''''. All the habitats suggested so far require [[anaerobes]] - lifeforms that don't require oxygen.
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Other authors also cite:
* '''''Lack of nitrogen'''''. All life on Earth requires nitrogen. Also there are theoretical reasons for expecting alien organic life to use nitrogen, as the weaker nitrogen based amide bonds are essential for the processes by which DNA is replicated. Mars, compared with Earth, has little nitrogen, either in the air or in the soil. Levels of nitrogen in the air are low, possibly too low for nitrogen fixation to be possible. But they can form in Martian conditions by non biological processes - either brought to Mars by meteorites (some carbonaceous chondrites are rich in nitrogen<ref>{{cite web|title=Meteorite with abundant nitrogen for life on Earth?|url=
Life on Mars may be limited to locations with local abundance of nitrates. Or, it may also be able to take advantage of fixation of nitrogen in monolayers of water, a process that can happen in present-day Mars conditions, and may be able to produce enough nitrates to supply a subsurface biosphere.<ref name="BoxeHand2012">{{cite journal|url=
Schuerger also mentions:
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* '''''Cosmic radiation''''' - this is not limiting of surface life in the short term (similar to the levels inside the ISS) but prevents it from reviving if kept dormant for periods of order of hundreds of thousands of years.<ref>[http://plantpath.ifas.ufl.edu/faculty/statewide/schuerger/Schuerger_2012_PSS-3371.pdf Biotoxicity of Mars soils: 1. Dry deposition of analog soils on microbial colonies and survival under Martian conditions], Andrew C. Schuerger, D.C. Golden, Doug W. Ming, Planetary and Space Science, 20 July 2012</ref> Martian surface or near surface life is likely to be strongly resistant to cosmic radiation, with repair mechanisms to repair the damage.
Curiosity measured ionizing radiation levels of 76 mGy a year.<ref>{{cite journal|last1=Donald M. Hassler, Cary Zeitlin, Robert F. Wimmer-Schweingruber, Bent Ehresmann, Scot Rafkin, Jennifer L. Eigenbrode, David E. Brinza, Gerald Weigle, Stephan Böttcher, Eckart Böhm, Soenke Burmeister, Jingnan Guo, Jan Köhler, Cesar Martin, Guenther Reitz, Francis A. Cucinotta, Myung-Hee Kim, David Grinspoon, Mark A. Bullock, Arik Posner, Javier Gómez-Elvira, Ashwin Vasavada, and John P. Grotzinger, and the MSL Science Team|title=Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover|journal=Science|date=12 November 2013|page=7|url=http://authors.library.caltech.edu/42648/1/RAD_Surface_Results_paper_SCIENCE_12nov13_FINAL.pdf}}</ref> This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. However, 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, which is possible, then our rovers on Mars could find dormant but still viable life at a depth of only one meter below the surface, according to an estimate in the paper that published the Curiosity ionizing radiation measurements.<ref>{{cite journal|last1=Donald M. Hassler, Cary Zeitlin, Robert F. Wimmer-Schweingruber, Bent Ehresmann, Scot Rafkin, Jennifer L. Eigenbrode, David E. Brinza, Gerald Weigle, Stephan Böttcher, Eckart Böhm, Soenke Burmeister, Jingnan Guo, Jan Köhler, Cesar Martin, Guenther Reitz, Francis A. Cucinotta, Myung-Hee Kim, David Grinspoon, Mark A. Bullock, Arik Posner, Javier Gómez-Elvira, Ashwin Vasavada, and John P. Grotzinger, and the MSL Science Team|title=
Modern researchers do not consider that ionizing radiation is a limiting factor in habitability assessments for present-day non-dormant surface life. The level of 76 mGy a 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=21 November 2012|url=https://www.newscientist.com/article/dn22520-mars-is-safe-from-radiation-but-the-trip-there-isnt/}}</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-->|bibcode=2014AsBio..14..887R}}</ref>
Line 654 ⟶ 758:
* Or be able to reproduce in surface or near surface conditions with dormancy periods never longer than 500,000 years or so.
In the 2014 MEPAG classification of special regions, ionizing radiation was not considered limiting for classifying the "Special regions" where present day surface life might survive.<ref name="RummelBeaty2014SpecialRegionsConclusion">{{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|bibcode=2014AsBio..14..887R|url=|quote=Finding 3-8: From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars" and "Over a 500-year time frame, the martian surface could be estimated to receive a cumulative ionizing radiation dose of less than 50 Gy, much lower than the LD 90 (lethal dose where 90% of subjects would die) for even a radiation-sensitive bacterium such as E. coli (LD 90 of *200–400 Gy). Accordingly, it can be stated that the RAD data show that the total surface flux of ionizing radiation is so low as to exert only a negligible impact on microbial viability during a 500-year time frame. These findings were in very good agreement with modeling studies"}}</ref>
{{quote|" From MSL RAD measurements, ionizing radiation from
In more detail they explain:
{{quote|"Over a 500-year time frame, the martian surface could be estimated to receive a cumulative ionizing radiation dose of less than 50 Gy, much lower than the LD 90 (lethal dose where 90% of subjects would die) for even a radiation-sensitive bacterium such as E. coli (LD 90 of * 200–400 Gy) (Atlan, 1973). Accordingly, it can be stated that the RAD data show that the total surface flux of ionizing radiation is so low as to exert only a negligible impact on microbial viability during a 500-year time frame"}}
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* '''''Unlikely''''' - these authors cite the inability of microbes to survive dormancy on the surface between periods when the atmosphere is thicker, due to ionizing radiation, the ephemeral nature of surface habitats, low temperatures, or low relative humidity, and the difficulty of colonization in surface conditions of high UV...<ref name=Morais>[https://books.google.com/books?id=TZKeBQAAQBAJ&pg=PA153 Planetary Exploration and Science: Recent Results and Advances], Antonio de Morais M. Teles, page 153, 27 Nov 2014</ref><ref name="PlaxcoGross2011_2"/><ref name=Quine2013/>
* '''''Possible, recolonized from below''''', these point out the ability of micro-organisms to repair damage by ionizing radiation and capability to remain dormant for up to several million years in the deep subsurface, suggesting that these short lived surface habitats, such as the Recurring Slope Lineae, could be recolonized from the subsurface.<ref name=Westall>[https://books.google.com/books?id=VYjEBAAAQBAJ&pg=PA192 Habitability of other planets and satellites - Habitability and Survival], Francis Westall, page 192, 30 Jul 2013
{{quote|'''''"This presupposes that the ephemeral surface habitats could be colonized by viable life forms, that is, that a subsurface reservoir exists in which microbes could continue to metabolize and that, as noted above, the viable microbes could be transported into the short-lived habitat.... Although there are a large number of constraints on the continued survival of life in the subsurface of Mars, the astonishing biomass in the subsurface of Earth suggests that this scenario as a real possibility."'''''}}
</ref>
* '''''Possible, open question if it occurs on the surface''''' these are investigating the possibility with experiments in simulated Mars conditions, theoretical models and study of the observations from Mars, and treat it as an open question for now, whether the present day surface and near sub surface is habitable.<ref name=Kilmer>{{cite journal|pmc=3989109|last1=Kilmer|first1=Brian R.|last2=Eberl|first2=Timothy C.|last3=Cunderla|first3=Brent|last4=Chen|first4=Fei|last5=Clark|first5=Benton C.|last6=Schneegurt|first6=Mark A.|title=Molecular and phenetic characterization of the bacterial assemblage of Hot Lake, WA, an environment with high concentrations of magnesium sulphate, and its relevance to Mars|journal=International Journal of Astrobiology|volume=13|issue=1|year=2014|pages=69–80|issn=1473-5504|doi=10.1017/S1473550413000268|pmid=24748851|bibcode=2014IJAsB..13...69K}}</ref>
<ref name=RummelandConley>
Rummel, J.D. and Conley, C.A., 2017. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5655418/ Four fallacies and an oversight: searching for martian life]. Astrobiology, 17(10), pp.971-974.
{{quote|'''''"Claims that reducing planetary protection requirements wouldn't be harmful, because Earth life can't grow on Mars, may be reassuring as opinion, but the facts are that we keep discovering life growing in extreme conditions on Earth that resemble conditions on Mars. We also keep discovering conditions on Mars that are more similar—though perhaps only at microbial scales—to inhabited environments on Earth, which is where the concept of Special Regions initially came from."'''''}}
</ref><ref name=Davila>Davila, A.F., Skidmore, M., Fairén, A.G., Cockell, C. and Schulze-Makuch, D., 2010. New priorities in the robotic exploration of Mars: the case for in situ search for extant life. Astrobiology, 10(7), pp.705-710.
{{quote|'''''"We argue that the strategy for Mars exploration should center on the search for extant life. By extant life, we mean life that is active today or was active during the recent geological past and is now dormant. As we discuss below, the immediate strategy for Mars exploration cannot focus only on past life based on the result of the Viking missions, particularly given that recent analyses call for a re-evaluation of some of these results. It also cannot be based on the astsumption that the surface of Mars is uniformly prohibitive for extant life, since research contributed in the past 30 years in extreme environments on EArth has shown that life is possible under extremes of cold and dryness."'''''}}}</ref><ref name=Fairen>Fairén, A.G., Parro, V., Schulze-Makuch, D. and Whyte, L., 2017. [https://www.liebertpub.com/doi/full/10.1089/ast.2017.1703 Searching for life on Mars before it is too late]. Astrobiology, 17(10), pp.962-970.
{{quote|'''''"The case of ExoMars is particularly dramatic as the first priority of the rover is to search for signs of past and present life on Mars ... however, it has been explicitly banned to go to Special Regions because it will not comply with the minimum cleanliness requirements. As a consequence, the billion-dollar life-seeking mission ExoMars will be allowed to search for life everywhere on Mars, except in the very places where we suspect that life may exist."'''''}}</ref><ref name=RummelandConley></ref> and many others. Selected quotes:<ref name="MorozovaMöhlmann2006">{{cite journal|url=http://epic.awi.de/14473/1/Mor2006e.pdf|last1=Morozova|first1=Daria|last2=Möhlmann|first2=Diedrich|last3=Wagner|first3=Dirk|title=Survival of Methanogenic Archaea from Siberian Permafrost under Simulated Martian Thermal Conditions|journal=Origins of Life and Evolution of Biospheres|volume=37|issue=2|year=2006|pages=189–200|issn=0169-6149|doi=10.1007/s11084-006-9024-7|pmid=17160628|quote='''''The observation of high survival rates of methanogens under simulated Martian conditions supports the possibility that microorganisms similar to the isolates from Siberian permafrost could also exist in the Martian permafrost.'''''|bibcode=2007OLEB...37..189M}}</ref>
{{quote|"The observation of high survival rates of methanogens under simulated Martian conditions supports the possibility that microorganisms similar to the isolates from Siberian permafrost could also exist in the Martian permafrost"}}
Also <ref name=Crisler>{{cite journal|pmc=3277918|last1=Crisler|first1=J.D.|last2=Newville|first2=T.M.|last3=Chen|first3=F.|last4=Clark|first4=B.C.|last5=Schneegurt|first5=M.A.|title=Bacterial Growth at the High Concentrations of Magnesium Sulfate Found in Martian Soils|journal=Astrobiology|volume=12|issue=2|year=2012|pages=98–106|issn=1531-1074|doi=10.1089/ast.2011.0720|pmid=22248384|quote='''''Our results indicate that terrestrial microbes might survive under the high-salt, low-temperature, anaerobic conditions on Mars and present significant potential for forward contamination. Stringent planetary protection requirements are needed for future life-detection missions to Mars'''''|bibcode=2012AsBio..12...98C}}</ref>
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Gary M. King, PNAS March 23, 2015, {{DOI|10.1073/pnas.1424989112}}</ref> || O<sub>2</sub>: partial pressure too low
|-
| organics: meteoritic
|-
| organics: endogenous available in subsurface || ClO{{su|b=4|p=−}}: available but not shown to support life
|}
A candidate metabolism would use one of the electron donors in the first column paired with one of the electron acceptors on the right as a source of energy.
See also the presentations in: [http://planets.ucla.edu/meetings/past-meetings/mars-habitability-2013/program/ Redox Potentials for Martian Life]
Line 727 ⟶ 836:
*'''''black yeast''''' Exophiala jeanselmei, also adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.<ref name="ZakharovaMarzban2014"/>
* '''''Methanogens such as [[Methanosarcina barkeri]]'''''<ref name=Nixon/> - only require CO<sub>2</sub>, hydrogen and trace elements. The hydrogen could come from geothermal sources, volcanic action or action of water on basalt.
* '''''Lichens''''' such as [[Xanthoria elegans]], Pleopsidium chlorophanum,<ref name="DLRLichenHabitableQuote">{{cite journal|url=https://core.ac.uk/download/pdf/31019036.pdf|last1=de
40th COSPAR Scientific Assembly. Held 2–10 August 2014, in Moscow, Russia, Abstract F3.3-9-14. Bibliographic Code:
2014cosp...40E2015M</ref><ref>[http://adsabs.harvard.edu/abs/2014cosp...40E.650D Survival of the lichen model system Circinaria gyrosa before flight to the ISS (EXPOSE R2 mission)], Rosa De la Torre Noetzel,
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{{Attrib-Water-on-Mars}}
[[Category:Present day Mars habitability]]
{{Possible present day habitats for life on Mars}}
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