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[[File:Phoenix landing2.jpg|thumb|Artist's impression of the Phoenix Lander settling down on Mars.<br><br>Its measurements of isotope ratios of carbon and oxygen gave evidence for liquid water on the surface now or in the recent geological past.<ref name=phoenixisotope>[http://uanews.org/story/phoenix-mars-lander-finds-surprises-about-planet%E2%80%99s-watery-past Phoenix Mars Lander Finds Surprises About Planet’s Watery Past] University of Arizona news, By Daniel Stolte, University Communications, and NASA's Jet Propulsion Laboratory | September 9, 2010</ref>.<br><br>Its observations of possible droplets on its legs suggested new ways that water could be stable temporarily on Mars.<ref name=phoenix_droplets_2009>[https://www.newscientist.com/article/dn16620-first-liquid-water-may-have-been-spotted-on-mars.html
This is a question of great interest in [[astrobiology]]. Does [[Mars]] in its present state have any potential habitats for native microbes, lichens, or other living organisms<ref>[https://scholar.google.co.uk/scholar?hl=en&as_sdt=0%2C5&q=present+day+Mars+habitability Google scholar search for: present day Mars habitability]</ref>? If so, are these habitats on or near the surface or only deep underground, perhaps next to geological hotspots or in the deep hydrosphere? Objective B of NASA's first Mars Science Goal is to investigate these potential habitats:<ref>Hamilton, V.E., Rafkin, S., Withers, P., Ruff, S., Yingst, R.A., Whitley, R., Center, J.S., Beaty, D.W., Diniega, S., Hays, L. and Zurek, R., [https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_Document_2015_v18_FINAL.pdf Mars Science Goals, Objectives, Investigations, and Priorities: 2015 Version].
:''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.'''''▼
</ref>
▲
▲
[[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> 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:
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* 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]]
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.
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== 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
*Theme: Solar System Sites
* 2019, 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>[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>▼
*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==
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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.<ref name=sanddunesbioreactor/><ref name=LevinMarsLifeIdea>[http://www.astrobio.net/news-exclusive/the-viking-files/ The Viking Files] Astrobiology Magazine (NASA) - May 29, 2003, astrobio.net (summary of scientific research)</ref>
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/>
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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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* "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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|}
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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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:
Line 641 ⟶ 705:
* '''''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 694 ⟶ 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 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"}}
In more detail they explain:
Line 710 ⟶ 773:
{{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=
{{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>
Line 745 ⟶ 812:
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]
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