What was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae,[2][3] may be grains of flowing sand and dust slipping downhill to make dark streaks.[4] While most water ice is buried, it is exposed at the surface across several locations on Mars. In the mid-latitudes, it is exposed by impact craters, steep scarps and gullies.[5][6][7] Additionally, water ice is also visible at the surface at the north polar ice cap.[8] Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian south pole. More than 5 million km3 of ice have been detected at or near the surface of Mars, enough to cover the whole planet to a depth of 35 meters (115 ft).[9] Even more ice might be locked away in the deep subsurface.[10][11]
Some liquid water may occur transiently on the Martian surface today, but limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for known life.[3][12][13] No evidence of present-day liquid water has been discovered on the planet's surface because under typical Martian conditions (water vapor pressure <1 Pa [14] and ambient atmospheric pressure ~700 Pa [15]), warming water ice on the Martian surface would sublime at rates of up to 4 meters per year.[16] Before about 3.8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures,[17][18][19][20] potentially allowing greater amounts of liquid water on the surface,[21][22][23][24] possibly including a large ocean[25][26][27][28] that may have covered one-third of the planet.[29][30][31] Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars' history.[32][33][34]Aeolis Palus in Gale Crater, explored by the Curiosity rover, is the geological remains of an ancient freshwater lake that could have been a hospitable environment for microbial life.[35][36][37][38] The present-day inventory of water on Mars can be estimated from spacecraft images, remote sensing techniques (spectroscopic measurements,[39][40]radar,[41] etc.), and surface investigations from landers and rovers.[42][43] Geologic evidence of past water includes enormous outflow channels carved by floods,[44] ancient river valley networks,[45][46]deltas,[47] and lakebeds;[48][49][50][51] and the detection of rocks and minerals on the surface that could only have formed in liquid water.[52] Numerous geomorphic features suggest the presence of ground ice (permafrost)[53] and the movement of ice in glaciers, both in the recent past[54][55][56][57] and present.[58]Gullies and slope lineae along cliffs and crater walls suggest that flowing water continues to shape the surface of Mars, although to a far lesser degree than in the ancient past.
Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago,[59] the current environment at the surface is dry and subfreezing, probably presenting an insurmountable obstacle for living organisms. In addition, Mars lacks a thick atmosphere, ozone layer, and magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded. The damaging effects of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface.[60][61] Therefore, the best potential locations for discovering life on Mars may be in subsurface environments.[62][63][64] Large amounts of underground ice have been found on Mars; the volume of water detected is equivalent to the volume of water in Lake Superior.[65][66][67] In 2018, scientists reported the discovery of a subglacial lake on Mars, 1.5 km (0.93 mi) below the southern polar ice cap, with a horizontal extent of about 20 km (12 mi), the first known stable body of liquid water on the planet,[68][69] but subsequent work has questioned this detection.[70][71]
Understanding the extent and situation of water on Mars is vital to assess the planet's potential for harboring life and for providing usable resources for future human exploration. For this reason, "Follow the Water" was the science theme of NASA's Mars Exploration Program (MEP) in the first decade of the 21st century. NASA and ESA missions including 2001 Mars Odyssey, Mars Express, Mars Exploration Rovers (MERs), Mars Reconnaissance Orbiter (MRO), and Mars Phoenix lander have provided information about water's abundance and distribution on Mars.[72] Mars Odyssey, Mars Express, MRO, and Mars Science Lander Curiosity rover are still operating, and discoveries continue to be made. In September 2020, scientists confirmed the existence of several large saltwater lakes under ice in the south polar region of the planet Mars. According to one of the researchers, "We identified the same body of water [as suggested earlier in a preliminary initial detection], but we also found three other bodies of water around the main one ... It's a complex system."[73][74] In March 2021, researchers reported that a considerable amount of water on ancient Mars has remained but that, for the most part, has likely been sequestered into the rocks and crust of the planet over the years.[75][76][77][78] In August 2024, further analysis of data from NASA's InSight Mars Lander enabled researchers to discover a reservoir of liquid water at depths of 10–20 kilometres (6.2–12.4 mi) under the Martian crust.[79]
^Jakosky, B. M.; Haberle, R.M. (1992). "The Seasonal Behavior of Water on Mars". In Kieffer, H. H.; et al. (eds.). Mars. Tucson, Arizona: University of Arizona Press. pp. 969–1016.
^Martín-Torres, F. Javier; Zorzano, María-Paz; Valentín-Serrano, Patricia; Harri, Ari-Matti; Genzer, Maria (April 13, 2015). "Transient liquid water and water activity at Gale crater on Mars". Nature Geoscience. 8 (5): 357–361. Bibcode:2015NatGe...8..357M. doi:10.1038/ngeo2412.
^ abOjha, L.; Wilhelm, M. B.; Murchie, S. L.; McEwen, A. S.; Wray, J. J.; Hanley, J.; Massé, M.; Chojnacki, M. (2015). "Spectral evidence for hydrated salts in recurring slope lineae on Mars". Nature Geoscience. 8 (11): 829–832. Bibcode:2015NatGe...8..829O. doi:10.1038/ngeo2546. S2CID59152931.
^Baker, V. R.; Strom, R. G.; Gulick, V. C.; Kargel, J. S.; Komatsu, G.; Kale, V. S. (1991). "Ancient oceans, ice sheets and the hydrological cycle on Mars". Nature. 352 (6348): 589–594. Bibcode:1991Natur.352..589B. doi:10.1038/352589a0. S2CID4321529.
^Dohm, J. M.; Baker, Victor R.; Boynton, William V.; Fairén, Alberto G.; Ferris, Justin C.; Finch, Michael; Furfaro, Roberto; Hare, Trent M.; Janes, Daniel M.; Kargel, Jeffrey S.; Karunatillake, Suniti; Keller, John; Kerry, Kris; Kim, Kyeong J.; Komatsu, Goro; Mahaney, William C.; Schulze-Makuch, Dirk; Marinangeli, Lucia; Ori, Gian G.; Ruiz, Javier; Wheelock, Shawn J. (2009). "GRS Evidence and the Possibility of Paleooceans on Mars"(PDF). Planetary and Space Science. 57 (5–6): 664–684. Bibcode:2009P&SS...57..664D. doi:10.1016/j.pss.2008.10.008. Archived from the original(PDF) on September 22, 2017. Retrieved July 23, 2019.
^Clifford, S. M.; Parker, T. J. (2001). "The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains". Icarus. 154 (1): 40–79. Bibcode:2001Icar..154...40C. doi:10.1006/icar.2001.6671. S2CID13694518.
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^Fassett, C. I.; Dickson, James L.; Head, James W.; Levy, Joseph S.; Marchant, David R. (2010). "Supraglacial and Proglacial Valleys on Amazonian Mars". Icarus. 208 (1): 86–100. Bibcode:2010Icar..208...86F. doi:10.1016/j.icarus.2010.02.021.
^Heisinger, H.; Head, J. (2002). "Topography and morphology of the Argyre basin, Mars: implications for its geologic and hydrologic history". Planetary and Space Science. 50 (10–11): 939–981. Bibcode:2002P&SS...50..939H. doi:10.1016/S0032-0633(02)00054-5.
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^Fassett, C.; Head, III (2008). "Valley network-fed, open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology". Icarus. 198 (1): 37–56. Bibcode:2008Icar..198...37F. doi:10.1016/j.icarus.2008.06.016.
^Head, J.; Marchant, D. (2006). "Modifications of the walls of a Noachian crater in Northern Arabia Terra (24 E, 39 N) during northern mid-latitude Amazonian glacial epochs on Mars: Nature and evolution of Lobate Debris Aprons and their relationships to lineated valley fill and glacial systems (abstract)". Lunar and Planetary Science. 37: 1128.
^Head, J.; Marchant, D. (2006). "Evidence for global-scale northern mid-latitude glaciation in the Amazonian period of Mars: Debris-covered glacial and valley glacial deposits in the 30–50 N latitude band". Lunar and Planetary Science. 37: 1127.
^Dartnell, L. R.; Desorgher, L.; Ward, J. M.; Coates, A. J. (2007). "Martian sub-surface ionising radiation: biosignatures and geology"(PDF). Biogeosciences. 4 (4): 545–558. Bibcode:2007BGeo....4..545D. doi:10.5194/bg-4-545-2007. Archived(PDF) from the original on July 9, 2014. Retrieved September 1, 2019. This ionising radiation field is deleterious to the survival of dormant cells or spores and the persistence of molecular biomarkers in the subsurface, and so its characterisation. ... Even at a depth of 2 meters beneath the surface, any microbes would likely be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.
^de Morais, A. (2012). "A Possible Biochemical Model for Mars"(PDF). 43rd Lunar and Planetary Science Conference. Archived(PDF) from the original on July 6, 2021. Retrieved June 5, 2013. The extensive volcanism at that time much possibly created subsurface cracks and caves within different strata, and the liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals organic molecules, and geothermal heat – ingredients for life as we know on Earth.
^Didymus, JohnThomas (January 21, 2013). "Scientists find evidence Mars subsurface could hold life". Digital Journal – Science. Archived from the original on December 13, 2013. Retrieved June 16, 2013. There can be no life on the surface of Mars, because it is bathed in radiation and it's completely frozen. Life in the subsurface would be protected from that. – Prof. Parnell.
^Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center. NASA. Archived from the original on January 17, 2009. Retrieved June 16, 2013. If microscopic Martian life is producing the methane, it likely resides far below the surface, where it's still warm enough for liquid water to exist
^Wright, Vashan; Morzfeld, Matthias; Manga, Michael (August 12, 2024). David Kohlstedt (ed.). "Liquid water in the Martian mid-crust". PNAS. 121 (35): e2409983121. doi:10.1073/pnas.2409983121. PMID39133865.