Water on Mars

Almost all water on Mars today exists as polar permafrost ice, though it also exists in small quantities as vapor in the atmosphere.^[1]

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]