Water on Mars

Although very small amounts of liquid water may occur transiently on the surface of Mars, limited to traces of dissolved moisture from the atmosphere and thin films, large quantities of ice are present on and under the surface. Small amounts of water vapor are present in the atmosphere, and liquid water may be present under the surface. In addition, a large quantity of liquid water was likely present on the surface in the distant past. Currently, ice is mostly present in polar permafrost.
More than 5 million km³ of ice have been detected at or near the surface of Mars, enough to cover the planet to a depth of. Even more ice might be locked away in the deep subsurface. The chemical signature of water vapor on Mars was first unequivocally demonstrated in 1963 by spectroscopy using an Earth-based telescope. In 2008 and 2013, ice was detected in soil samples taken by the Phoenix lander and Curiosity rover. In 2018, radar findings suggested the presence of liquid water in subglacial lakes and in 2024, seismometer data suggested the presence of liquid water deep under the surface.
Most of the ice on Mars is buried. However, ice is present at the surface at several locations. In the mid-latitudes, surface ice is present in impact craters, steep scarps and gullies. At latitudes near the poles, ice is present in glaciers. Ice is visible at the surface at the north polar ice cap, and abundant ice is present beneath the permanent carbon dioxide ice cap at the Martian south pole.
The present-day inventory of water on Mars can be estimated from spacecraft images, remote sensing techniques, and surface investigations from landers and rovers including x-ray spectroscopy, neutron spectroscopy and seismography.
Before about 3.8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures, potentially allowing greater amounts of liquid water on the surface, possibly including a large ocean that may have covered one-third of the planet. Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars's history. 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.
Geologic evidence of past water includes enormous outflow channels carved by floods, ancient river valley networks, deltas, and lakebeds; and the detection of rocks and minerals on the surface that could only have formed in liquid water. Numerous geomorphic features suggest the presence of ground ice and the movement of ice in glaciers, both in the recent past and present. Gullies and slope lineae along cliffs and crater walls suggest that flowing water may continue to shape the surface of Mars, although what was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae, may be grains of flowing sand and dust slipping downhill to make dark streaks.
Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago, no definite evidence of life, past or present, has been found on Mars. The best potential locations for discovering life on Mars may be in subsurface environments. A large amount of underground ice, equivalent to the volume of water in Lake Superior, has been found under Utopia Planitia. In 2018, based on radar data, scientists reported the discovery of a possible [|subglacial lake on Mars], below the southern polar ice cap, with a horizontal extent of about, findings that were strengthened by additional radar findings in September 2020, but subsequent work has questioned this detection.
Understanding the extent and situation of water on Mars is important 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 in the first decade of the 21st century. NASA and ESA missions including 2001 Mars Odyssey, Mars Express, Mars Exploration Rovers, Mars Reconnaissance Orbiter, and Mars Phoenix lander have provided information about water's abundance and distribution on Mars. Mars Odyssey, Mars Express, MRO, and Mars Science Lander Curiosity rover are still operating, and discoveries continue to be made.
In August 2024, researchers reported that analysis of seismic data from NASA's InSight Mars Lander suggested the presence of a reservoir of liquid water at depths of under the Martian crust.

Historical background

The notion of water on Mars preceded the space age by hundreds of years. Early telescopic observers correctly assumed that the white polar caps and clouds were indications of water's presence. These observations, coupled with the fact that Mars has a 24-hour day, led astronomer William Herschel to declare in 1784 that Mars probably offered its inhabitants "a situation in many respects similar to ours."
By the start of the 20th century, most astronomers recognized that Mars was far colder and drier than Earth. The presence of oceans was no longer accepted, so the paradigm changed to an image of Mars as a "dying" planet with only a meager amount of water. The dark areas, which could be seen to change seasonally, were then thought to be tracts of vegetation. The person most responsible for popularizing this view of Mars was Percival Lowell, who imagined a race of Martians constructing a network of canals to bring water from the poles to the inhabitants at the equator. Although generating tremendous public enthusiasm, Lowell's ideas were rejected by most astronomers. The majority view of the scientific establishment at the time is probably best summarized by English astronomer Edward Walter Maunder who compared the climate of Mars to conditions atop a peak on an arctic island where only lichen might be expected to survive.
In the meantime, many astronomers were refining the tool of planetary spectroscopy in hope of determining the composition of the Martian atmosphere. Between 1925 and 1943, Walter Adams and Theodore Dunham at the Mount Wilson Observatory tried to identify oxygen and water vapor in the Martian atmosphere, with generally negative results. The only component of the Martian atmosphere known for certain was carbon dioxide identified spectroscopically by Gerard Kuiper in 1947. Water vapor was not unequivocally detected on Mars until 1963, at the Mount Wilson Observatory.
The composition of the polar caps, assumed to be water ice since the time of Cassini, was questioned by a few scientists in the late 1800s who favored CO₂ ice, because of the planet's overall low temperature and apparent lack of appreciable water. This hypothesis was confirmed theoretically by Robert Leighton and Bruce Murray in 1966. Today it is known that the winter caps at both poles are primarily composed of CO₂ ice, but that a permanent cap of water ice remains during the summer at the northern pole. At the southern pole, a small cap of CO₂ ice remains during summer, but this cap too is underlain by perennial water ice as shown by spectroscopic data from 2004 from the Mars Express orbiter.
The final piece of the Martian climate puzzle was provided by Mariner 4 in 1965. Grainy television pictures from the spacecraft showed a surface dominated by impact craters, which implied that the surface was very old and had not experienced the level of erosion and tectonic activity seen on Earth. Little erosion meant that liquid water had probably not played a large role in the planet's geomorphology for billions of years. Furthermore, the variations in the radio signal from the spacecraft as it passed behind the planet allowed scientists to calculate the density of the atmosphere. The results showed an atmospheric pressure less than 1% of Earth's at sea level, effectively precluding the existence of liquid water, which would rapidly boil or freeze at such low pressures. Thus, a vision of Mars was born of a world much like the Moon, but with just a wisp of an atmosphere to blow the dust around. This view of Mars would last nearly another decade until Mariner 9 showed a much more dynamic Mars with hints that the planet's past environment was more clement than the present one.
For many years it was thought that the observed remains of floods were caused by the release of water from a global water table, but research published in 2015 reveals regional deposits of sediment and ice emplaced 450 million years earlier to be the source. "Deposition of sediment from rivers and glacial melt filled giant canyons beneath primordial ocean contained within the planet's northern lowlands. It was the water preserved in these canyon sediments that was later released as great floods, the effects of which can be seen today."

Aqueous and hydrated minerals

It is widely accepted that Mars had abundant water very early in its history. Minerals that incorporate water or form in the presence of water are generally termed "aqueous minerals". Hydrated minerals are minerals which have undergone a chemical reaction which adds water to their crystal structure.

Water in weathering products (aqueous minerals)

The primary rock type on the surface of Mars is basalt, a fine-grained igneous rock which on Mars is made up mostly of the mafic silicate minerals olivine, pyroxene, and plagioclase feldspar. When exposed to water and atmospheric gases, these minerals chemically weather into new minerals, some of which may incorporate water into their crystalline structures, either as H₂O or as hydroxyl. Examples of hydrated minerals include the iron hydroxide goethite ; the evaporite minerals gypsum and kieserite; opaline silica; and phyllosilicates, such as kaolinite and montmorillonite. All of these minerals have been detected on Mars.
One direct effect of chemical weathering is to consume water and other reactive chemical species, taking them from mobile reservoirs like the atmosphere and hydrosphere and sequestering them in rocks and minerals. The amount of water in the Martian crust stored as hydrated minerals is currently unknown, but may be quite large. For example, mineralogical models of the rock outcroppings examined by instruments on the Opportunity rover at Meridiani Planum suggest that the sulfate deposits there could contain up to 22% water by weight.
On Earth, all chemical weathering reactions involve water to some degree. Many secondary minerals do not actually incorporate water, but still require water to form. Some examples of anhydrous secondary minerals include many carbonates, some sulfates, and metallic oxides such as the iron oxide mineral hematite. On Mars, a few of these weathering products could theoretically form without water or with scant amounts present as ice or in thin molecular-scale films.
Aqueous minerals are sensitive indicators of the type of environment that existed when the minerals formed. The ease with which aqueous reactions occur depends on the pressure, temperature, and on the concentrations of the gaseous and soluble species involved. Two important properties are pH and oxidation-reduction potential. For example, the sulfate mineral jarosite forms only in low pH water. Phyllosilicates usually form in water of neutral to high pH. E_h is a measure of the oxidation state of an aqueous system. Together E_h and pH indicate the types of minerals that are thermodynamically most stable and therefore most likely to form from a given set of aqueous components. Thus, past environmental conditions on Mars, including those conducive to life, can be inferred from the types of minerals present in the rocks.