Mars holds water in ice caps, buried glaciers, hydrated minerals, thin atmospheric vapor, and possibly as liquid deep within its crust. The planet is far from bone-dry. What has changed over billions of years is where that water resides and what form it takes. Early Mars likely had rivers, lakes, and perhaps a hemisphere-spanning ocean, but the loss of its protective magnetic field allowed the solar wind to strip away much of the atmosphere, taking vast quantities of water with it. The water that remains is scattered across the planet in forms that range from easily visible polar ice to liquid potentially locked kilometers underground.
The Polar Ice Caps
The most obvious reservoirs of Martian water sit at the poles. Both the north and south poles host thick stacks of layered deposits made of water ice mixed with dust, recording climate cycles much the way tree rings record seasons on Earth. At the north pole, the residual cap that persists through summer is water ice. The south pole is more complex: its outermost residual layer is frozen carbon dioxide, but water ice is exposed around the edges and lies beneath the carbon dioxide veneer.
1Icarus. A comparison of CO2 seasonal activity in Mars’ northern and southern hemispheresBelow these thin residual caps, the polar layered deposits extend to considerable thickness. In the south, the layered deposits are composed mostly of ice and dust, topped by a perennial carbon dioxide cover that prevents the underlying water ice from sublimating into the atmosphere during summer. In the north, seasonal carbon dioxide frost comes and goes, but the water ice underneath is exposed each summer and interacts directly with the atmosphere.
2Geophysical Research Letters. Mars Without the Southern Perennial CO2 CoverIf you melted all the water ice in both polar layered deposits and spread it evenly across the planet, estimates suggest it would form a layer tens of meters deep. That is a lot of water by any practical measure, but it is a fraction of what Mars once had.
Buried Ice at Lower Latitudes
You do not have to go all the way to the poles to find ice. Across the Martian mid-latitudes, thick deposits of water ice lie just below the surface, hidden under a thin blanket of dust and rock. Where erosion has carved steep scarps into the landscape, these ice sheets are exposed and visible in orbital images. Eight such locations have been studied in detail, revealing ice layers that can be over 100 meters thick, starting as shallow as one to two meters below the surface.
3PubMed. Exposed subsurface ice sheets in the Martian mid-latitudesRadar sounding from orbit has added to the picture. In the eastern Hellas region, the Mars Reconnaissance Orbiter’s radar returned signals entirely consistent with massive water ice buried under debris, confirming the long-suspected idea that some of Mars’s mid-latitude terrain is draped over remnant glaciers.
4PubMed. Radar sounding evidence for buried glaciers in the southern mid-latitudes of MarsThese buried ice deposits matter for two reasons. They are much more accessible than polar ice for any future visitors, sitting at latitudes where conditions are less extreme. And they tell a story about recent climate shifts: the ice was likely deposited during periods when Mars’s axial tilt was different, causing polar ice to migrate toward the equator.
Liquid Water Deep in the Crust
The most provocative recent findings suggest that Mars may harbor liquid water right now, not on the surface, but deep underground. In 2018, the European Space Agency’s Mars Express orbiter reported anomalously bright radar reflections beneath the south polar layered deposits, centered in a roughly 20-kilometer-wide zone. The radar signal’s properties matched what you would expect from water-bearing materials, leading the team to interpret the feature as a stable body of liquid water trapped under the ice.
5PubMed. Radar evidence of subglacial liquid water on MarsThat claim has been debated, with some researchers arguing the reflections could be caused by certain types of clay or volcanic rock rather than liquid. But a separate line of evidence arrived from the InSight lander’s seismometer, which listened to marsquakes and mapped the planet’s interior structure. Analysis of seismic wave speeds through the mid-crust, at depths of roughly 11.5 to 20 kilometers, found that fractured igneous rock saturated with liquid water best explains the data.
6PubMed Central. Liquid water in the Martian mid-crustAn independent study using the same InSight seismic data proposed that a transition from dry cracks to water-filled cracks could explain a sharp seismic discontinuity observed at about 10 kilometers depth, with no need to invoke changes in rock type or porosity.
7Geology. Seismic discontinuity in the Martian crust possibly caused by water-filled cracksIf liquid water does exist at these depths, it would be under enough pressure and possibly warm enough from residual internal heat to stay liquid despite the freezing surface temperatures. The volumes involved could be substantial. But accessing water kilometers deep in another planet’s crust is, to put it mildly, not straightforward.
Evidence of Ancient Lakes and a Possible Ocean
Modern Mars is cold and dry at the surface, but the geological record tells a very different story about the planet’s first billion years or so. The most detailed evidence comes from Jezero crater, where NASA’s Perseverance rover is currently working. Orbital observations had already shown a prominent fan-shaped body of sedimentary rock at the crater’s western margin. Perseverance confirmed that the inclined rock layers represent deltas that advanced into a lake, recording a transition from long-lived lake conditions to brief, energetic floods.
8PubMed. Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, MarsEarlier studies had already mapped the delta’s features in detail, identifying scroll bars and meandering channels on the delta plain that form only when water flows steadily over a prolonged period. The minerals detected there, including phyllosilicates (a family of clay minerals), require liquid water to form, confirming a regional wet period early in Martian history.
9Earth and Planetary Science Letters. Sedimentological evidence for a deltaic origin of the western fan deposit in Jezero crater, Mars and implications for future exploration10Journal of Geophysical Research: Planets. Characterization of phyllosilicates observed in the central Mawrth Vallis region, Mars, their potential formational processes, and implications for past climate
Beyond individual lakes, there is a persistent and increasingly well-supported hypothesis that Mars once had an ocean filling much of its northern lowlands. Topographic data from the Mars Orbiter Laser Altimeter found that a lowland-encircling geologic contact closely follows an equipotential surface, the terrain is smoother below that line than above it, and the volume it encloses falls within estimates of available Martian water.
11PubMed. Possible ancient oceans on Mars: evidence from Mars Orbiter Laser Altimeter dataMore recently, China’s Zhurong rover imaged rock layers along an extended traverse in the northern lowlands whose dip angles, lack of dissection by river channels, and low radar permittivity are consistent with coastal sedimentary deposits. The structure and thickness of these layers suggest a voluminous supply of sediment into a large body of water, not just a brief local melt event.
12PubMed Central. Ancient ocean coastal deposits imaged on MarsHow Mars Lost Most of Its Water
If Mars once had lakes, rivers, and perhaps an ocean, where did all that water go? Part of it is still on the planet, locked in the ice caps, buried glaciers, hydrated minerals, and possibly deep crustal reservoirs described above. But a large fraction escaped to space entirely, and the mechanism was the solar wind.
Mars lacks a strong global magnetic field, so the solar wind interacts directly with the upper atmosphere. Energetic solar wind particles can knock atmospheric molecules into space through a process called sputtering. Early in solar system history, the young Sun produced far more intense ultraviolet radiation and a much denser solar wind. Modeling of that early period suggests the solar wind could have removed a global water layer roughly 10 to 70 meters deep within the first 150 million years alone.
13PubMed. Atmosphere and water loss from early Mars under extreme solar wind and extreme ultraviolet conditionsOver the full 3.5 billion years since Mars lost its dynamo, sputtering models estimate that roughly 80 meters’ worth of water (spread planet-wide) could have been lost, along with about 3 bars of carbon dioxide.
14PubMed. Loss of atmosphere from Mars due to solar wind-induced sputteringScientists track this loss using the ratio of deuterium (heavy hydrogen) to regular hydrogen. When water molecules break apart in the upper atmosphere, the lighter hydrogen escapes to space more easily than the heavier deuterium, so the remaining water becomes progressively enriched in deuterium over time. The current deuterium-to-hydrogen ratio on Mars is about five to six times higher than Earth’s, a strong signal that large quantities of hydrogen from water have been lost to space over billions of years.
15Journal of Geophysical Research: Planets. Mars Water and D/H Evolution From 3.3 Ga to Present16Annual Review of Earth and Planetary Sciences. Atmospheric Loss to Space and the History of Water on Mars
Brines and the Question of Surface Liquid
Pure liquid water cannot last on the Martian surface today. The atmosphere is too thin (average surface pressure around 600 pascals, compared to Earth’s 101,325) and temperatures too low. Water exposed to those conditions would either freeze or boil away almost immediately. But dissolved salts change the equation. Mars’s soil contains perchlorates and other salts that can dramatically lower water’s freezing point and raise its boiling point.
Thermodynamic analysis shows that under average Martian atmospheric pressure, a saturated sodium perchlorate solution can remain liquid between about 240 and 275 kelvin, roughly −33°C to 2°C.
17PubMed Central. Stability of the Liquid Water Phase on Mars: A Thermodynamic Analysis Considering Martian Atmospheric Conditions and Perchlorate Brine SolutionsMagnesium and calcium perchlorate solutions are even more stubborn: laboratory experiments found that these solutions do not crystallize during slow cooling but instead remain in a supercooled liquid state until forming a glass near −120°C. Even adding soil particles to the mix did not trigger freezing. This means perchlorate brines could theoretically stay liquid through entire Martian day-night cycles and possibly through seasonal changes.
18Icarus. The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to MarsWhether these brines actually form on Mars today, and in what volumes, remains uncertain. Nighttime humidity near the surface can approach saturation, and modeling suggests water vapor condenses on or near the ground at night, possibly as thin brine films through a process called deliquescence, where hygroscopic salts pull moisture from the air.
19Icarus. Global seasonal variations of the near-surface relative humidity levels on present-day MarsThe amounts involved are tiny, perhaps enough to dampen the top layer of soil grains but far from anything you would call a puddle. Still, even trace quantities of transient liquid water on the surface would have implications for chemistry and, potentially, biology.
Water Vapor in the Atmosphere
Mars’s atmosphere contains water vapor, though in vanishingly small amounts compared to Earth’s. Measurements at the Phoenix lander site found that the total column of precipitable water stayed around 30 micrometers throughout the Martian day, meaning if you condensed every molecule of water vapor in the atmosphere above that spot, it would form a film thinner than a human hair.
20Icarus. Humidity observations and column simulations for a warm period at the Mars Phoenix lander site: Constraining the adsorptive properties of regolithRelative humidity swings wildly over a Martian day. During daylight hours it drops to very low values, but at night, as temperatures plummet, humidity near the surface can climb toward saturation. The soil itself acts as a sponge: porous regolith adsorbs water vapor at night and releases it each morning as temperatures rise, creating a daily moisture cycle between ground and air. This cycle is subtle but important because it moves water around and governs where frost and thin brines can form.
Why Martian Water Matters for the Search for Life
Every known form of life on Earth requires liquid water. That simple fact drives much of Mars exploration strategy. If Mars ever hosted long-lived lakes or an ocean, conditions may have supported microbial life. And if liquid water persists today, even as deep crustal reservoirs or transient brine films, the question of present-day biology becomes more than theoretical.
Mars analog environments on Earth, places that resemble Martian conditions, show that microbial communities can thrive in surprisingly harsh settings. Extremely cold soils support microbial life, and springs fed by groundwater exist in virtually any climate. At the surface, chemical gradients between rock, soil, and atmosphere support diverse microbial metabolisms, and some organisms can even use high-energy radiation as an energy source.
21PubMed Central. Biosignature Preservation and Detection in Mars Analog EnvironmentsImpact craters add another dimension. Modeling of impact-driven hydrothermal systems on early Mars shows that a 180-kilometer crater could sustain warm, water-rich conditions in its subsurface for hundreds of thousands of years. A basin the size of Hellas could potentially host hydrothermal activity for around 10 million years after impact. Those timescales are more than enough for microbial colonization if life existed on early Mars.
22Journal of Geophysical Research: Planets. Impact‐induced hydrothermal activity on early MarsBecause of these possibilities, areas on Mars where liquid water could exist, even transiently, are designated “Special Regions” under international planetary protection guidelines. Spacecraft exploring these zones must meet strict sterilization requirements to prevent Earth microbes from hitchhiking to Mars and contaminating environments that might already harbor life.
23PubMed. Planetary Protection and Mars Special Regions–A Suggestion for Updating the DefinitionWater as a Resource for Human Exploration
Water on Mars is not just a scientific curiosity. For any human mission, local water extraction is a practical necessity. Shipping water from Earth would be prohibitively expensive, so every serious mission architecture relies on harvesting Martian resources. Water can be split into hydrogen and oxygen for rocket propellant, used for drinking and agriculture, and employed as radiation shielding.
The buried ice deposits at mid-latitudes are particularly attractive because they combine relatively accessible water with less extreme conditions than the poles. Analysis of growing colony water demands shows that small-scale extraction from regolith or atmosphere might support initial crews, but a large water extraction system tapping major ice deposits would be needed to sustain any expanding settlement.
24ScienceDirect. Water extraction on Mars for an expanding human colonyWhat Martian Meteorites Reveal About the Planet’s Interior Water
Some of what we know about Martian water comes not from spacecraft orbiting or rolling across Mars, but from rocks that Mars accidentally sent to Earth. Impacts on Mars occasionally launch fragments into space, and a handful have landed on Earth as meteorites. A group of these called nakhlites (named after the Egyptian town where one was first found) contain minerals that trapped tiny amounts of hydrogen from the Martian mantle before they were ejected.
By measuring the hydrogen content and deuterium-to-hydrogen ratio in the least-altered minerals within nakhlites, researchers estimated that the portion of the Martian mantle sampled by these rocks contains roughly 59 to 184 parts per million of water and has a deuterium signature significantly enriched relative to Earth.
25Geochimica et Cosmochimica Acta. Determination of the water content and D/H ratio of the martian mantle by unraveling degassing and crystallization effects in nakhlitesThose numbers help constrain how much water Mars started with and how its water budget evolved. The deuterium enrichment in the mantle itself means that loss to space has affected even the planet’s deep interior over geological time, not just the atmosphere. Continued refinement of these measurements from multiple Martian meteorites gives scientists an independent check on the orbital and lander-based estimates of Martian water history.
26Earth and Planetary Science Letters. Evolution of water reservoirs on Mars: Constraints from hydrogen isotopes in martian meteoritesMars in the Broader Story of Planetary Habitability
Mars is not studied in isolation. Together with Venus and Earth, it forms a natural laboratory for understanding how rocky planets gain, keep, or lose their water and atmospheres. All three started with broadly similar ingredients, yet their fates diverged dramatically. Earth kept its water and thick atmosphere. Venus lost its water and developed a runaway greenhouse. Mars lost most of its atmosphere and froze.
Studying these divergent outcomes helps scientists evaluate the habitability of rocky planets orbiting other stars. A key insight from comparing the three is that a planet’s ability to cycle gases between its interior, surface, and atmosphere makes simple models based only on how much atmosphere a star strips away probably incorrect. Mars teaches us that even a planet that loses most of its surface water can sequester significant amounts underground and in polar ice for billions of years, a finding with real implications for how we assess distant worlds we will never visit with rovers.
27Journal of Geophysical Research: Planets. Using Venus, Earth, and Mars to Understand Exoplanet Volatile and Climate Evolution