Quick Answer
Yes, there is water on Mars — but almost all of it is frozen. Mars holds vast amounts of water ice in its polar caps and buried beneath its surface, and billions of years ago it had rivers, lakes, and possibly an ocean. Today the planet is too cold and its atmosphere too thin for liquid water to last on the surface, though briny, salty water may still exist underground. Water is the single most important resource for any future Mars colony.
Few questions about the Red Planet are searched as often as “is there water on Mars?” The answer matters enormously, because water is the master key to everything — drinking, breathing, growing food, making rocket fuel, and the possibility of life itself. This guide traces Mars’ water through its past, present, and future, and explains why a planet that once had oceans is now a frozen desert.
Did Mars Once Have Oceans?
The evidence that ancient Mars was a wet world is overwhelming. Roughly 3.5 to 4 billion years ago, Mars appears to have been warmer, wetter, and wrapped in a thicker atmosphere. Liquid water flowed across its surface, carving features we can still see today and leaving chemical fingerprints in its rocks.
Evidence from riverbeds and minerals
Orbiters and rovers have mapped sprawling networks of dried-up river valleys, ancient lake basins, and enormous outflow channels far larger than anything on Earth. NASA’s Perseverance rover is exploring Jezero Crater, which contains a textbook river delta — sediment fanned out exactly as it would where a river once emptied into a standing lake.
The mineral evidence is just as compelling. Spacecraft have detected clays and sulfate salts that only form in the presence of water, and the Opportunity rover famously found tiny hematite spheres — nicknamed “blueberries” — that form in watery conditions. Many scientists think a shallow ocean may once have covered much of the low-lying northern hemisphere. To understand how that warm, wet world was lost, see our companion article on why Mars lost its atmosphere.
Where Is Mars’ Water Now?
Mars did not lose all of its water — much of it simply froze or retreated underground. The planet still holds enough ice that, if melted and spread evenly, it could cover the whole surface in a shallow global sea.
Polar ice caps
Mars has bright polar caps at both poles. They are made mostly of water ice, capped by a seasonal layer of frozen carbon dioxide (dry ice) that grows and shrinks with the Martian seasons. The permanent caps, especially in the south, lock away a huge reservoir of water ice — a substantial fraction of the planet’s known water.
Subsurface ice and possible briny lakes
Beyond the poles, orbiters using ground-penetrating radar have found buried ice across vast areas, including thick deposits at mid-latitudes. The Phoenix lander dug into shallow soil near the north pole in 2008 and exposed white ice that vanished by sublimation over a few days, confirming ice lies just centimetres beneath the surface in places.
There are also tantalising hints of liquid water. In 2018, radar data suggested a possible pond of briny liquid water beneath the south polar ice, though later studies argued the bright radar signal might instead come from certain clays or frozen materials — so it remains debated. Salty brines can stay liquid at lower temperatures than pure water, which keeps the door open for small amounts of underground liquid today.
Why Liquid Water Can’t Last on the Surface Today
The reason Mars cannot keep surface water is a matter of pressure and temperature. The Martian atmosphere is extremely thin — only about 0.6% the pressure of Earth’s at sea level — and the average surface temperature is a frigid −60°C. Under such low pressure, water cannot remain a stable liquid: it either freezes or, when warmed, boils and sublimates straight into vapour even at low temperatures.
This is a consequence of physics called the triple point of water. On most of Mars, conditions sit below the pressure at which liquid water is stable, so water skips the liquid phase almost entirely. Any liquid that did appear — say, from melting ice on a warm day — would not last long before freezing or evaporating away.
Could We Melt Mars’ Ice to Create Water?
If Mars has so much frozen water, could we simply melt it and bring the planet back to life? This is the central idea behind terraforming, and it is exactly the scenario we explore in what if a space mirror melted Mars’ ice caps. The concept is to use giant orbital mirrors, or to deliberately thicken the atmosphere, to warm the planet enough that ice turns to liquid and vapour.
The physics is genuinely tricky. Melting the ice is one thing; keeping the water liquid is another, because that requires raising both the temperature and the atmospheric pressure at the same time. Releasing frozen carbon dioxide could thicken the air and trap heat in a greenhouse effect, but studies suggest Mars may not have enough accessible CO2 to do this on its own. Still, the thought experiment captures why water is the linchpin of any plan to make Mars habitable.
What Water Means for Future Mars Colonists
For the first humans on Mars, finding water locally would be transformative. Hauling water from Earth is prohibitively expensive, so a colony would need to “live off the land,” extracting water from buried ice and the atmosphere.
- Drinking and farming: the obvious necessity for survival and growing food.
- Breathable oxygen: water can be split into hydrogen and oxygen by electrolysis.
- Rocket fuel: that same hydrogen and oxygen make potent propellant for the trip home.
- Radiation shielding: water and ice are effective at blocking harmful space radiation.
This is why mission planners care so much about where the ice is. A base built near accessible subsurface ice could become largely self-sufficient in water, oxygen, and fuel — turning a frozen liability into the foundation of a settlement.
Missions Hunting for Martian Water
Tracking Mars’ water has been a goal of nearly every recent mission. Mars Odyssey first mapped large amounts of subsurface hydrogen (a sign of ice) from orbit. The Mars Reconnaissance Orbiter’s radar and high-resolution cameras revealed buried glaciers and seasonal flows. The Phoenix lander touched Martian ice directly, and Curiosity and Perseverance study rocks shaped by ancient water. Europe’s ExoMars Trace Gas Orbiter continues to map subsurface water-rich layers, while the MAVEN orbiter studies how Mars lost its water and air to space over billions of years.
Q&A
Not straight from the ground. Martian ice and soil are mixed with perchlorate salts and dust that are toxic to humans, so the water would need to be melted and purified first. Once filtered and treated, however, it could be made safe to drink.
Yes. Mars gets two kinds of snow: ordinary water-ice snow high in the atmosphere, and carbon-dioxide (“dry ice”) snow that falls near the poles in winter. Spacecraft have detected snow falling from Martian clouds, though it often sublimates before reaching the ground.
Possibly, but unconfirmed. Seismic data from the InSight lander has been interpreted as hinting at liquid water deep in the Martian crust, and brines may exist near the south pole. There is no proof of a large underground ocean, but the deep subsurface remains one of the best places to keep liquid water stable.
The wettest period was roughly 3.5 to 4 billion years ago, during what geologists call the Noachian era. After that, Mars gradually lost its thick atmosphere and cooled, and surface water froze or escaped — a transition explored in our article on why Mars lost its atmosphere.
The Bigger Question
Mars proves that a planet can have oceans and then lose them. The water is still there, locked in ice — which raises the tantalising possibility of melting it to wake the planet up. That is the dream, and the engineering nightmare, at the heart of what if a space mirror melted Mars’ ice caps. Whether such terraforming is even possible depends on the very atmosphere Mars lost long ago, which we unpack in why Mars lost its atmosphere.
For more on humanity’s future beyond Earth, visit our Space & Cosmos hub.
Watch the terraforming scenario to see what would really happen if we tried to melt the Red Planet’s ice.