Water on Mars
Follow the Water
“Follow the water” has been a guiding principle for Mars exploration. Water is essential for life as we know it, and it is equally essential for future human settlement. The story of water on Mars is one of the most compelling chapters in planetary science — a tale of a world that once had rivers, lakes, and possibly even an ocean, and that still holds vast reserves of ice today.
Evidence of a Watery Past
Multiple lines of evidence point to a time when liquid water flowed across the Martian surface. Orbital images reveal ancient river channels, some stretching hundreds of kilometers, with branching tributary patterns strikingly similar to river systems on Earth. These channels were carved by flowing water billions of years ago.
Deltas and lakebeds provide further evidence. NASA’s Perseverance rover landed in Jezero Crater, a 45-kilometer-wide (28-mile) impact basin that once held a lake fed by at least two river channels. A clearly visible delta — a fan-shaped deposit of sediment — marks where a river once entered the lake. Jezero Crater is one of the best-preserved examples of an ancient Martian lake environment.
NASA’s Curiosity rover, exploring Gale Crater, has found layers of sedimentary rock that formed at the bottom of a lake that existed for millions of years. The rover has also detected clay minerals and sulfate minerals that only form in the presence of water, confirming sustained wet conditions.
The Ancient Ocean Hypothesis
Some scientists believe that Mars once had a vast ocean covering much of its northern hemisphere. This idea is supported by features along the boundary between the northern lowlands and southern highlands that resemble ancient shorelines. Data from NASA’s MAVEN mission and mineral analysis support the hypothesis that enough water existed on early Mars to fill an ocean hundreds of meters deep across the northern plains.
If confirmed, this ancient ocean — sometimes called Oceanus Borealis — would have made early Mars a world with a hydrological cycle similar to Earth’s, with evaporation, clouds, precipitation, and flowing rivers feeding into a great northern sea.
Where Is the Water Now?
Mars lost most of its surface water over billions of years. As the atmosphere thinned and temperatures dropped, liquid water could no longer persist on the surface. But it did not all escape into space — much of it is still on Mars, locked away as ice.
Polar Ice Caps
Mars has prominent ice caps at both poles. The north polar cap is composed primarily of water ice and spans about 1,000 kilometers (620 miles) across. During winter, a seasonal layer of carbon dioxide frost (dry ice) covers it. The south polar cap has a permanent layer of CO2 ice on top, with water ice beneath.
Together, the polar caps contain enough water ice that, if melted, could cover the entire planet in a layer of water about 11 meters (36 feet) deep.
Subsurface Ice
Beyond the poles, vast deposits of subsurface ice exist across the mid-latitudes of Mars. Radar instruments on orbiters like Mars Express and Mars Reconnaissance Orbiter have detected extensive ice sheets buried just meters below the surface. In some locations, erosion has exposed clean ice in cliff faces, visible in orbital photographs.
In 2018, scientists using the Mars Express orbiter’s radar instrument reported evidence of a subsurface lake of liquid water beneath the south polar ice cap, about 1.5 kilometers (1 mile) below the surface. While this finding is still debated, if confirmed, it would represent the first known body of standing liquid water on Mars today.
Seasonal Brines
Even on the surface, water may play a role in modern Mars. Dark streaks called recurring slope lineae (RSL) appear on steep slopes during warm seasons and fade in colder months. Initially, scientists proposed these could be caused by briny (very salty) water flowing downhill. More recent studies suggest dry processes may also explain some of these features. The question of whether small amounts of liquid water exist on Mars today remains an active area of research.
Water and Human Settlement
Water on Mars is not just a scientific curiosity — it is a critical resource for future human settlers. Water can provide:
- Drinking water for crews
- Oxygen for breathing (by splitting H2O into hydrogen and oxygen)
- Rocket propellant (liquid hydrogen and liquid oxygen)
- Agriculture support for growing food in greenhouses
Access to Martian water ice will be a major factor in choosing landing sites for future human missions. Locations with accessible subsurface ice near the equator would be ideal, combining warmer temperatures with water resources.
A Story Still Unfolding
Every mission to Mars adds new chapters to the water story. Perseverance is collecting rock samples from Jezero Crater’s ancient lakebed that will eventually be returned to Earth for detailed analysis. These samples may contain preserved organic molecules or even traces of ancient microbial life that once inhabited Martian waters. The search for water on Mars is ultimately a search for the conditions that could support life — past, present, and future.