Mars does not have oceans today. Its surface is cold, dry, and exposed to near-vacuum atmospheric pressure, making stable liquid water impossible. But roughly 3.5 to 4 billion years ago, Mars almost certainly had a large ocean covering much of its northern hemisphere, and the evidence for that ancient body of water has grown substantially in recent years.
What the Surface Looks Like Now
Present-day Mars has an atmospheric pressure less than 1% of Earth’s. At that pressure, liquid water on the surface would either freeze or boil away almost instantly. The average surface temperature sits around minus 60°C. There are no lakes, rivers, or oceans anywhere visible on the planet.
What Mars does have is ice. The south polar layered deposits are a formation of nearly pure water ice reaching more than 3 kilometers thick. The north pole has a similar ice cap. Together, these hold a significant amount of frozen water, but nothing close to the volume needed to fill an ocean basin.
Evidence for an Ancient Ocean
The strongest case for a past Martian ocean centers on the vast northern lowlands, a basin so large it covers roughly a third of the planet’s surface. Scientists have long noticed features along the edges of this basin that look like ancient shorelines, and in 2025, data from China’s Zhurong rover provided some of the most direct evidence yet. The rover imaged structures consistent with beach ridges, the kind of layered sediment deposits created by tidal action. The shoreline-perpendicular length of these deposits stretches more than 1.3 kilometers, suggesting they formed in a persistent, stable ocean rather than a temporary flood.
One challenge with the shoreline hypothesis is that the proposed boundaries sit at multiple elevations, which wouldn’t happen along the edge of a flat body of water. Scientists have proposed explanations for this: the planet’s axis has shifted over billions of years (a process called true polar wander), and the massive volcanic region known as Tharsis deformed the crust under its weight. Either process could have warped the original shoreline into the uneven elevations seen today.
How Much Water Mars Once Had
Scientists estimate the total water needed to sustain a Martian ocean and its accompanying ice sheets at around 700 meters of global equivalent layer. That means if you spread all the water evenly across the entire planet, it would form a layer about 700 meters deep. Roughly half of that would have been in the ocean itself and half locked in polar ice sheets, a balance not unlike Earth’s water cycle.
The ratio of deuterium (heavy hydrogen) to regular hydrogen in the Martian atmosphere offers another way to estimate past water. Today, Mars has a deuterium-to-hydrogen ratio five to eight times higher than Earth’s ocean water. That enrichment happens because lighter hydrogen atoms escape to space more easily, leaving the heavier deuterium behind. Working backward from that ratio, scientists estimate Mars has lost enough water to cover its entire surface to a depth of roughly 300 meters, on top of whatever remains frozen today.
Why the Water Disappeared
Mars lost its ocean primarily because it lost its atmosphere. Early Mars likely had a thick atmosphere capable of trapping enough heat to keep water liquid. But unlike Earth, Mars has no global magnetic field. Without that shield, the solar wind (a constant stream of charged particles flowing from the Sun) slams directly into the upper atmosphere, accelerating gas molecules fast enough to escape the planet’s gravity entirely.
NASA’s MAVEN spacecraft confirmed this process by directly measuring ions being stripped away from Mars in real time. Over billions of years, this steady erosion thinned the atmosphere to the point where surface pressure dropped too low for liquid water to persist. As temperatures plummeted, surface water either escaped to space, froze into polar caps, or seeped underground.
The Timeline of Martian Water
Mars’s wet period maps onto two geological eras. During the Noachian period (4.1 to 3.7 billion years ago), valley networks carved the surface, lakes filled craters and basins, and a shallow ocean likely covered at least part of the northern lowlands. This was Mars at its most Earth-like.
During the Hesperian period (3.7 to 2.9 billion years ago), the climate cooled and most remaining water froze into permafrost or subsurface ice. Occasional catastrophic floods still occurred when underground water was heated by volcanic activity or asteroid impacts, surging across the surface before refreezing. By the end of the Hesperian, Mars had transitioned into the dry, frozen world we see today.
Water That May Still Exist
While the surface is bone dry, there are tantalizing hints of liquid water hidden beneath it. In 2018, researchers using the MARSIS radar instrument on the Mars Express spacecraft reported a 20-kilometer-wide zone beneath the south polar ice cap with radar reflections consistent with liquid water. The feature sits deep below the ice and would need to be extremely salty (briny) to remain liquid at those temperatures. More recent analysis has suggested the bright radar reflections could also be explained by variations in ice composition and dust layers, so the underground lake interpretation remains debated.
Seasonal dark streaks called recurring slope lineae appear on steep Martian slopes during warmer months and fade in winter. These were initially thought to be evidence of briny water trickling down hillsides. Research now suggests they could originate from briny aquifers roughly 750 meters below the surface, with water rising along geological faults and reaching the surface during summer when shallow ice melts enough to open the pathways. Hydrated salts detected at several of these sites support the idea that brine, not pure water, is involved. But dry granular flows remain an alternative explanation, and the question is unresolved.
Seasonal marsquakes detected by NASA’s InSight lander have added another layer to the story. Some researchers interpret patterns in these quakes as consistent with near-surface liquid brines expanding and contracting with temperature changes, though this evidence is indirect and open to other interpretations.
How Mars Compares to Earth
Earth’s oceans cover about 71% of the surface and average 3.7 kilometers deep. Mars’s ancient ocean, by comparison, would have covered roughly a third of the surface at a fraction of that depth. It was more comparable to a vast shallow sea than to Earth’s deep ocean basins. The total water inventory on ancient Mars (around 700 meters GEL) is modest next to Earth’s roughly 2,700 meters GEL, but it was enough to sustain a water cycle with evaporation, precipitation, ice sheets, and tidal coastlines for hundreds of millions of years.