The image of Mars as a desolate, red world is familiar, yet scientific inquiry points to a past where it was submerged beneath a vast ocean. This transition from a water-rich world to the arid landscape observed today poses one of the great mysteries in planetary science. Understanding what happened to this water drives much of the ongoing exploration of Mars.
Evidence for a Past Ocean
Evidence for a past Martian ocean comes from several sources. Orbiters have identified vast geological features resembling ancient shorelines that trace the edges of the northern lowlands. Images have also revealed deltas, like the one explored by the Perseverance rover in Jezero Crater. These fan-shaped deposits of sediment form when a river slows and empties into a larger body of water.
Further evidence comes from minerals on the Martian surface. Rovers like Curiosity and Perseverance discovered hydrated minerals, such as clays and sulfates, in the rocks and soil. These minerals only form through the interaction of rock with liquid water. Their widespread presence, particularly in suspected ancient lakebeds, indicates water was a significant part of the environment for an extended period.
Atmospheric data provides another clue. The Martian atmosphere has a high ratio of deuterium to hydrogen compared to Earth. Deuterium, a heavier form of hydrogen, is less likely to escape a planet’s gravity. This high ratio suggests that a significant amount of lighter hydrogen—a component of water (H2O)—has been lost to space over billions of years.
Mapping the Martian Seascape
The prevailing hypothesis places this ancient ocean, sometimes called Oceanus Borealis, in the northern hemisphere. This region, the Vastitas Borealis basin, is a massive, low-lying plain. The sharp contrast in elevation between the smooth northern lowlands and the cratered southern highlands, known as the Martian dichotomy, provides a natural basin for an ocean.
This ocean likely existed during Mars’s Noachian and early Hesperian periods, roughly 4.1 to 3.0 billion years ago. During this era, a warmer climate and denser atmosphere would have allowed liquid water to remain stable on the surface. While its exact dimensions are debated, some estimates suggest the ocean could have covered nearly a third of the planet.
The depth of Oceanus Borealis would have varied. Models based on the volume of ancient river valleys flowing into it suggest depths of over a mile in some areas. Its scale would have been immense, potentially rivaling the volume of Earth’s Arctic Ocean. Data from China’s Zhurong rover in the Utopia Planitia basin has also identified subsurface structures resembling coastal sediments, supporting this picture.
The Great Martian Disappearance
Mars’s transformation from a wet to a dry world was driven by changes deep within its core. Early in its history, a molten iron core generated a global magnetic field, much like Earth’s. This field shielded the planet from the solar wind—a stream of charged particles from the Sun. This protection was what maintained a thick atmosphere capable of supporting liquid water on the surface.
About four billion years ago, the Martian core cooled and solidified, causing its global magnetic field to shut down. Without this protective field, the solar wind began to directly impact the atmosphere. Over millions of years, the solar wind stripped away lighter atmospheric gases. This process thinned the atmosphere, causing surface pressure to plummet.
As the pressure dropped, the water in the northern ocean began to boil away into vapor. Once in the upper atmosphere, this vapor was vulnerable to solar radiation. The radiation broke the water molecules apart, and their constituent hydrogen atoms were lost to space.
While a significant portion of the water was lost to the cosmos, another portion was absorbed into the planet’s crust. Water molecules chemically bonded with minerals in the rock. This process locked the water away in a solid state.
Water on Mars Today
Although the great northern ocean is gone, its water has not vanished. The largest visible reservoirs of water on Mars are its polar ice caps, composed of water ice and frozen carbon dioxide. The northern polar cap is predominantly water ice and contains a significant volume of water.
Beyond the poles, large quantities of water are frozen beneath the surface as subsurface ice, especially at the mid-latitudes. Ground-penetrating radar from orbiters has detected these extensive deposits, often just a few feet below the dusty surface. Seismic data from the InSight lander also suggests a possible deep reservoir of liquid water trapped within rock layers many kilometers down.
Hydrated minerals scattered across the planet’s soil represent another form of trapped water. This water is not liquid or ice but is chemically bound within the mineral structure of the Martian regolith. The presence of water in these various forms—as polar ice, subsurface ice, and locked in minerals—is a driver for the search for evidence of past microbial life.