Mars, the cold, arid world of today, bears little resemblance to the planet it once was billions of years ago. Geological evidence strongly indicates that early Mars hosted a climate warm enough to support vast bodies of liquid water on its surface. This ancient reservoir, often referred to as Oceanus Borealis, represents a massive episode in the planet’s history. The profound transformation from a water-rich environment to the desiccated desert seen by modern rovers is a central mystery in planetary science.
The Evidence for a Martian Ocean
The scientific case for a primordial Martian ocean is built upon multiple lines of evidence gathered from orbiters and surface missions. A fundamental observation is the Martian dichotomy, a stark difference in elevation between the lower northern hemisphere and the higher, heavily cratered southern highlands. This suggests the northern plains were once filled by a liquid. Furthermore, orbital imagery has revealed extensive features resembling ancient river deltas and long-abandoned shorelines, such as those found at Aeolis Dorsa. These topographical markers indicate the presence of a stable, long-lived body of water in the northern lowlands.
The ground-level evidence gathered by rovers confirms that water was abundant and chemically active. Minerals like hydrated silicates (clays) and carbonates, which form only in the presence of liquid water, have been identified within ancient Martian rock formations. For example, the Curiosity rover found layered sedimentary rocks in Gale Crater that suggest the existence of an ancient lake. These mineral signatures act as a chemical record of the aqueous processes that once shaped the surface.
The most compelling evidence is found in the ratios of water isotopes in the atmosphere and rocks. Scientists measure the ratio of deuterium (a heavier isotope of hydrogen) to normal hydrogen (D/H ratio). Normal hydrogen, being lighter, escapes Mars’s gravity more easily when water vapor is broken down in the upper atmosphere. The remaining water is highly enriched in deuterium, a signature explained only by the massive, preferential loss of lighter hydrogen to space over billions of years.
Scale and Location of the Ancient Ocean
The ancient ocean, known as Oceanus Borealis, was primarily situated in the northern lowlands, filling the Vastitas Borealis basin. This region is topographically the lowest on the planet, lying four to five kilometers below the mean planetary elevation. The basin’s topography would have naturally collected surface water that flowed from the southern highlands through massive outflow channels.
Estimates suggest this ocean could have covered nearly one-third of the planet’s surface. The volume of water is thought to have been comparable to that of Earth’s Arctic Ocean, potentially reaching 60 million cubic kilometers. If this water were spread uniformly across the Martian globe, it would create a layer more than 500 meters deep.
The evidence suggests the northern ocean may have existed in at least two distinct phases. The first was a large, deep ocean, potentially 4.1 to 3.8 billion years ago. This was followed by a period of desiccation and then a possible later, smaller body of water. Sedimentary deposits found at sites like Aeolis Dorsa support the idea of a dynamic and changing coastline, showing evidence of sea-level rise and fall over time.
How Mars Lost Its Water
The disappearance of the massive ocean is attributed to a combination of geological and atmospheric factors. The first significant event was the demise of Mars’s global magnetic field, which shut down around four billion years ago. This magnetic field, generated by a churning core, once acted as a protective shield, deflecting the solar wind—a stream of charged particles emanating from the Sun.
Once the magnetic field vanished, the solar wind began to bombard the upper atmosphere directly. This stripped away atmospheric gases, including water vapor broken down by solar ultraviolet radiation. The lighter hydrogen atoms were then accelerated by the solar wind and lost to space, a process confirmed by measurements from missions like MAVEN. This atmospheric escape mechanism was the primary driver that turned the planet into the cold, low-pressure world it is today.
The remaining water was lost through sequestration, where it became locked up in the planet itself. A significant portion froze into massive underground ice sheets, forming a planetary permafrost layer. Additionally, some water chemically reacted with surface rocks to form hydrated minerals, such as the clays and sulfates observed by rovers. These processes effectively removed the water from the active surface cycle, trapping it as ice in the subsurface or as a permanent chemical component of the crust.
Where Martian Water Exists Today
While the ancient oceans are gone, vast reserves of water ice remain on Mars today. The most visible reserves are the planet’s polar ice caps, which are composed primarily of water ice, though they are seasonally covered by frozen carbon dioxide. The north polar cap alone contains an estimated 1.6 million cubic kilometers of ice. If melted, this is enough to cover the entire planet in a 35-meter-deep layer.
Even more extensive reserves are found buried beneath the surface, particularly at mid-to-high latitudes. Data from orbiters show that water ice concentrations exceed 20% in the near-surface soil poleward of 60° latitude in both hemispheres. This subsurface ice extends surprisingly close to the equator, with shallow ice layers exposed by fresh impact craters. This confirms that a substantial portion of the lost ocean is frozen beneath a protective layer of dust and rock.
There is also compelling evidence for small pockets of liquid water that may exist today. Radar and topographic data suggest the possibility of subglacial liquid water beneath the south polar ice cap. This water would need a source of geothermal heat to remain liquid in the frigid environment, implying Mars still possesses some internal warmth. Furthermore, seasonal features known as Recurring Slope Lineae (RSL) are thought to be caused by the temporary flow of briny, or salty, water, which can remain liquid at lower temperatures than pure water.