A spectacular orbital view imagining what Mars would have been like with a vast ocean in its northern hemisphere. The ultimate contrast between what could have been and the dusty desert it is today.
The current Martian landscape is a cold, desolate desert, swept by abrasive dust winds and subjected to an atmosphere so thin that liquid water would instantly evaporate on its surface. However, looking at the topography of its northern hemisphere, we encounter a vast plain of surprisingly smooth lowlands that contrasts drastically with the rugged, crater-filled highlands of the southern hemisphere. This geological dichotomy has led the astrobiological community to ask one of the most momentous questions in planetary science: did a giant ocean really exist in the northern hemisphere of Mars in its distant past? The clues written in Martian rock suggest that, about four billion years ago, the northern half of the Red Planet may have hosted Oceanus Borealis, a colossal body of liquid water that would completely redefine our understanding of planetary habitability.
Waterprints: ancient deltas and fossil shorelines
The most irrefutable physical evidence of this wet past is found in the ancient river deltas and valley networks that empty into the northern plains. Space missions equipped with high-resolution cameras have mapped sedimentary structures identical to terrestrial deltas in craters like Jezero, the exploration site of the Perseverance rover. These deltas —which resemble the alluvial fans that form at the mouth of Earth's rivers when they deposit their mud as they lose speed— prove that rushing rivers flowed constantly for eons. Furthermore, several planetary geologists claim to have identified traces of ancient shorelines, such as the Arabia and Deuteronilus formations, which run for thousands of kilometers along the boundary of the Martian dichotomy, marking the water level of an extinct sea.
Curiously, the interpretation of these shorelines remains a subject of intense scientific debate. Wind and the constant impact of meteorites over billions of years have eroded and deformed these reliefs, making their exact correlation difficult. Some geophysicists suggest that the apparent deformation of this fossil coasts is due to true polar wander on Mars caused by a shift in its rotation axis, a colossal phenomenon that relocated the weight of the water mass on the planet. Despite these discrepancies, the accumulation of smooth sedimentary deposits in the northern lowlands remains difficult to explain without the presence of a planet-wide body of water acting as the hemisphere's great sedimentary sink.
The chemical signature of clays and hydrated minerals
Beyond physical topography, spectroscopy from space has provided key chemical evidence in favor of the Martian ocean. Mineralogical maps show an unusual abundance of hydrated minerals, specifically phyllosilicates and iron- and magnesium-rich clays, concentrated along the edge of the northern plains. These minerals cannot form under dry conditions; they require the prolonged and peaceful interaction of liquid water with primordial basaltic rock. It is exactly the same chemical reaction that transforms terrestrial granite into the moist, malleable clay that sculptors work with in their studios.
The presence of these hydrated silicates indicates that Mars' water was not a transitory phenomenon or one caused by catastrophic and short-lived floods, but that the liquid interacted with the surface for hundreds of millions of years in a neutral and temperate environment. This alkaline water chemistry is especially exciting for astrobiology, as it is the perfect liquid medium for the development of the complex organic chemistry that gives rise to life. Martian clays function as a geological record of the planet's internal heat and of an era when Mars had an active hydrological cycle, similar to that of early Earth.
The thermal dilemma of the faint young Sun
Despite the overwhelming geological evidence, the hypothesis of the Martian ocean runs into a puzzling physical paradox: the faint young Sun paradox. Four billion years ago, our Sun shone with about 30% less intensity than it does today. Under that weak starlight, and with Mars' orbit located farther from the Sun than Earth's, climate models predict that the planet should have been a frozen snowball, unable to maintain liquid water. This is the great knot that space climatology is trying to untie today.
To resolve this paradox, scientists theorize about primitive atmospheres rich in carbon dioxide and hydrogen that generated a powerful greenhouse effect, or about colossal volcanic eruptions that released sulfurous gases that warmed the climate intermittently. The natural terraforming of early Mars was a dynamic and delicate process, where liquid water could have existed in a stable state only during warm transient intervals. The study of this forgotten sea is not only a look into the past of our neighboring planet, but a window to understanding the fate of planetary biospheres throughout the universe. The history of Mars teaches us that water is a precious and fleeting gift of cosmic time. Until our next cosmic lesson, fellow travelers.