A Martian geological structure could provide evidence of an ocean that once covered a third of the red planet. This unexpected finding transforms our understanding of Mars' history.
Previous research hit an obstacle: the observed shorelines were located at different altitudes, which contradicts the idea of a stable ocean. Scientists therefore changed their strategy: rather than looking for beaches, they identified continental plateaus, similar to those that border oceans on Earth.
To imagine what a vanished ocean might look like, researchers virtually drained Earth's oceans using simulations. They observed that coastal plains and continental plateaus — those flat, low-lying strips of land — remain stable even when sea levels fluctuate. These formations are therefore reliable markers of ancient oceans.
Applying this reasoning to Mars, the team identified a flat area in the northern hemisphere. This area has the shape of a continental plateau and indicates that an ocean covered a third of the planet for millions of years.
This plateau is all the more remarkable because Martian river deltas align perfectly with it, just as on Earth. If life ever existed on Mars, the sediments there could preserve traces of it, similar to terrestrial fossils.
One question remains unanswered: how did this plateau form? Even on Earth, scientists are not certain of the exact mechanism. Future Martian missions could analyze these rocks and perhaps understand the phenomenon of liquid water on the red planet.
How do continental plateaus form on Earth?
Continental plateaus are flat, shallow areas that border continents, underwater. Their formation is linked to rock erosion and the deposition of sediments brought by rivers and waves. They sometimes extend for hundreds of miles (kilometers) before plunging into the abyss.
These plateaus are stable over long geological periods, unlike shorelines that change with glacial cycles. Their relative altitude remains constant, making them excellent indicators for detecting ancient oceans, even those that disappeared millions of years ago.
On Earth, they host great biodiversity and are rich in resources such as oil. Studying them helps us understand the history of sea levels and tectonic plate movements, but their exact origin is still debated among geologists.
What is the northern ocean hypothesis on Mars?
The idea of an ocean in Mars' northern hemisphere, called the northern ocean, has been circulating since the 1980s. Images from probes show low, flat terrains that could be ancient seabeds. But the evidence remained indirect because shorelines were not at the same altitude.
This new study provides a strong argument by identifying a typical continental plateau, suggesting the ocean was stable for a very long time. This raises hope that organic sediments may have accumulated there, preserving possible traces of life.
If this ocean existed, it would have transformed the Martian climate. Water modulates the carbon cycle and could have allowed warmer periods. Understanding its disappearance will help explain why Mars became a frozen desert.