We call it the Red Planet. But three billion years ago, parts of it may have been blue.
A new study led by the University of Bern, published in the journal *npj Space Exploration*, has provided some of the most compelling evidence yet that Mars was once covered by a vast ancient ocean — one that blanketed roughly half the planet's surface and could have been up to a kilometre deep.
The discovery centres on a close examination of the Valles Marineris canyon system — the largest canyon in the solar system, stretching more than 4,000 km across the Martian surface — specifically a region called Coprates Chasma in the southeast. Using high-resolution images from multiple Mars orbiters, including the CaSSIS instrument built in Bern aboard ESA's ExoMars Trace Gas Orbiter, as well as NASA's Mars Reconnaissance Orbiter and ESA's Mars Express, the team identified unmistakable geological structures called scarp-fronted deposits.
On Earth, these formations are well understood. They form at the edges of river deltas — where sediment-laden rivers empty into standing water, depositing their load in characteristic curved, layered shapes. They are geological fingerprints of coastlines.
On Mars, in the depths of Valles Marineris, the team found exactly these structures. Now buried under wind-sculpted dunes and ancient sediment, they allowed researchers to reconstruct a former sea level and trace what may be an ancient Martian coastline.
'This is a smoking gun for a Martian coastline,' researchers said.
The implications are profound. Mars today is a frozen, airless desert. But if liquid water once pooled in ocean-scale quantities on its surface, the planet would have needed a very different atmosphere — warmer, denser, and potentially capable of sustaining the chemistry of life.
Mars was not always like this. For a billion years or more, it may have been somewhere a great deal more welcoming.
The evidence has been waiting three billion years for someone to look.
We looked. 🔴🌊