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Geological features on Mars suggest the planet once boasted rivers and broad coastlines around a vast ocean. The discovery provides the most direct evidence yet for the Red Planet’s former blue vistas.

“The presence of liquid water [on Mars] is a broad topic that includes rains, rivers, lakes, as well as oceans,” says , a geochemist at Jackson State University in Mississippi who wasn’t involved in the research. “In my opinion, this paper addresses the most important one: the ocean.”

A research team including planetary geologist and geoscientist , both at the University of Bern in Switzerland, made the discovery by analysing data gathered from different probes orbiting Mars, including NASA’s Mars Reconnaissance and the European Space Agency’s Mars Express and ExoMars Trace Gas Orbiter. The ExoMars vessel is equipped with a specialised camera, the Bernese Mars, which can take high-resolution colour images. It was instrumental to the new work.

“These images allow us to distinguish subtle differences in surface materials that are invisible in black-and-white images,” says Indi. When combined with topographic data from the other probes, these tools become “geological time machines” that paint a clearer picture of how the Martian landscape formed, he says.

To apply the imagery to the question of putative past water sources on Mars, the researchers focused on Valles Marineris, a channel more than 4000 kilometres long that cuts along the Martian equator. They paid particular attention to a south-eastern region of the channel called Coprates Chasma. Its deep features are around 3.3 billion years old.

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By combining the new imagery with geomorphological analyses, the researchers identified many structures comparable to those that form on Earth where rivers pour into oceans or where alpine lakes materialise at the base of mountains.

“The Nile delta is a classic example,” says Schlunegger. If you removed the Mediterranean waters just beyond the terminus of the Nile, he says, you would see structures very similar to those detected on Mars.

The new data also enabled the researchers to trace ancient coastlines around a former ocean, allowing them to measure its approximate size. They concluded that it was about as large as Earth’s Arctic Ocean. This would have been the largest ocean on Mars.

“Our research suggests that around 3 billion years ago, Mars may have hosted long-lasting bodies of surface water inside Valles Marineris, the largest canyon in the Solar System,” says Indi. “Even more exciting, these water bodies may have been connected to a much larger ocean that once covered parts of Mars’ northern lowlands.”

Previous research has given many scholars reason to believe Mars once held water, but this work has primarily been indirect. One study found Martian in the distant past. Another found signs that ancient asteroid impacts triggered massive Martian tsunamis. While much evidence has hinted at a past humid world, definitive data is difficult to come by.

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“The idea that Mars once had large oceans remains controversial – in part because, if they existed, their record is not fresh because they are so old,” says , a geoscientist at the University of California, Berkeley who wasn’t involved in the research.

The discovery has tantalising implications for the search for extraterrestrial life, but also offers a cautionary tale that Earth’s most indispensable resource might one day similarly disappear.

“The paper… addresses an issue that is in the mind of anyone who conducts research on the evolution of the planet Mars,” says Heydari. “Oceans on Mars would have acted just like oceans on Earth, and they are vital to the health of the planet.”

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