Vast oceans on Mars formed thousands of millions of years earlier than previously thought, according to a new study by planetary scientists. 

The seas, covering much of Mars’ northern hemisphere, appeared 4 billion years ago. That was before or at the start of a period of violent volcanic activity that deformed the planet’s crust.

Evidence for ancient seas and lakes has been recorded by NASA and ESA spacecraft in orbit around Mars. But the previous suggestions for a huge ocean flooding the northern plains ran up against a couple of snags.

One was the claim that such an ocean would have been too big to match estimates of how much water has escaped into space and disappeared underground as permafrost.

The other was that shorelines said to have been left by the ocean are irregular, rather than level like those on Earth, and at elevations varying by as much as a kilometer.

The new study answers this by suggesting that the early oceans on Mars formed before, or at the same time as, a plateau called Tharsis that produced three of the biggest volcanoes in the Solar System: Arsia Mons, Pavonis Mons, and Ascraeus Mons.

Tharsis formed 3.7 billion years ago. Today it is around 5,000 km (3,000 miles) wide and a notable bulge on the martian surface. But billions of years ago, Tharsis was a lot smaller than it is now. Because of that, it distorted the shape of the crust less, and so the oceans would have been shallower, containing around half as much water as previously estimated.

The volcanic activity at Tharsis played a key role in warming the martian atmosphere, allowing liquid water to form oceans on Mars, according to the research by a team of geophysicists at the University of California, Berkeley, and published in Nature this week.

Senior author Professor Michael Manga said: “Volcanoes may be important in creating the conditions for Mars to be wet.”

He believes that volcanic gases filling the atmosphere created a greenhouse effect that warmed Mars. On top of that, eruptions created channels that allowed underground water to reach the surface and flood the northern plains.

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Professor Manga added: “The assumption was that Tharsis formed quickly and early, rather than gradually, and that the oceans came later,” Manga said. “We’re saying that the oceans predate and accompany the lava outpourings that made Tharsis.”

Scientists have named the first of the oceans on Mars Arabia, and say it existed for as much of the first fifth of Tharsis’ growth. As the volcanic plateau grew, it depressed the land around it, causing Arabia’s shoreline to deform, leaving the irregular pattern seen today.

A similar explanation is offered for the development of a later ocean, called Deuteronilus, about 3.6 billion years ago during the last 17 per cent of Tharsis’ growth.

UC Berkeley graduate student Robert Citron, who is first author of the new paper, said: “These shorelines could have been emplaced by a large body of liquid water that existed before and during the emplacement of Tharsis, instead of afterwards.”

Planetary scientists are looking forward to the launch, in May 2018, of NASA’s latest mission to Mars, called InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) because it will place a seismometer on the martian surface to learn more about what lies beneath it in the planet’s crust, mantle and core. The mission was due to launch in 2-16 but was delayed two years.

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