It no longer comes as any surprise to people that there is water on Mars, We know there are vast amounts of underground ice at the poles and water frost through much of the surface. 

But despite geological features such as valleys that were clearly carved by running water, plus the presence of surface deposits left by the stuff, there is still some controversy over whether the Red Planet ever had vast seas like Earth today.

Now fresh clues have arrived from Mars Express, the European Space Agency’s orbiting probe. Its powerful radar, MARSIS, has produced what the agency is describing as strong evidence for an ancient ocean that once covered part of Mars. The radar picked out sediments that are reminiscent of an ocean floor and which are bordered by shorelines previously been identified by surface studies.

MARSIS has been collecting data since 2005 and the information was analysed by a team led by Jérémie Mouginot, of the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) and the University of California, Irvine.

Their studies reveal that the northern plains of Mars are covered in a low-density material which appears to have been left by an ancient sea. Announcing the find, Dr Mouginot said: “We interpret these as sedimentary deposits, maybe ice-rich. It is a strong new indication that there was once an ocean here.”

Wlodek Kofman, leader of the radar team at IPAG, said: “MARSIS penetrates deep into the ground, revealing the first 60–80 metres of the planet’s subsurface. Throughout all of this depth, we see the evidence for sedimentary material and ice.”

According to planetary scientists’ theories, Mars had oceans twice in its history. The first was four billion years ago, when the planet enjoyed a warmer climate. Then, three billion years ago an impact by a giant asteroid caused ice below the surface to melt and it as channeled to fill low-lying plains. The signature of the sediments detected by MARSIS suggest that they are low-density granular materials that were eroded away by water and carried to their final destination.

Related: New evidence for oceans on Mars

The more recent ocean would not have lasted long. Dr Mouginot believes it would have disappeared underground to freeze once more, or evaporated away into space. Disappointingly for hunters of Martians, he adds: “I don’t think it could have stayed as an ocean long enough for life to form.”

Mars cannot hold onto liquid water today because of its thin, low-pressure atmosphere. But the climate has been very different in the past, allowing it to lie on the surface. Skymania News spoke to planetary scientist Dr Susan Conway, of the Laboratoire de Planétologie et Géodynamique, Université de Nantes, France, who is carrying out separate research looking for evidence of it flowing more recently.

She told us: “What would have made Mars more favourable for liquid water is a thicker atmosphere. Mars’ axis wobbles a lot. As recently as 10,000 years ago, the planet was more tilted than it is today and its poles got a lot more sunlight. The polar caps would melt – or rather sublimate – and because the caps are made of water and carbon dioxide, it would make the atmosphere a lot thicker. That would be enough to make water stable in the martian summer at mid latitudes.

“What this means is that Mars hasn’t long been this really dry, arid planet and these kind of variations could have been happening over the planet’s entire history. We could have had this climate variation giving us liquid water quite regularly, causing erosion, but also possibly giving us small micro environments for life.”