ESA says its Mars Express probe has returned some of the strongest evidence yet that an ocean once covered part of Mars.
Using two years' worth of data from its MARSIS radar system, it's detected sediments reminiscent of an ocean floor within the boundaries of previously identified, ancient shorelines on Mars.
"We interpret these as sedimentary deposits, maybe ice-rich," says Dr Jérémie Mouginot, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) and the University of California, Irvine. "It is a strong new indication that there was once an ocean here."
Evidence of possible ancient shorelines have led scientists to propose the existence of oceans at two different periods in Mars' past - four billion years ago, when conditions were warmer, and also three billion years ago when subsurface ice melted following a large impact, creating outflow channels that drained the water into low-lying areas.
MARSIS penetrates deep into the ground, revealing the first 60 to 80 metres of the planet's subsurface - and throughout this depth, the team's found evidence for sedimentary material and ice.
Such sediments are typically low-density granular materials that have been washed away by water and carried to their final destination, and support the theory of oceans three billion years ago.
However, this ocean would have lasted no more than a million years, before either freezing back in place and being preserved underground again, or turning into vapour.
"I don't think it could have stayed as an ocean long enough for life to form," says Dr Mouginot.
A much better candidate for life is the possible earlier ocean, when liquid water would have existed for much longer periods.