The ocean would have covered the northern plains billions of years ago. The radar detected sediments reminiscent of an ocean floor inside previously identified, ancient shorelines on the red planet

• Oceans across north pole of planet
• Identified by radar scan of surface from Mars orbiter
• 'Oceans' fall within boundaries of 'shores' previously seen by space scans
• Area covered by oceans twice - last time three billion years ago

Today, Mars's surface is an arid desert - but the European Space Agency's Mars Express spacecraft has returned strong evidence for an ocean once covering part of the red planet's surface.

Using radar, the orbiter has detected sediments reminiscent of an ocean floor within the boundaries of areas previously picked out as ancient shorelines on Mars.

Jérémie Mouginot of University of California, Irvine, and colleagues have analyzed more than two years of data and found that the northern plains are covered in low-density material.

'We interpret these as sedimentary deposits, maybe ice-rich,' says Dr. Mouginot. 'It is a strong new indication that there was once an ocean here. The existence of oceans on ancient Mars has been suspected before - and spacecraft have previously identified 'shores'.

'Our radar penetrates deep into the ground, revealing the first 60-80 meters of the planet’s subsurface,t says Wlodek Kofman, leader of the radar team.

'Throughout all of this depth, we see the evidence for sedimentary material and ice.'

The sediments revealed by MARSIS are areas of low radar reflectivity

Such sediments are typically low-density granular materials that have been eroded away by water and carried to their final destination.

The second ocean, three billion years ago, would however have been temporary.

Within a million years or less, Dr. Mouginot estimates, the water would have either frozen back in place and been preserved underground again, or turned into vapor and lifted gradually into the atmosphere.

'I don’t think it could have stayed as an ocean long enough for life to form.'

In order to find evidence of life, astrobiologists will have to look even further back in Mars’ history when liquid water existed for much longer periods.

'Previous Mars Express results about water on Mars came from the study of images and mineralogical data, as well as atmospheric measurements.

Now we have the view from the subsurface radar,' says Olivier Witasse, ESA’s Mars Express Project Scientist. 'This adds new pieces of information to the puzzle but the questionremains: where did all the water go?'