WASHINGTON, April 17 -- There is a lot of evidence from orbiting satellites and surface rovers indicating that liquid water was present long ago on the surface of Mars in the form of ponds, lakes and rivers. But the idea that Mars once had a huge and long-lived ocean on its northern plains has remained a matter of debate.

Scientists have now detected what might be the contours of this putative ocean on Earth's planetary neighbour, using data collected by a Nasa probe that orbited Mars for a decade. Their research indicates the presence of the Martian equivalent of the continental shelf that defines the boundaries of Earth's oceans.

They compared it to a huge "bathtub ring" showing where water may have once met land on Mars. Because Mars...