The spacecraft has also produced evidence that hot hydrothermal vents once bubbled on the planet's surface.

Scientists believe one clear contour in the images that varies in height by less than half a kilometre marks the rim of an ancient ocean covering the northern Martian lowlands.

The probe has also found the first clear signs that water collected in flooded craters, and revealed the existence of chasms up to a kilometre deep with terraced sides.

The findings, presented at a meeting of the American Geophysical Union in Boston, USA, are reported by New Scientist magazine.

One region of the Martian equator shows signs of hydrothermal systems. Nasa scientists believe hydrothermal vents could have incubated Martian life, and the zone has been suggested as a possible good target for a future lander.