Mars' great flood was size of the Med

Had the Pathfinder spacecraft landed where it is now 1 billion years ago, it would have been hundreds of feet under water. The Ares Vallis site was once the scene of a colossal flood that would have been big enough to fill the Mediterranean basin.

The question intriguing scientists is where all the water that was once on Mars has gone and whether it might have been able to support life. Photographs taken by Pathfinder on its third day showed boulders stacked by currents, ripples in the rocky, salmon-pink landscape and stains left behind by puddles which evaporated long ago.

Geologists have known since the Viking missions 21 years ago that floods once swept the planet. The new pictures are powerful evidence confirming that. The water would have covered a swath of Mars hundreds of miles wide, with churning water that reshaped the surface of the planet. "This was huge," said Michael Malin, one of the scientists at the US space agency, Nasa. He estimated that the flood happened between 1 billion and 3 billion years ago.

If, as some scientists contend, large bodies of water - rather than the rushing torrents described yesterday - ever existed on the Martian surface, life probably could have survived. "I think we're fairly confident that there was liquid water on Mars," said Matthew Golombek, a project scientist.

Modern conditions on Mars are less welcoming. The highest day temperature measured by the lander on Monday was -12C; the lowest at night was -76C. Atmospheric pressure, at less than 10 millibars, is now too low for liquid water to exist on the surface. The missing water may have boiled off into space millions of years ago, been trapped in rocks, or frozen into icecaps and underground ice-sheets.

Data received yesterday from the Sojourner rover's X-ray spectrometer suggests that "Barnacle Bill", a curiously shaped rock a few feet from Pathfinder, might be a kind of andesite, the second most common type of lava found on Earth, which is typically found in the Andes mountain range in South America. A spokeswoman said last night that Nasa was very excited by the discovery, but emphasised there was no absolute confirmation that the rock was formed by volcanic action.

The rover is about the size of a microwave oven and is the first movable craft sent to explore the surface of another planet. So far it has only gone about 6ft from the lander. But it has delighted the scientists, who are as proud as parents seeing their child take its first steps. The lander's camera has taken hundreds of photographs, many in colour and 3-D, leaving scientists astounded by the variety of colours, and thus rock types, in view.

Because it bears on the issue of whether life existed on Mars, scientists said the evidence of floods at the Pathfinder landing site will be investigated intensively. But to answer definitively whether life did evolve on Mars, scientists will have to collect and study the rocks on Earth. Nasa plans a mission to do just that, in 2005.