Images taken by the US space probe Mariner 9 in 1971-72 showed channel and valley networks similar to river and stream features on Earth. These were assumed to show that Mars had water running across its surface at some time in its history. The 1997 Pathfinder mission then recorded images of a rock-strewn plain showing rounded pebbles and layered structures, which was interpreted as evidence of catastrophic flooding.

The most recent high-resolution images of Mars' surface, taken from 2001 onwards, come from Mars Global Surveyor. These show the channels and valley networks in even greater detail. Analysis suggests either that there was once ice on the surface, rather than liquid water, or that the flow was below the surface. But whatever form it took, it still seems that, perhaps as recently as one million years ago, water was relatively abundant.

For water to have been present on the surface of Mars, the atmosphere must have been much thicker and surface temperatures much warmer than they are today. A thicker atmosphere would have ensured greater protection from solar radiation, and with wetter conditions, all the conditions necessary for the emergence of life, in theory, would have been in place.

When Mars lost its atmosphere, any water on the surface would have vaporised. But water is still locked up in the polar caps and below the surface elsewhere, like permafrost, and also trapped in minerals.

	This is an extract from 'Live from Mars', chapter 4 of the Voyage in Space
	and Time booklet