Vast amounts of water are distributed across the surface of Mars in the form of sand dunes mixed with ice and snow, a researcher for Nasa, the US space agency, told the British Association science festival in Dublin.

These "niveo-aeolian" dep-osits could provide water reserves for astronauts visiting the red planet - and might be a fruitful place to look for signs of primitive Martian life, said Mary Bourke, of the Planetary Science Institute in Arizona.