Scientists think they can now tie dark streaks seen on the surface of Mars to periodic flows of liquid water.
Data from a Nasa satellite shows the features, which appear on slopes, to be associated with salt deposits.
Crucially, such salts could alter the freezing and vaporisation points of water in Mars’s sparse air, keeping it in a fluid state long enough to move.
Michael Meyer, the lead scientist on Nasa’s Mars exploration programme, and Dr Jim Green, Nasa’s planetary science director, made the announcement.
Mars has liquid water just below its surface, according to new measurements by Nasa’s Curiosity rover.
Until now, scientists had thought that conditions on the red planet were too cold and arid for liquid water to exist, although there were known to be deposits of ice.
The latest findings suggest that Martian soil is damp with liquid brine, due to the presence of a salt that significantly lowers the freezing point of water. When mixed with calcium perchlorate liquid water can exist down to around -70C, and the salt also soaks up water vapour from the atmosphere.
Liquid water is traditionally considered an essential ingredient for life as we know it, but Mars remains hostile for other reasons, the scientists said. The latest findings are unlikely to change the view that if life ever blossomed on Mars, it probably died out more than a billion years ago.