Martian water is prime candidate

Gullies have been discovered in impact craters

By Dr David Whitehouse BBC News Online science editor

Water is the liquid that cut the fresh gullies seen on Mars in 2000, suggests a new analysis - this despite claims that other liquids may be responsible.

The fleeting presence of water flowing on the Red Planet once again raises hopes that primitive life may exist just below the surface.

Looking at ancient canyons and shorelines, most scientists agree that Mars was wetter and warmer billions of years ago.

But the fresh gullies would indicate that running water, and perhaps life, may be a feature of Mars today.

Alternative ideas

The discovery of features looking as if they were caused by erosion by some liquid flowing across the surface of Mars caused a sensation when it was announced in 2000.

Gullies on Mars may have been formed by water

At higher latitudes on the Red Planet on steep slopes along the walls of impact craters, polar pits and valleys, usually facing the poles, sharply cut gullies were seen that were obviously caused by some kind of flowing liquid.

At first, most considered the liquid concerned must be water because the Martian gullies looked very similar to those on Earth and because there is much evidence that water flowed on Mars in the distant past.

But some scientists expressed doubts. It was possible, they argued, that the gullies were not caused by water but by liquid carbon dioxide.

One of the reasons for favouring liquid CO2 was that computer models of the Martian crust indicated that water could exist only at depths of several kilometres, but liquid carbon dioxide could persist much nearer the surface.

Not enough CO2

Deciding what form of liquid really was responsible for the features is a pressing question that has important consequences for the likelihood of life on Mars.

A new study of the Martian crust by Sarah Stewart of Caltech, US, and Francis Nimmo of University College London, UK, looks at the carbon dioxide hypothesis but cannot find enough CO2 in the upper Martian crust to form one gully let alone a planet-wide network of them.

This leaves water as the prime candidate, particularly now that the Odyssey probe reports vast quantities of water-ice just below the surface across great swathes of the planet.

One problem faced by both the water and CO2 hypotheses is that neither can exist in liquid form on the surface of Mars under current conditions.

This has led to suggestions that the water - if it is responsible - must be mixed with minerals and salts that make it more stable.