Mars 'once warm and wet'

'Visiting astronauts would not be short of water'

The ice cap at Mars' south pole is made almost entirely of ice made from water, rather than from carbon dioxide as had previously been thought, scientists say.

New data from an unmanned spacecraft in orbit around the planet show that the south pole is very similar to the north pole, which is made up of frozen water, with just a thin covering of frozen carbon dioxide.

The findings - from scientists at the California Institute of Technology - indicate that any astronauts visiting the planet would not be short of water.

But it also means that it would be difficult to make Mars habitable in the future because there is less carbon dioxide there than previously thought, and carbon dioxide is what is needed to trap heat and warm the planet up.

New goal

Writing in the magazine Science, Professor Andy Ingersoll and his graduate student, Shane Byrne, argue that the old model is inaccurate.

They say the south polar cap is too warm to be carbon dioxide, or dry ice, as previously believed.

According to the new study, the south pole's dry ice cover is slightly thicker than the one found in the north and does not disappear entirely in the summertime.

The layer of dry ice on the south pole is about eight metres - which would indicate that the planet has a only a small fraction of the carbon dioxide found on Earth and Venus, the researchers say.

"Mars has all these flood and river channels, so one theory is that the planet was once warm and wet," professor Ingersoll said.

This could suggest that there once was a large amount of carbon dioxide in the planet's atmosphere - enough to produce a greenhouse effect that would allow liquid water to exist.

The scientists say that finding the missing carbon dioxide - or accounting for its absence - is now a major goal of Mars research.