The southern Martian ice cap is mostly made of water, according to those clever boffins at MIT.
The polar caps on the red planet were thought to be composed largely of a thin layer of frozen CO2, resting over a dust and ice mixture. But new analysis by MIT researchers, led by Maria Zuber, MIT professor of geophysics, suggests that the largest constituent is in fact bog-standard H2O.
More: http://www.theregister.co.uk/2007/09/24/mars_water/