There is enough water frozen at Mars' south pole to cover the entire planet to a depth of about 11 metres, new data collected by the Mars Express orbiter suggests.

The new estimate comes from measurements made by a radar instrument that can see through layers of ice to the bottom of the polar cap, about 3.7 kilometres down.

"The south polar layered deposits of Mars cover an area bigger than Texas. The amount of water they contain has been estimated before, but never with the level of confidence this radar makes possible," Jeffrey Plaut of NASA's Jet Propulsion Laboratory (JPL), in Pasadena, Calif., said in a written statement. Plaut is co-lead investigator for the radar and principal author of a report on the findings in the online edition of the journal Science.

The radar tool, developed by the U.S. National Aeronautics and Space Administration and the Italian Space Agency, is also mapping the depth of the ice cap at the planet's north pole.

The echo from the MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) instrument aboard the European Space Agency probe suggests to researchers that as much as 90 per cent of the ice at the south pole is frozen water.

An area at the base of the cap has left scientists scratching their heads because it appears to be a thin layer of liquid water - but it is so cold there, they don't believe that is a plausible explanation for their observations.

The polar caps contain most of the water known to be on Mars but other parts of the planet look as if they may have seen vast amounts of the liquid long ago.

Scientists hope that they will be able to determine whether Mars ever supported life by probing the history and eventual fate of water there.