The latest data returned from Europe's Mars Express orbiter confirms there are substantial quantities of water-ice held at the Red Planet's south pole.
Mars Express will soon look for the subsurface water reservoir
Scientists say the spacecraft has seen the water-ice in three distinct zones: mixed with carbon dioxide, all on its own and in vast tracts mixed with dust.
The data was collected during the local summer when the southern cap was small.
Jean-Pierre Bibring and colleagues tell the journal Nature that the probe will map the water-ice changes over time.
The data was collected by a French-led instrument called Omega. This visible and infrared spectrometer is able to identify chemical elements thanks to the way sunlight is reflected off the planet's surface.
The Omega researchers believe the most interesting find is the mix of water-ice and dust which is present "along vast zones expanding down slope in stratified terrains, tens of kilometres wide, and tens of kilometres away" from the bright polar cap.
The darkness of the ice-dust mix explains why these reserves have not been observed before, the team says.
The ratio of dust to frozen water varies and there appears to be no correlation with local topography. It is thought the dust is included with the ice when the water precipitates out of the atmosphere.
Although a range of studies have now established the southern cap contains huge quantities of water-ice, the question has remained about the extent to which this is mixed with frozen carbon dioxide.
"We know now that carbon dioxide's only a very thin veneer of ice over a much larger body of water," Professor Bibring, from the Institute of Space Astrophysics, Orsay, told BBC News Online.
He said there appeared to be much more water in the southern polar ice cap than anywhere observed so far on Mars - but this did not mean it was necessarily the prime water reservoir on Mars.
A better idea of how much water-ice is retained on the planet will be gained when Mars Express deploys the Marsis (Mars Advance Radar for Subsurface and Ionospheric Sounding) instrument from May.
It should enable a volume to be determined for the surface water-ice seen by Omega at the south pole and compare this with the volume of water thought to lurk below the Martian surface.
It is this subterranean permafrost that has been considered up until now as the major Mars water reservoir.
Omega is now looking at Mars' northern polar region. Its cap has long been thought to be composed mainly of water-ice.