By Paul Rincon
Science reporter, BBC News, Houston
Enough water is locked up at Mars' south pole to cover the planet in a liquid layer 11m (36ft) deep.
The Mars Express probe used its radar instrument to map the thickness of Mars' south polar layered deposits.
Analysis of the Marsis radar data shows that the polar deposits consist of almost pure water-ice.
The findings appear in the journal Science and were also presented this week at the Lunar and Planetary Science Conference in Houston, Texas.
It was known by the 1970s that the north and south polar regions of the Red Planet were blanketed by thick accumulations of layered material.
Based upon data from the Mariner and Viking projects, the polar layered deposits were considered to be accumulations of dust and ice.
Deep and wide
Today, polar layered deposits hold most of the known water on Mars, though other areas of the planet appear to have been very wet at times in the past. The south polar layered deposits alone are the size of the US state of Texas.
Understanding where the water went is considered crucial to knowing whether the Red Planet could once have supported life.
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (Marsis) consists of two 20m-long (65ft) hollow fibreglass "dipole" booms to make a primary antenna.
The layered deposits cover an area similar to the US state of Texas
It sends out pulses of radio waves from the antenna to the planet's surface and analyses the time delay and strength of the waves that return.
Analysis of those waves that penetrate the soil and bounce back will give information on transitions between materials with different electrical properties, such as rock and liquid water, beneath the Martian surface.
The instrument gathered data on the south polar region over the course of about 300 orbits of Mars Express.
It was able to reach through the icy layers to the lower boundary, which can be as deep as 3.7km (2.3 miles) below the surface.
The radar penetrated through the chaotic, lumpy deposits with very little attenuation (reduction in signal strength), suggesting they were almost 90% water-ice; the rest being dust.
The radar cannot tell whether there is carbon dioxide mixed in with the water-ice, but lead author Jeff Plaut told BBC News that the thickness of the ice also pointed to a composition of nearly pure frozen water.
Researchers traced the base of the south polar layered deposits and found a set of buried depressions within 300 km of the pole that may be ancient impact craters.
"We didn't really know where the bottom of the deposit was," Dr Plaut, from the US space agency's (Nasa) Jet Propulsion Laboratory in California, explained.
"We can see now that the crust has not been depressed by the weight of the ice as it would be on Earth.
"The crust and upper mantle of Mars are stiffer than the Earth's, probably because the interior of Mars is so much colder."
One area with an especially bright reflection from the base of the deposits posed a puzzle for the researchers. It resembled what a thin layer of liquid water might look like to radar, but the conditions are so cold that the presence of melted water was considered highly unlikely.
Marsis was developed jointly by the Italian Space Agency (Asi) and Nasa.
The radar was successfully deployed in June 2005, after a delay of more than a year amid concerns that the booms might swing back and damage the spacecraft.