Zuber said scientists have suspected that the southern polar cap of Mars is comprised of a thin veneer of carbon dioxide that rests atop a layer of dust and ice. However, scientists have also observed a surrounding area much larger than the polar cap that is dark and smooth, and it was uncertain whether that region was also composed of dust or ice--or both.

"What we found is that water ice is the dominant constituent beneath a thin dust veneer," said Zuber, lead author of a paper on the work appearing in the Sept. 21 issue of Science.

Ever since carved channels were first observed on the surface of Mars, scientists have suspected that water once flowed across the surface.

Scientists also wondered whether the Martian poles held large reserves of water. However, because the Mars atmosphere is 95 percent carbon dioxide with only trace amounts of water, some researchers theorized that the polar caps were frozen carbon dioxide, or dry ice.

Zuber's team identified the composition of the southern polar cap by calculating its density. Their results show the density of the polar cap as well as the surrounding smooth layered deposit region is about 1,220 kilograms per cubic meter, which indicates that it is made of mostly water, with about 15 percent silicate dust mixed in.

(The density of water ice is 1,000 kilograms per cubic meter, and the density of dry ice is 1,600 kilograms per cubic meter.)

Zuber and her colleagues used topographical and gravitational data gathered by three Mars orbiters to find the volume and mass of the ice cap, allowing them to calculate its density.

"It's a really simple experiment but you have to measure things very precisely," Zuber said, who is head of MIT's Department of Earth, Atmospheric, and Planetary Sciences.

The experiment reveals that the southern Martian polar region is the largest body of frozen water on the planet and the largest, outside of Earth, in the inner solar system, which includes Mars, Earth, Venus and Mercury.

Until now, scientists were puzzled by the observation that a large percentage of the southern polar region surface does not reflect much light, as it would if there were ice on the surface. This study shows that much of the ice is covered in a layer of dust, but it remains unknown why the dust only covers certain areas, Zuber said.

She plans to undertake a similar density study of the northern polar cap, which does not appear to have a covering of dust, but which is abuts against a large apparent dune field that is not now thought to contain significant ice.

Zuber is the lead investigator for gravity for the Mars Reconnaissance Orbiter, and deputy principal investigator for the altimetry experiment aboard the Mars Global Surveyor. The team also used data from the Mars Odyssey satellite.

Such collaborations between teams "really increase the value of what any single experiment could show on its own," Zuber said.

Jeffrey Andrews-Hanna, an MIT postdoctoral associate in the Department of Earth, Atmospheric and Planetary Sciences, is also an author on the paper. Other authors are Roger Phillips of Washington University; Sami Asmar, Alexander Konopliv, Jeffrey Plaut and Suzanne Smrekar of the Jet Propulsion Laboratory at Caltech; and Frank Lemoine and David Smith of the Planetary Geodynamics Laboratory at the NASA Goddard Space Flight Center.

The research was funded by the NASA Mars Program.

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