Planetary scientists agree that the Red Planet has plenty of water, but the big question remains: where is it? Both Martian poles are capped by bright, enduring ice caps that contain frozen water. But given the vast flood channels that have been carved into the planet's ancient terrain, there must be far more water ice stashed elsewhere.
Since mid-2005 the European Space Agency's Mars Express orbiter has been probing beneath the planet's landscape, hunting for hidden reservoirs of frozen ice. By this time last year its radar sounder, dubbed MARSIS (for Mars Advanced Radar for Subsurface and Ionosphere Sounding), had identified vast slabs of relatively pure ice buried beneath the broad plains (known as layered deposits) surrounding both poles.
Now the MARSIS team has refined its estimates of just how much polar ice exists. In the online edition of Science for March 15th, Jeffrey J. Plaut (Jet Propulsion Laboratory) and 23 colleagues describe how the instrument's radar energy encountered little attenuation as it penetrated the plains surrounding the south pole, an indication that the subsurface ice layer contains no more than 10% dust. This vast buried glacier extends to depths of up to 3.7 kilometers (2.3 miles) — roughly twice the thickness of the ice mass that underlies the north pole. All told, the south polar layered deposits hold an estimated 1.6 million cubic kilometers of ice — 130 times the volume of Lake Superior. That's enough water to cover the entire planet to a depth of about 11 meters (36 feet).
At a meeting of planetary scientists in Texas last week, Plaut offered details about a second stash of ice near the south pole. This additional find, which underlies the enigmatic plain known as Dorsa Argentea, likewise runs clear and deep, perhaps totaling another million cubic kilometers of buried ice. "Apart from the south polar layered deposits," Plaut told his audience, "this is our best and most widespread detection to date." Moreover, he adds, the Dorsa Argentea formation is billions of years old, suggesting that its ice deposits could have been emplaced rather early in Martian history.
Geologists now have another means of probing these deposits besides MARSIS, which stands for Mars Advanced Radar for Subsurface and Ionosphere Sounding. On board NASA's Mars Reconnaissance Orbiter, which reached the Red Planet last year, is SHARAD (short for Shallow Subsurface Radar). The two systems complement each other: MARSIS can penetrate to depths of up to 5 km (3 miles) but can't see fine-scale layering; SHARAD can probe distinguish individual ice and high-density rock layers layers within 300 meters (1,000 feet) of the surface and as thin as 15 meters (50 feet).