Image: NASA/MOLA Science Team

Astronomers have long known Mars has polar ice caps, but until recently the larger, south polar cap was thought to be mainly frozen carbon dioxide — dry ice. Observations by ESA's Mars Express orbiter show that the cap is almost pure water ice, more than two miles (3.7 km) thick in places. There's more frozen water in the ground surrounding the ice cap. All told, there's enough water to cover the red planet with an ocean 36 feet (11 meters) deep, if spread evenly over the surface, according to an ESA report posted Thursday (March 15, 2007).

The new estimate comes from mapping the thickness of the deposits with the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), developed by the Italian Space Agency and NASA. The 24-member MARSIS science team reported its analysis in the March 15 edition of Science magazine.

"The polar layered deposits extend beyond and beneath a polar cap of bright-white frozen carbon dioxide and water at Mars' south pole. Dust darkens many of the layers. However, the strength of the echo that the radar receives from the rocky surface underneath the layered deposits suggests the composition of the layered deposits is at least 90 percent frozen water," according to the ESA report.

Such vast deposits of frozen water raise hopes of finding it in liquid form somewhere on Mars, and maybe life — which should raise the question of whether human exploration risks contaminating a fragile ecosystem. On the other hand, it means plenty of water and a relatively easy source for oxygen and hydrogen for breathing and rocket fuel. And Martian explorers might not have to live in cramped tin cans, but could spread out in spacious, pressurized igloos — that is, if they can put up with nine months of polar night at temperatures that would make Earth's south pole seem like the tropics.

Anyway, expect to hear more about Martian ice. The MARSIS team is still mapping the planet's north pole.