Martian landscaping: spacecraft eyes evidence of a frozen sea.

A flat region near the Red Planet's equator may hold a frozen ocean that was once as deep and big as the North Sea. The region's relatively craterless facade suggests that water gushed to the surface and froze recently, raising the possibility that life might exist today on or just beneath the surface, says Mars Express researcher John Murray. Last week, his team reported its analysis of images that were taken by the orbiting Mars Express spacecraft.

In this region, "you've had water above freezing beneath the face of Mars for thousands of millions of years," says Murray, a volcanologist and planetary scientist at the Open University in Milton Keynes, England. "This is the place to go look for life" he adds.

His group, which examined images taken with the spacecraft's High Resolution Stereo Camera, reported its findings on Feb. 25 at a European Space Agency briefing in Noordwijk, the Netherlands. The team will present further details in the March 17 Nature.

Other researchers caution that the flat region, known as Elysium Planitia, might have been sculpted by flowing lava or a mixture of lava and water. This set of scenarios is far less amenable to the presence of life on Mars.

But Murray maintains that water, not lava, shaped Elysium Planitia. Solidified lava and ice would leave different marks on the surface, he says. For every criterion that he has applied, he told Science News, "the Elysium features correspond to the morphology of ice."

The few craters poking above the surface, for example, suggest a frozen sea, Murray says. The craters appear as if they had once been filled with liquid to 45 meters but that the level later dropped to about 30 m. Water can disappear by evaporation or by seeping into the ground, but lava filling a crater would remain trapped there, he argues.

Moreover, "the surface [of Elysium Planitia] is flat to a remarkable degree over more than 50 kilometers, the same as a sea surface where the tide is coming in," Murray says.

Exposed ice at the Martian equator would rapidly sublimate. Murray's team suggests that a layer of ash or dust 1 to 20 m thick has blanketed the ice and preserved it. Such a layer could explain why the spectrometers on several Mars orbiters haven't detected the signature of ice, leading scientists to conclude that the planet has been dry for billions of years.

Jim Rice of Arizona State University in Tempe, who previously suggested that water played a role in forming features at Elysium Planitia, says that a thick deposit of ash on top of a frozen sea seems far-fetched. He argues that a complex interplay of lava and water is more likely to have shaped the area and that the ice that might once have been there vanished long ago.

Images taken by the Mars Reconnaissance Orbiter, slated for launch this summer, could add data to the debate, but a lander may be needed to settle the geological story behind Elysium Planitia, Rice says.