SAN FRANCISCO--A striking altitude survey of the north pole of Mars suggests that its ice cap is only half the expected size. Despite sitting in a huge basin that would have once collected most of the planet's running water, the ice cap holds just 10% of the water needed to account for a hypothesized early ocean on Mars, researchers reported here yesterday at a meeting of the American Geophysical Union. The findings, which will appear in the 11 December issue of Science, may throw more cold water on the prospect of Martian life.
Water is the red planet's most slippery enigma. Clues to a wet past abound in photographs, radar images and other data from Mars Global Surveyor, which has been looping around the planet since September 1997. Sharp images of sinuous valleys emerging from crater walls and appearing in the middle of plains seem to confirm that water flowed underground at one time, and perhaps on the surface as well. The planet's ice caps, small cousins to Earth's polar ice fields, preserve some of that water in full view today.
The orbiter's laser range finder charted the crests and swales of the 1200-kilometer-wide northern cap this summer. The relief map shows a grooved dome 3 kilometers thick, less than previous estimates of 5 to 6 kilometers. This dome is not the vast icy reservoir that some researchers envisioned, concludes planetary scientist Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "If you want [to have had] an ocean on Mars, you have to come up with at least a factor of 10 more water," she says. Zuber suspects that most of the ancient water may lurk underground on Mars today as ice sheets shrouded by eons of dust.
The map also exposes a giant depression in the northern hemisphere of Mars, about 5 kilometers deep. The ice cap rests in the middle "like a hockey puck," says team leader David Smith of NASA's Goddard Space Flight Center in Greenbelt, Maryland. If water did flow on Mars, Smith says, it would have migrated north and stayed there. "That makes it difficult to understand how water could have moved around easily on the surface," he says.
The laser results are settling disputes about ice cap size, but they raise new questions about water's history on Mars, says project scientist Arden Albee of the California Institute of Technology in Pasadena. Although a warm and wet early Mars billions of years ago is still possible, "We keep pushing evidence of liquid water further and further back," he says. As a result, any Martian life would have had even less time to arise before the deep freeze, Albee notes, although water still may flow more than 2 kilometers beneath the surface.