Cold shoulder. Region of water ice (arrow) flanking a vast sheet of frozen carbon dioxide.

Martian Ice on the Rocks

Space scientists have discovered a new hiding place for water on Mars. A kilometer-wide patch of frozen water at the edge of the planet's southern polar cap was exposed when an upper layer of carbon dioxide ice (“dry ice”) evaporated. This discovery might literally be the tip of an iceberg: Some Mars scientists believe that the entire southern polar cap could be water ice, covered by a veneer of dry ice.

The orbiting Mars Odyssey spacecraft first glimpsed the water ice in February. What caught scientists' attention was a relatively flat piece of land that was colder than the adjacent exposed soil. More-detailed measurements made with the spacecraft's infrared camera revealed that the plain absorbed more heat than the surrounding terrain during the day and radiated more heat at night. That strongly suggested that the surface was pure water ice.

Tracking this region's history, Phillip Christensen of Arizona State University, Tempe, and colleagues examined old photographs of the area taken by NASA's Viking orbiter mission in the 1970s. Sure enough, the photos showed sharp delineations between bright dry ice, medium-bright water ice, and dark rock. The transitions between thermally distinct regions seen by the Mars Odyssey corresponded to transitions between visibly distinct regions in the Viking photos. The icy plain, the researchers conclude, is a regular feature that has reappeared every martian summer for at least 25 years as the dry ice periodically melts off, the researchers report online 5 December in Science. Viking saw many similar medium-brightness patches around the edges of the southern ice cap, so seasonal plains of water ice might be fairly common.

The ice deposits might someday provide a record of Mars's climatic history, just as glaciers do on Earth. “In many ways, Mars should be a simpler system than Earth for understanding climate change,” says Ken Herkenhoff of the U.S. Geological Survey. “There are no oceans on Mars, and no biological community that we know of.” Thus Mars could serve as a laboratory for understanding the effects of orbital mechanics and of the sun's variations on climate.

Related sites
The Mars Odyssey Web page
THEMIS, the instrument that made the observations