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... And an Icy Patch at Mars's South Pole

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Science  06 Dec 2002:
Vol. 298, Issue 5600, pp. 1866-1867
DOI: 10.1126/science.298.5600.1866b

Space scientists have discovered a new hiding place for water on Mars. In a paper published online by Science this week (www.sciencemag.org/cgi/content/abstract/1080497), three geologists report that they have found a kilometer-wide patch of water ice at the edge of the southern polar cap. The orbiting Mars Odyssey spacecraft first glimpsed the water ice in February, exposed when an upper layer of carbon dioxide ice (“dry ice”) evaporated in the −20°C heat of the martian summer. 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 thin layer of dry ice.

When Mars Odyssey began photographing the martian surface, says Phillip Christensen of Arizona State University, Tempe, “this region was one of the first pictures we took.” 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 tundralike plain absorbed more heat than did the surrounding terrain during the day and radiated more heat at night. That high “thermal inertia” strongly suggested that the surface was pure water ice.

Cold shoulder.

Region of water ice (arrow) flanking a vast sheet of frozen CO2, photographed by the Mars Global Surveyor, may be typical of the fringes of Mars's southern ice cap.

CREDIT: NASA/JPL/MALIN SPACE SCIENCE SYSTEMS

Christensen's team, which included Timothy Titus and Hugh Kieffer of the U.S. Geological Survey (USGS) in Flagstaff, Arizona, also examined old visible-light 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, in exactly the same places where their infrared camera had seen them. The icy plain, the researchers concluded, is a regular feature that has reappeared every martian summer for at least 25 years. 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. This suggests that the permanent layer of carbon dioxide ice might be relatively thin—perhaps only meters thick.

Other researchers say the find is like a Christmas present you have asked for: not a big surprise but good news nevertheless. “It's important to me because I predicted it,” says David Paige, a planetary scientist at the University of California, Los Angeles. Several years ago, he and two other scientists studying data from the 1971 Mariner 9 mission found that the spectrum of light reflected from the south pole did not match that of dry ice alone. They speculated that the other ingredient was water ice, but their instruments could not pinpoint its location.

If the ice deposits are indeed accessible from the surface, they 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 USGS. “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.

But that understanding will come only if NASA sends a mission to the polar regions of Mars, to replace the Polar Lander that failed to reach its destination in 1999. The inaugural Mars Scout mission, to be launched in 2007, might provide an opportunity. Two of the 10 finalists for this mission, including Paige's “Artemis” proposal, involve polar landings. (The winning proposal was expected to be announced on 5 December.) “I see a groundswell of interest in going to the poles,” Paige says. “The poles are where a lot of the action on Mars is.”

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