A darkening of the Martian surface may have slowly warmed the planet over the past 20 years. Based on a model of the Red Planet's climate, researchers report that the brightness or darkness of its sands have a strong effect on its atmospheric temperature. They found that the heat absorbed by dark rock kicks up winds that blow away shiny dust, leaving behind even darker rock. But the predicted warming is hard to confirm, researchers say, and could shift with the sands at any time.

Snapshots over the past three decades have shown vast regions of the Red Planet's surface have brightened or darkened by 10 percent or more, reflecting about 20 percent of incoming sunlight in total. To determine if albedo (reflectivity) changes affect the climate, researchers compared Viking orbiter photos from 1976 to 1978, which mapped the planet's bright and dark spots, to those from 1999 to 2000, when the Mars Global Surveyor discovered a darker Mars.

They plugged the albedo data into a Martian climate model, which simulated temperature and wind pattern changes over the course of a year. In the model, darkened surfaces heated the air above them, encouraging the formation of local dust devils that sweep up bright dust particles, says team member Lori Fenton, a planetary scientist at the Carl Sagan Center in Mountain View, Calif. The dimmer regions also experience stronger winds, she says, because warm air gusts toward cooler regions and rises, causing turbulence.

The difference in albedo between the two eras corresponds to a warming of about 0.65 kelvin (one degree Fahrenheit), Fenton and her colleagues report in this week's Nature. Rising summertime temperatures near the planet's south pole might explain why the polar ice cap has been shrinking for the past four years, they note. The warming process, Fenton says, would in effect be cleaning up after the giant dust storms of the 1970s, which rained dust all over Mars.

Documenting the trend, however, may be tricky. The planet's atmospheric temperature swings up and down too much from year to year to spot a gradual warming. "It's impossible," Fenton says, "to detect our signal among the other processes that happen on Mars, such as big dust storms. "So the data doesn't prove us wrong, but it also is too limited to prove us right."

The result demonstrates the importance of albedo to Martian climate and provides better documentation that albedo changes actually occur, says Mars atmospheric modeler Tim Michaels of the Southwest Research Institute in Boulder, Colo. "I certainly think that it has generally warmed the atmosphere," he says, assuming the albedo changed gradually.

But unlike Earth's global warming, the Martian kind would depend entirely on dust patterns—which can shift, he notes. "If we have a big dust storm tomorrow, this could all change like that."