Slight variations in the hue of the Red Planet appear to drive the martian climate. The color changes alter Mars's reflectivity, or albedo, and could be responsible for a curious increase in the planet's temperature in recent years, a team of planetary scientists reports.
Martian temperatures depend in part on how much sunlight is absorbed or reflected. Dark features such as exposed bedrock and boulders catch rays, causing local temperatures to rise. On the other hand, bright surfaces such as varying layers of planetary dust bounce sunlight back into space, keeping things cool. Such uneven heating then stirs up strong winds in the thin, dry martian atmosphere, and the winds then sweep the dust around, burying darker features in some places even as they uncover them in others. Spacecraft images show that more than one-third of its surface area has shown seasonal variations that brighten or darken by at least 10%. Now, Lori Fenton of NASA's Ames Research Center in Moffett Field, California, and colleagues have linked such shifting patterns of reflectivity to a curious warming of the martian climate. The researchers mapped albedo changes on the martian surface over 20 years, comparing images from the late-1970s Viking mission to the Mars Global Surveyor mission from 1999. On average, Mars reflects 84% of the sunlight that strikes it, but the researchers have found that the martian albedo has generally decreased. The overall reduction in albedo, in part, corresponds to the planet's warming of 0.65°C over 20 years, the team reports this week in Nature. In addition, the changes could whip up intense weather, including global dust storms and spiral disturbances called dust devils, which act like little vacuum cleaners, reduce albedo, and help increase temperature. The findings introduce one more factor in the complicated equations that govern martian climate, Fenton says: "Albedo has been regarded as a secondary concern, but we've shown you can't really ignore it if you want to understand [the climate] there."
"For short-term climate change on Mars, I think it's a very nice study," says Philip Christensen, a geologist at Arizona State University in Tempe. However, he adds that "undoubtedly there are other factors that can affect climate" over longer time spans and that are difficult to measure, including erosion of carbon dioxide deposits on the southern polar cap.