SAN FRANCISCO, CALIFORNIA--If there were a martian phrase for "It's a twister!" then martians would say it a lot. It turns out there may be up to 300 times more dust devils on Mars than scientists previously predicted, which explains why there's so much dust in Mars's atmosphere.
Dust devils form on Mars much like they do on Earth: The sun heats the terrain and low-lying air, which rises into the cooler air sitting on top. The result is a vortex of spinning air that sucks up dust. The dust absorbs the sun's heat, warms the atmosphere, and intensifies global winds. So dust devils are an important component in climate models for Mars. Scientists were counting the tracks on images taken by a satellite orbiting Mars; but now, images from a rover have shown those counts to be wrong.
A new study from the U.S. Geological Survey in Flagstaff, Arizona, compares high-resolution images from the Mars Reconnaissance Orbiter with surface video taken by the Mars Rover Spirit, which is stuck in the Gusev crater. (Watch a Rover video from the crater here.) The videos show dust devils are smaller and significantly more frequent than predicted by tracks on the orbiter images. This means only larger dust devils--whirling monsters as many as 9 kilometers high--suck up enough dust to expose the dark dirt layer underneath and leave tracks.
"We found roughly one in 300 dust devils leaves a track in the Gusev crater," says Paul Geissler, a research geologist at the U.S. Geological Survey, a member of the study team who presented the research at the American Geophysical Union conference on Tuesday. "It was a big surprise," Geissler says. "Now we know tracks are the exception rather than the rule. Further exploration will tell us how applicable this is to the rest of the planet." The data also show that there are more dust devils in the martian summer, and they're much bigger than the ones occurring in the winter. The research results are under review with the Journal of Geophysical Research.
"This study is very important because [dust devils] are a big source of dust in the atmosphere on Mars," but the methods of counting them are "primitive," says Jeffery Hollingsworth, a research scientist who models the martin climate at the NASA Ames Research Center Planetary Systems Branch in Moffett Field, California. "Who knows if we're getting [our models] right, if we're not getting the dust cycle right? There's a lot to unravel, and this is a good step in the right direction."