SAN FRANCISCO--Dust devils might not cause the giant dust storms that envelop Mars, but there are enough of them to cast a hazy pall over the red planet, according to reports here on 6 December at a meeting of the American Geophysical Union. A satellite camera has spotted thousands of these vortices, which scour narrow tracks along the planet's surface. Dust devils might affect Mars's atmosphere by whipping particles to high altitudes, where the fine aerosols help control the planet's dry climate.
Dust devils arise in the martian spring and summer when sunlight heats the ground, forcing warm cells of air to rise. In the same process that occurs over Earth's deserts, the rising cells form spinning columns while cool air descends through the middle. When the winds grow fast enough, they suck up dust. The Viking orbiters saw dust devils in the mid-1970s, but researchers weren't sure how much dust they threw into the atmosphere.
When the Mars Global Surveyor began snapping detailed photos of the surface in March 1999, it caught many devils in action. So far, the camera has imaged more than 2500 dust devils, reported planetary scientist Bruce Cantor of Malin Space Science Systems in San Diego, California. The funnels--white circular clouds that cast a shadow--appear everywhere, from the volcanic summit of Olympus Mons to the lowest craters. By etching paths in the planet's veneer of fine dust, the devils usually expose darker material beneath. There's enough crisscrossing to cause a seasonal "wave of darkening" on parts of the planet during summer, Cantor says, making the surface retain more heat. Independent analysis of dust devils by graduate student Jenny Fisher of the California Institute of Technology (Caltech) in Pasadena shows that some vortices span hundreds of meters and tower 8.5 kilometers--probably tall enough to suspend dust at high altitudes.
Although some scientists thought dust devils might trigger the occasional dust storms that can shroud the entire planet, Cantor's analysis revealed no correlation. The satellite sees only a fraction of the millions of whirlwinds on Mars, he notes, so dust devils might inject as much dust aloft as a few raging storms. Caltech planetary geologist Mark Richardson agrees: "They are good candidates for maintaining the atmosphere's background level of dust, even though they are individually small."