Mars Photo Essay S SLEEK EDIFICE This crescent-shaped dune (left in image) is known as a barchan. It forms when the wind blows in one direction, in this case east to west, over long stretches of time. By monitoring such dunes over years, scientists can follow the speed and manner of their creep across the surface, like tracking glaciers' progress on Earth. 18 OCTOBER 2 02 0 * SK Y & TELESCOPE T MAKING TRACKS Dust devils on Mars create their own calligraphy, scraping dust off the surface to reveal underlying darker materials. The twisters take shape from the sun-warmed surface's heat reradiating into the air. As the warm air rises, the devils spin and contract, much as figure skaters spin faster as they bring their arms into their sides.