Dust Devils are cleaning Spirit

Caption NASA originale:"At the Gusaev site recently, skies have been very dusty, and on its 421st Sol (March 10, 2005) NASA's Mars Exploration Rover Spirit spied 2 dust devils in action. This is an image from the Rover's NavCam. Views of the Gusaev landing region from orbit show many dark streaks across the landscape - tracks where dust devils have removed surface dust to show relatively darker soil below - but this is the first time Spirit has photographed an active Dust Devil. Scientists are considering several causes of these small phenomena. Dust Devils often occur when the Sun heats the surface of Mars. Warmed soil and rocks heat the layer of atmosphere closest to the surface, and the warm air rises in a whirling motion, stirring dust up from the surface like a miniature tornado. Another possibility is that a flow structure might develop over craters as wind speeds increase. As winds pick up, turbulence eddies and rotating columns of air form. As these columns grow in diameter they become taller and gain rotational speed. Eventually they become self-sustaining and the wind blows them down range".