This is the first dust devil that NASA's Mars Exploration Rover Opportunity has observed in the rover's six and a half years on Mars. The whirlwind appeared in a routine drive-direction image taken by Opportunity's panoramic camera right after a drive during the 2,301st Martian day, or sol, of the rover's mission on Mars (July 15, 2010).
Contrast has been stretched and the image has been carefully calibrated to make the dust devil easier to see against the Martian sky.
Opportunity's twin, Spirit, has observed dozens of dust devils at its location in Gusev Crater halfway around Mars from Opportunity's location in the Meridian Planum region. Opportunity conducted systematic searches for dust devils in past years without seeing any. A rougher and dustier surface at Gusev makes dust devils form more readily there than at Meridiani.