This movie clip shows a large, distant dust devil -- a whirlwind that lofts dust into the air -- as a dark shape on the horizon near the right side of the images. This dust devil was about 5 kilometers (3 miles) away from NASA's Mars Exploration Rover Spirit, and may have been up to 200 meters or yards in diameter. Smaller dust devils closer to the rover appear bright against the dark ground. Spirit's navigation camera took these images on the rover's 468th martian day, or sol (April 27, 2005.) Contrast has been enhanced for anything in the images that changes from frame to frame, that is, for the dust devil. The number of seconds elapsed since the first frame is indicated at lower left of the images, typically 20 seconds between frames.
Scientists expected dust devils since before Spirit landed. The landing area inside Gusev Crater is filled with dark streaks left behind when dust devils pick dust up from an area. It is also filled with bright "hollows," which are dust-filled miniature craters. Dust covers most of the terrain. Winds flow into and out of Gusev crater every day. The Sun heats the surface so that the surface is warm to the touch even though the atmosphere at 2 meters (6 feet) above the surface would be chilly. That temperature contrast causes convection. Mixing the dust, winds, and convection can trigger dust devils.