This movie clip shows a several dust devils -- whirlwinds that loft dust
into the air -- moving across a plain below the hillside vantage point of
NASA's Mars Exploration Rover Spirit. Several of the dust devils are
visible at once in some of the frames in this sequence. The local solar
time was about 2 p.m., when the ground temperature was high enough to
cause turbulence that kicks up dust devils as the wind blows across the
plain. The number of seconds elapsed since the first frame is indicated
at lower left of the images, typically 20 seconds between frames. Spirit's
navigation camera took these images on the rover's 463rd martian day, or
sol (April 22, 2005.) Contrast has been enhanced for anything in the
images that changes from frame to frame, that is, for the dust devil.
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.