The swirling vortex of dust is visible near the center of the image. The shadow cast by this column of dust can be seen in the upper-left while the dark track left by the passage of the dust devil is evident in the lower-right.
Dust devils on Mars form the same way that they do on Earth. The ground heats up during the daytime, warming the air immediately above the surface. This hot layer of air rises and the cooler air above falls, creating vertical convection cells. A horizontal gust of wind will cause the convection cells to rotate, resulting in a dust devil.
As the dust devil moves across the surface of Mars, it can pick up and disturb loose dust leaving behind a darker track.
Note: The subimage is non-map projected, while the above image is oriented as the observation is (see table below).
Written by: Mindi Searls (12 August 2009)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_013545_1110
Image: NASA/JPL/University of Arizona