Researchers used the navigation camera on NASA's Mars Exploration Rover
Spirit to look for dust devils near the rover during the mission's 1,919th
Martian day, or sol (May 27, 2009). This shot from that day's sequence,
presented here with three different levels of processing, caught a large
dust devil about 1.5 kilometers (1 mile) northwest of Spirit.
The top frame is the original image, the middle frame has been processed
enhance the visibility of the dust devil, and the bottom frame is a merged
version. The image was taken in the early afternoon from Spirit's position
at the "Troy" sand trap beside "Home Plate," looking northwest across the
floor of Gusev crater. The large dust devil shows a typical central core
(brightest area) surrounded by a more diffuse sand and dust "skirt" about
415 meters (about 1,350 feet) across. The dust devil is moving toward the
northeast (toward the right in this image) at about 0.75 meter per second
(1.7 miles per hour). This dust devil is some 20 times larger than the
average dust devil on Earth. A smaller dust devil is seen on the right
leading the larger dust devil.
More than 650 dust devils have been recorded by Spirit since its operation
began in 2004. The mission is currently in its third "season" for dust
devils on Mars, which typically begin in Martian spring.
Dust devils occur on both Mars and on Earth when solar energy heats the
surface, resulting in a layer of warm air just above the surface. Since
the warmed air is less dense than the cooler atmosphere above it, it
rises, making a swirling thermal plume that picks up the fine dust from
the surface and carries it up into the atmosphere. This plume of dust
moves with the local wind.