This movie clip shows a dust devil scooting across a plain inside Gusev
Crater on Mars as seen from the NASA rover Spirit's hillside vantage point
during the rover's 456th martian day, or sol (April 15, 2005). The
individual images were taken about 20 seconds apart by Spirit's navigation
camera, and the contrast has been enhanced for anything in the images that
changes from frame to frame, that is, for the dust devil.
The movie results from a new way of watching for dust devils, which are
whirlwinds that hoist dust from the surface into the air. Spirit began
seeing dust devils in isolated images in March 2005. At first, the rover
team relied on luck. It might catch a dust devil in an image or it might
miss by a few minutes. Using the new detection strategy, the rover takes a
series of 21 images. Spirit sends a few of them to Earth, as well as
little thumbnail images of all of them. Team members use the 3 big images
and all the small images to decide whether the additional big images have
dust devils. For this movie, they specifically told Spirit to send back
frames that they knew had dust devils.
The images were processed in three steps. All images were calibrated to
remove known camera artifacts. The images were then processed to remove
stationary objects. The result is a gray scene showing only features that
change with time. The final step combined the original image with the
image that shows only moving features, showing the martian scene and the
enhanced dust devils.
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 should trigger dust devils.
Scientists will use the images to study several things. Tracking the dust
devils tells which way the wind blows at different times of day.
Statistics on the size of typical dust devils will help with estimates of
how much dust they pump into the atmosphere every day. By watching
individual dust devils change as they go over more-dusty and less-dusty
terrain, researchers can learn about the turbulent motion near the
surface. Ultimately, that motion of wind and dust near the surface
relates these small dust devils with Mars' large dust storms.