Figure 1 Annotated
At the Gusev site recently, skies have been very dusty, and on its 421st
sol (March 10, 2005) NASA's Mars Exploration Rover Spirit spied two dust
devils in action. This pair of images is from the rover's rear
hazard-avoidance camera.
Views of the Gusev landing region from orbit show many dark streaks across
the landscape -- tracks where dust devils have removed surface dust to
show relatively darker soil below -- but this is the first time Spirit
has photographed an active dust devil.
Scientists are considering several causes of these small phenomena. Dust
devils often occur when the Sun heats the surface of Mars. Warmed soil
and rocks heat the layer of atmosphere closest to the surface, and the
warm air rises in a whirling motion, stirring dust up from the surface
like a miniature tornado. Another possibility is that a flow structure
might develop over craters as wind speeds increase. As winds pick up,
turbulence eddies and rotating columns of air form. As these columns grow
in diameter they become taller and gain rotational speed. Eventually they
become self-sustaining and the wind blows them down range.
One sol before this image was taken, power output from Spirit's solar
panels went up by about 50 percent when the amount of dust on the panels
decreased. Was this a coincidence, or did a helpful dust devil pass over
Spirit and lift off some of the dust?
By comparing the separate images from the rover's different cameras, team
members estimate that the dust devils moved about 500 meters (1,640 feet)
in the 155 seconds between the navigation camera and hazard-avoidance
camera frames; that equates to about 3 meters per second (7 miles per
hour). The dust devils appear to be about 1,100 meters (almost
three-quarters of a mile) from the rover.
