NASA's Mars Exploration Rover Spirit, which remains in good health after 14 months on the Red Planet, has witnessed a pair of miniature tornado-like dust devils for the first time as they swirled across Gusev Crater.

Dust devils were seen by Spirit's rear hazard-avoidance camera. Credit: NASA/JPL, enhanced by Greg Smye-Rumsby Download a larger version here.

Skies have been very dusty at the site. Tau, a measure of how much sunlight cannot penetrate the atmosphere, rose to a high of 1.5 on the afternoon of sol 418, but the opacity of the atmosphere has since dropped off.

On its 421st sol (March 10) Spirit spied two dust devils in action. Images were taken by the rover's rear hazard-avoidance camera and navigation 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.

Dust devils were seen by Spirit's navigation camera. Credit: NASA/JPL, enhanced by Greg Smye-Rumsby Download a larger version here.

One sol before these 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?

"As Spirit and Opportunity are the first solar-powered vehicles on the surface of Mars during the dust storm season, this is a learning experience," mission controllers said in this week's status report.

"There are likely large transient dust storm events that reduce solar energy due to dust deposition on the solar arrays and blocking some sunshine, but also may sometimes raise energy levels by cleaning dust from arrays, possibly by winds associated with dust storms. The impact on other rover systems, such as cameras, will also be closely monitored."

Solar energy continues to be very high: more than 700 watt-hours, officials said. The last time Spirit had this much energy was around sol 80.

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.