NASA / JPL-Caltech / UA / TAMU
An animated sequence of images from NASA's Phoenix Mars Lander shows a dust devil, or mini-tornado, moving from left to right across the Martian polar horizon. Click on the photo to see a larger version from NASA.
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updated 9/12/2008 12:27:33 PM ET 2008-09-12T16:27:33

Dust devils raging across the arctic plains of Mars were caught on film by NASA's Phoenix Mars Lander.

Phoenix captured images of at least six different dust devils that danced across the planet's surface last week, and sensed a dip in air pressure as one passed near the lander.

These whirlwinds, which are somewhat like gentle tornadoes and are common in the American Southwest, too, had been expected in Phoenix's landing site near the north pole (they've been seen from above by orbiting spacecraft) but not confirmed by the spacecraft until now.

Dust devils are whirlwinds that often occur when the sun heats the surface of Mars (this also happens in some areas on Earth). The warmed surface heats the layer of atmosphere closest to it, and the warm air rises in a whirling motion, stirring dust up from the surface and whisking it high into the air. They have been photographed by the Mars rovers operating near the equator, too.

Phoenix's Surface Stereo Imager camera took 29 images of the western and southwestern horizon on Monday, during midday hours of the lander's 104th Martian day. When the Phoenix science team analyzed the images the next day, they noticed a dust devil immediately.

"It was a surprise to have a dust devil so visible that it stood with just the normal processing we do," said Mark Lemmon of Texas A&M University, lead scientist for the stereo camera. "Once we saw a couple that way, we did some additional processing and found there are dust devils in 12 of the images."

The dust devils seen range in diameter from about 7 feet to about 16 feet (2 to 5 meters).

"It will be very interesting to watch over the next days and weeks to see if there are lots of dust devils or if this was an isolated event," Lemmon said.

The day the camera saw dust devils, Phoenix's pressure meter recorded a sharp dip in the thin atmosphere. The drop was less than the overall daily change in air pressure from daytime to nighttime, but this dip occurred over a much shorter time.

"Throughout the mission, we have been detecting vortex structures that lower the pressure for 20 to 30 seconds during the middle part of the day," said Peter Taylor of York University in Toronto, a member of the Phoenix science team. "In the last few weeks, we've seen the intensity increasing, and now these vortices appear to have become strong enough to pick up dust."

Still, the dust devils observed by Phoenix are much smaller than those seen closer to the planet's midsection by NASA's Mars Exploration Rover Spirit.

"We expected dust devils, but we are not sure how frequently," said Phoenix project scientist Leslie Tamppari of NASA's Jet Propulsion Laboratory, in Pasadena, Calif. "It could be they are rare and Phoenix got lucky. We'll keep looking for dust devils at the Phoenix site to see if they are common or not."

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