Mars lander observes its first dust devils

At least six dust devils have been imaged by the Phoenix Mars lander – the first ever observed by the probe. The spiralling vortices of dust, which do not pose a danger to the lander, may have been triggered by a growing difference between daytime and nighttime temperatures.

Whirling dust devils towering nearly a kilometre high had been spotted at Phoenix’s landing site by orbiting spacecraft before the probe touched down on Mars in late May.

But it was not clear how common the devils were. On Monday, Phoenix’s Surface Stereo Imager imaged at least six dust devils. They are smaller than those seen from orbit, ranging in diameter from 2 to 5 metres (watch an animated movie made from the images here).

“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,” says Mark Lemmon of Texas A&M University in College Station.

Dust devils are created when vortices of air – set in motion when warm air rises from the surface on an otherwise still day – pick up dust from the ground. The dust reaches such great heights because of the Red Planet’s relatively low gravity.

Pressure drop

On the same day the dust devils were imaged, Phoenix’s pressure meter recorded a sharper dip than ever before.

“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,” says Peter Taylor of York University in Toronto, Canada. “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.”

The change may be due to a widening gap between daytime and nighttime temperatures. It is summertime now at Phoenix’s site in the northern polar region of Mars, but autumn is approaching and will begin in December.

Daytime highs have recently been holding steady at about -30 °C, but nighttime lows have been dropping slightly, getting close to -90 °C.

The vortices are not powerful enough to endanger any spacecraft. “With the thin atmosphere on Mars, the wind loads we might experience from dust devil winds are well within the design of the vehicle,” says Ed Sedivy, Phoenix programme manager at Lockheed Martin Space Systems, which built the lander.

In fact, dust devils have cleaned dust off of the Mars rovers Spirit and Opportunity, allowing more sunlight to reach their solar panels.

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