Always Chasing Dust Devils

August 10, 2008

Matt Balme has one of the coolest jobs in science. A research scientist with the Planetary Science Institute (PSI), Balme will be spending the next three years chasing dust devils across desert playas all over the Southwest, the better to understand how such whirlwinds shape the climate on Mars -- and what effect the little devils might have on local or regional air quality on Earth. Fans of the movie Twister, take note: tornado chasers finally have some competition in the madcap-race-against-nature department.

Dust devils are basically miniature tornadoes, insofar as both involve a vertical, rotating column of air that tosses up dust and debris. But they develop from very different atmospheric conditions. Tornadoes are related to storm systems; dust devils form when ground temperatures are warmer than the surrounding air; the moving air then triggers the telltale swirling column (a convective vortex). To the Navajo, dust devils are known as chiindii -- ghosts or spirits of dead Navajos -- and the direction they spin (clockwise or counterclockwise) indicates whether those spirits are good or evil.

While spinning dust devils on Earth reach maximum velocities of around 50 MPH, the thinner atmosphere on Mars means they can get up to 200 MPH. (Much higher wind speeds are needed to lift the same amount of  dust into Mars' thin air to form dust devils, compared to Earth.) They can also be as much as 50 times as wide and 10 times as high as their terrestrial counterparts. So they could pose a potential threat to future probes sent to the Red Planet, which is a very dusty place.

Or they could prove beneficial: Both the Spirit and Opportunity Mars rovers encountered dust devils during their exploration of the planet. Far from causing damage, the whirlwinds handily cleared off layers of accumulated dust on the solar panels powering each rover, greatly increasing their power levels and productivity.

Still, scientists are unwilling to leave such things to chance. They want to better understand what makes these whirling dervishes tick. There are no weather stations on Mars (not yet, anyway), so scientists can only study Martian climate via computer models. The more data they have, the better the models, and the better scientists can design future rovers. Earth-bound dust devils are useful analogs for their Martian siblings, so the data Balme collects can be used to test equations to describe how dust devils form, developed by Nilton Renno of the University of Michigan.

That doesn't make them easy to catch. Balme and his colleagues first set up three meteorology stations in a triangle formation, which measure wind speed, air temperature and air pressure. Then they attach a mobile sampling platform to the front of a four-wheel drive truck. Balme hops in the truck and when dust devils appear, he races ahead of them within the triangulated area covered by the three weather station, then slams on the brakes and quickly lowers his instruments to the ground, collecting samples of the whirlwind's wind speed, temperature, size, and amount of dust as the storm passes over the truck. If he's not fast enough, no worries! He just hops back in the truck and chases the dust devil down again.

Fun though it might, from Balme's perspective, "Chasing just gives you more information about dust devils," he says.  The next step is to use the new data to link the formation of dust devils with the specific climactic conditions that cause them. That's why it's useful to collect data not just on the dust devils themselves, but also the ambient conditions monitored by the triangulated weather stations. "Having two different test areas gives us two different climatic regions to study," according to Balme.

Balme admits "it's a bit of an art" figuring out to drive safely across dry lake beds at very high speeds, and occasionally a bit hairy to boot, even with four-wheel drive. But it's definitely a useful skill. If this whole science thing doesn't work out, Balme could have a nice career as a Hollywood stunt driver, or perhaps nab a guest stint on BBC's Top Gear, which regularly features wacky stunts involving high-end automobiles. Better yet, bring the Top Gear crew to the Nevada desert. If you're going to chase after dust devils, why not do it in the latest Jaguar?

Photos: (top) Image of a Martian dust devil taken by Spirit. Source: NASA/Jet Propulsion Laboratory. A short movie of the dust devil in motion can be found here. (bottom) 2009 Jaguar XKR convertible. Source: Wikimedia Commons.

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[Lab Lemming]

OK, you've inspired me to post a picture and a question:
http://lablemminglounge.blogspot.com/2008/08/willy-willy.html