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Devilish Dust Packs a Charge
23 April 2004 (All day)
Dust devils have an electric secret. These funnels of whirling dust that haunt dry parking lots, fields, and deserts can pack an electrical punch of several thousand volts. And although terrestrial dust devils are just a curiosity, their martian cousins may drive the lightning and dust storms of our neighbor planet.
Dust devils spin into life when a difference in temperature, like that created by the edge of a hot asphalt road meeting a cooler patch of sand, sets the air rising and whirling into a funnel. The swirling air catches dust as it travels. The largest dust devils can be as much as 200 meters high and 100 meters wide, and last for an hour or two, though most are much smaller and subside within minutes. On Mars, dust devils can be enormous, as much as 10 kilometers tall, and they may have a hand in martian weather, according to William Farrell, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
After spending the past few summers chasing dust devils around the deserts of the American southwest, Farrell and colleagues report a surprising finding in the 20 April Journal of Geophysical Research. Dust devils carry voltages--large ones. The team measured voltages of more than 4 kilovolts per meter. The voltage builds up, they found, because lighter dust particles, which tend to be negatively charged, get blown higher up into the top of the funnel, while heavier, positively charged particles sink to the bottom. The mechanism is loosely analogous what happens in thunderstorms. And on Mars, the biggest ones can reach the size of thunderstorms on Earth and could easily have voltages of 20 kilovolts per meter, says Farrell. That's enough to ionize the thin atmosphere and cause lightning storms, he says.
"I would never have expected to see an electrical effect associated with dust devils," says Don Gurnett, a planetary scientist at the University of Iowa in Iowa City. He'd be surprised to see electrical activity as violent as lightning on Mars, however, but he adds that high-voltage dust devils could complicate future martian missions. Electrical discharges could fry computers, and static charge could make potentially toxic dust cling to everything.