Image: Dust devil
University of Michigan
An artist's conception shows a dust devil swirling on Mars. The glow near the bottom is the result of an electrical discharge. 
By Senior Science Writer
updated 4/20/2004 5:40:44 PM ET 2004-04-20T21:40:44

Whirling dust devils on Mars probably generate high-voltage electric fields and associated magnetic fields and would need to be considered by future human explorers, scientists said Tuesday.

The conclusion is based on studies in Arizona and Nevada, where researchers raced across the deserts to catch dust devils and drive right through them. They found unexpectedly large electric fields exceeding 4,000 volts per meter.

Dust devils are like miniature tornadoes, though their formation is completely different. The smallest variety can be kicked up by gentle breezes in a vacant city lot. More notable versions can be as wide as a house or a football field with winds up to 60 mph (96 kilometers per hour). They are created by wind that swirls around a column of warm, rising air.

Bigger on Mars
On Mars, dust devils can be five times wider and soar 5 miles (8 kilometers), much higher than even full-blown tornadoes on Earth.

On both planets, dust devils form when the ground heats up during the day, warming the air immediately above the surface. Pockets of warm air rise and interfere with each other, sometimes causing one pocket or another to begin a swirling motion.

"If Martian dust devils are highly electrified, as our research suggests, they might give rise to increased discharging or arcing in the low-pressure Martian atmosphere, increased dust adhesion to spacesuits and equipment, and interference with radio communications," said William Farrell of NASA's Goddard Space Flight Center.

Farrell is lead author on a paper describing the results in the latest issue of the Journal of Geophysical Research.

A dust devil was spotted in imagery from NASA's 1997 Pathfinder mission. Tracks of Martian dust devils are common in pictures made by orbiting spacecraft. Global dust storms can shroud the entire planet.

Surprising result
Scientists had not expected dust devils to be charged.

Dust particles in a devil become electrified because they rub against one another. It's like shuffling your feet across the carpet, the researchers explained. But they figured the positive and negative particles would be evenly mixed in a dust devil, keeping the overall electrical charge in balance.

Instead, it turns out smaller particles tend to gain negative charge, and the wind carries them higher.

Heavier, positive particles remain nearer the surface. The separation of charges creates a giant battery. And because the particles are in motion, a magnetic field is generated by the moving electrical charges, the researchers explained.

They don't yet know for sure what to expect on Mars.

If dust on the Red Planet comes in a variety of sizes and compositions, as expected, then dust devils there ought to be similarly electrified, the scientists said. NASA could equip a future Mars landing craft with an instrument to detect a dust devil's electric and magnetic fields.

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