Martian dust includes magnetic, composite particles, with a mean size of one micron--the equivalent to powdered cement or flour in consistency. This size range is about five percent the width of a human hair.

By comparison to how a dust devil in Arizona might stir up uncultivated farmland, the scale on Mars is much more daunting.

"These martian dust devils dwarf the five-to-10 meter terrestrial ones, can be greater than 500 meters in diameter and several thousand meters high. The track patterns are known to change from season to season, so these huge dust pipes must be a large factor in transporting dust and could be responsible for eroding landforms," said Peter Smith of the University of Arizona (Tucson)

Mars has only a faint atmosphere [less than one percent of terrestrial pressures], yet offers up its history of dust devils as swirling tracks in a remarkable landscape of wind-swept and carved terrain. These tiny twisters tend to appear in the middle afternoon on Mars, when solar heating is maximum and when warm air rises and collides with other pressure fronts to cause circulation.

In his first press conference after the Spirit rover landed, the principal investigator for the rover's science package, Cornell's Steven Squyres, described one instance his team has been discussing: the intriguing possibility that at Gusev, over their mission, the rover's camera may actually be able to animate a dust devil in action.

Squyres informally proposed a mini-series of frames, or twister movie which with some meterological luck, might offer a rare example of surface weather on another planet.

"At the Pathfinder site during its 83 sol mission, approximately thirty dust devils were either sensed by the pressure drop as they passed over the lander, or were imaged by the Pathfinder camera," says Smith.

"Based on these observations, one might expect to see several dust devils per hour from an active site on Mars between 10 am and 3 pm. Few, if any dust devils will be present at other times. Dust devils typically form during late spring and summer and can be found at all latitudes. Exactly, how their population density varies around the planet is currently unknown."

In addition to Pathfinder's run-in with a dust devil, previous missions to Mars have run into very dusty days. For instance, there was a dust storm covering the Viking Lander I (VL-1) site on Martian day (1742) or sol 1742 (1 Martian year=669 Earth days). In 1971, Mariner 9 and 2 USSR missions all arrived during a dust storm.

"Rovers and other robots must be carefully designed to withstand the sandblasting that they will endure from dust devils," said Smith. "Bearing surfaces and solar panels must be protected and dust accumulation on solar panels will lower their efficiency."

Actual mini-tornadoes of this magnetic dust, or dust devils, have been caught in the act by orbital cameras are highlighted by images below. These miniature tornadoes can span about 10 to 100 meters wide with 20- to 60-mile-per-hour (32- to 96-km/hr) winds swirling around a heated column of rising air. One might expect to see several dust devils per hour from an active site on Mars between 10 am and 3 pm, when rising afternoon air is hottest.

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