Science News - October 28, 2006

dust storms that carry material across oceans (SN: 9/29/01, p. 200). However, the small whirlwinds, in aggregate, pump more material into the atmosphere than large storms do, says Koch. Massive dust storms sweep the world’s deserts only a few times each month and contribute about 8 percent of the mineral dust that reaches the atmosphere each year. The hundreds of dust devils spawned daily in deserts throughout the summer together loft about three times that much, Koch and Renno reported last December at the American Geophysical Union meeting in San Francisco.

No area of Mars may be safe from the whirlwinds. Satellites have recorded dust devil tracks in all regions of Mars and at all elevations—even inside the crater atop the 24-km-tall Olympus Mons, the largest volcano known in the solar system. However, some Martian regions seem to be more afflicted by the whirlwinds than others are, says Patrick L. Whelley, a geologist at Arizona State University in Tempe.

In the Red Planet’s southern hemisphere, orbital images show

an average of about 0.6 dust devil track per square kilometer, but pictures of the northern hemisphere show only one-tenth as many, Whelley and his colleague Ronald Greeley reported at the San Francisco meeting.

That disparity probably stems from
the eccentricity of Mars’ orbit, says
Whelley. Summer comes to the north-
ern hemisphere when Mars is at its far-
thest from the sun, about 249 million
miles away. However, dust devil season
comes to the southern hemisphere at
the opposite side of Mars’ orbit, when
the planet is only 207 million miles
from the sun. Because the southern
hemisphere thus receives 40 percent
more solar energy per square meter in
summertime than the northern hemi-
sphere does, dust devils are more fre-
quent in the southern hemisphere.
Nevertheless, dust devil tracks
appear even in the high latitudes of
Mars’ northern hemisphere, above that
planet’s equivalent of Earth’s Arctic Cir-
cle. Scientists are now planning a Mars
mission that will put a lander down at
high latitudes, so they’re closely scruti-
nizing orbital images to get an idea of
MAKING TRACKS — A Martian dust devil the region’s geology and weather, says
(arrow), seen from orbit as it climbs a crater R. David Baker, an atmospheric scien-
wall, casts a small shadow and leaves a trail on tist at Austin College in Sherman, Texas.
the planet’s surface. None of the 1,558-or-so clear images
of sites in that latitude band shows a dust
devil in action. However, about 10 percent of those pictures include
dust devil tracks, says Baker. The trails range in length from 500
m to more than 16 km. He and his colleagues also reported their
findings in December at the San Francisco meeting.

“We were surprised at the number of dust devil [tracks] we saw at high latitude,” says Baker. “There was much more [past] activity than we expected.”

The lander that will set down in this polar region will carry an atmospheric-pressure sensor as well as an upward-looking laser-radar device, so it will be equipped to study any dust devils that happen past the craft, says Baker.

SPACE DUST On Earth, individual dust devils are usually no more than a nuisance. On Mars, however, such whirlwinds are larger and more common than their terrestrial kin. Martian dust devils may pose a threat to both robotic and human exploration.

As on Earth, dust devils on Mars arise from atmospheric turbulence. The temperature difference between the planet’s surface and the atmosphere just above it can be much higher on Mars than on Earth, making the dust devils larger and stronger, says William M. Farrell, a geophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. He’s a member of a NASA scientific panel assembled in 2004 to analyze risks to human missions to the Red Planet.

Dust devils within 10 km of a spaceship port on Mars could be a hazard for takeoffs and landings, Farrell speculates. Before creating such a Red Planet base, space agencies should send landers, rovers, and other instruments to monitor dust devils and larger dust storms to determine whether those phenomena pose a threat, he and his colleagues suggested in a June 2005 report.

Many studies indicate that dust devils scour much of the Red Planet’s surface, which covers as much area as Earth’s continents do. Cameras on Mars landers have seen hundreds of the dusty whirlwinds, says chief rover scientist Steven W.

Squyres of Cornell University.

The shadows of monstrous whirlwinds thick with dust have even been seen from craft orbiting Mars. The dimensions of those shadows indicate that some Martian dust devils grow to be several hundred meters across and up to 9 km tall, about 10 times the size of their cousins on Earth, says Paul E. Geissler, a planetary geologist with the U.S. Geological Survey in Flagstaff, Ariz.

The largest of the massive Martian whirlwinds, 5 km across at high altitude, can rival earthly tornadoes and “look like mountains” in the orbital images, he notes.

NASA, JPL, MALIN SPACE SCIENCE S YS TEMS

Evidence of past Martian dust devils can be detected from orbit too. A whirlwind leaves linear or looping trails as it sweeps away light-colored dust to reveal darker material, says Timothy I. Michaels, an atmospheric scientist at Southwest Research Institute in Boulder, Colo. Some orbital images have caught dust devils in the act of making such tracks. Similar tracks appear in satellite images of Earth’s southern Sahara but aren’t obvious to observers on the ground (SN: 5/8/04, p. 302).

Rover-based analyses of dust devil tracks on Mars indicate that most such trails are no more than a few micrometers deep, he notes. Because dust devils are pushed along by other weather systems, researchers can use the tracks to deduce the strength and consistency of the prevailing winds in areas of Mars.

STRANGE CHEMISTRY The denser an atmosphere, the more effectively its molecules block the flow of charged particles. On Earth, where the atmosphere is dense, electric fields inside dust devils aren’t strong enough to accelerate dust particles to speeds where they strip electrons off molecules.

On Mars, however, the atmosphere is less than 1 percent as dense as Earth’s, so speeding charged particles begin to break down atmospheric gases when electric fields build up to 25 kV/m. That’s well below the value of the electric fields that build up in terrestrial dust devils, says Gregory T. Delory, a planetary scientist at the University of California, Berkeley. If Martian dust devils generate such fields, they may spark significant changes in atmospheric chemistry, he notes.