Looking at dust devils on Earth and Mars
How to study a dust devil: Sit in your truck on a dry lake bed and wait for a mini-tornado to spring up within sight. Eyeball its path and guess where it’s headed. Drive to a spot in that direction, shut off the engine, and hope that the vortex sweeps over you and your truckful of instruments.
“It’s really passive-aggressive,” says Gregory T. Delory, a planetary scientist at the University of California, Berkeley. Unlike the pursuit of monster Midwestern tornadoes, which can have two-by-fours and cows flying around in 500-kilometer-per-hour winds, dust devil studies don’t have much excitement in the chase. The desert whirlwinds are typically only a few hundred meters tall and feature 60-km/hr gusts.
But within dust devils, electric fields can be surprisingly strong. New research suggests that those electric fields help the whirlwind lift material off the ground, enabling dust devils to pump enough dust into Earth’s atmosphere to possibly affect climate (SN: 9/29/01, p. 200: Dust, the Thermostat).
Results of these earthbound investigations have implications for explorations of other planets. On Mars, a desert planet where dust devils are common and unusually large, the whirlwinds result from severe atmospheric turbulence. Recent studies suggest that the electric fields in dust devils on the Red Planet are strong enough to cause chemical changes in the atmosphere there, including the creation of hydrogen peroxide. That reactive substance may sterilize Mars’ surface, and its presence could explain some of the odd soil chemistry observed by the Mars Viking landers in the 1970s.
Kicking up dust
Although quite different in size and strength, tornadoes and dust devils both result from atmospheric convection. When water vapor condenses inside a thunderstorm, the heat that’s released drives fast-rising air masses that can spawn tornadoes. On a smaller scale, the more languid ascent of air warmed at Earth’s surface—the thermals that buzzards and glider pilots use to gain altitude—produces the convection that triggers dust devils.
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Packets of ground-heated air typically rise between 3 and 6 km before they cool, spread, and fall back toward the ground, says Nilton O. Renno, an atmospheric scientist at the University of Michigan in Ann Arbor. Within the large mass of circulating air, called a convection cell, vortices often form in the region between the upwelling center and the downdrafts along its outer rim, he notes.
When those swirling air masses touch the ground and pick up loose material, they become visible as dust devils.
In recent years, scientists have measured strong electrical fields inside dust devils. The airborne particles become electrically charged as they bump and scrape each other, says Renno. The lighter particles in the whirlwind tend to develop a negative charge, and the heavier ones, a positive charge. Because the light particles typically rise higher and faster than heavier ones, the separation of charges creates an electrical field that can measure more than 80 kilovolts per meter (kV/m). The whirling charged particles also create small magnetic fields that fluctuate between 3 and 30 times each second (SN: 2/8/03, p. 94: Available to subscribers at Dust devils produce magnetic fields).
Lab experiments indicate that the strong electrical fields inside dust devils help the vortices boost material off the ground, Renno and his colleague Jasper F. Kok report in the Aug. 28 Geophysical Research Letters. An electrical field would need to measure at least 150 kV/m to overcome gravity and lift a grain of sand in the absence of wind, the tests suggest. However, a field half that value would enable wind to pick up many particles, says Kok.
With the aid of their internal electric fields, dust devils pump a lot of dust into the atmosphere. Field tests suggest that a dust devil lifts about 1 gram of dust per second from each square meter of ground over which it passes, says Jacquelin Koch, an atmospheric scientist at the University of Michigan in Ann Arbor. Therefore, a large dust devil—about 100 m across at its base—can lift about 15 metric tons of dust during its 30-minute life span.
Dust devils may seem innocuous compared with the immense dust storms that carry material across oceans (SN: 9/29/01, p. 200: Dust, the Thermostat). 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.
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.
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: Available to subscribers at Tracks of dust devils spotted from space).
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.
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 northern hemisphere when Mars is at its farthest 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 hemisphere does, dust devils are more frequent 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 Circle. Scientists are now planning a Mars mission that will put a lander down at high latitudes, so they’re closely scrutinizing orbital images to get an idea of the region’s geology and weather, says R. David Baker, an atmospheric scientist at Austin College in Sherman, Texas.
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.
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 Delory. If Martian dust devils generate such fields, they may spark significant changes in atmospheric chemistry, he notes.
Electrons stripped from the gas molecules in Martian air would be accelerated by the electric fields. Lab tests suggest that those charged particles would attach themselves to carbon dioxide molecules to make negative ions and would split carbon dioxide into carbon monoxide and oxygen ions. The speeding electrons would also split water vapor into hydroxyl and hydrogen ions, says Delory.
Reactions of hydrogen ions, oxygen atoms, and hydroxyl ions produce hydrogen peroxide, H2O2, the highly reactive chemical that’s used on Earth to bleach hair and disinfect scrapes.
The typical lifetime of a hydrogen peroxide molecule in Martian atmosphere is about 2 days, says Delory. However, in the presence of large electric fields, hydrogen peroxide wouldn’t remain in the atmosphere as a gas. So, the hydrogen peroxide that’s formed inside a dust devil would either crystallize in the air and fall as snow or crystallize on the surface of the whirling dust particles.
Either way, the peroxide would quickly fall to the ground, where, if protected from sunlight by a shallow layer of dust, it could survive for more than 4 years. Delory, Renno, and their colleagues reported their analyses in the June Astrobiology.
The presence of dust devil–produced peroxide could explain some of the odd results from a battery of soil chemistry experiments performed onboard the Mars Viking lander in the 1970s. Those tests detected highly reactive chemicals but didn’t find any sign of organic material, says Delory. Even if there hadn’t been life on Mars, scientists expected to find traces of organic chemicals brought to the planet by meteorites.
Highly reactive peroxide would scour organic chemicals from Martian soil, says Delory. That process would make the surface of the Red Planet hostile to life. Furthermore, because the planet lacks an ozone layer, large quantities of ultraviolet radiation reach Mars’ surface. Deep in the soil, where neither ultraviolet radiation nor peroxide infiltrates, however, life might survive.
“The jury’s still out as to whether there is life on Mars,” Delory notes.
Indeed, the jury’s still out on many things about the Red Planet. For instance, “there’s still a lot we don’t understand about the chemistry of the atmosphere and soils of the planet,” he adds. The researchers’ theory about dust devils generating peroxide, Delory notes, could be verified by future Mars rovers or landers if they’re equipped with an electric field sensor and can analyze Mars’ atmospheric chemistry.