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The powdery lunar soil was great for making footprints, but was a problem for astronauts like Charlie Duke, shown here during the Apollo 16 mission in 1972. It got in their eyes and throats, and clung stubbornly to every surface. (NASA)

Stronger Than Dirt

Lunar explorers will have to battle an insidious enemy—dust.

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(Continued from page 2)

That’s the other thing about dust: It isn’t always bad. On Mars, dust might actually help clean up its own mess. The solar panels on the twin rovers currently on Mars were expected to have been coated long ago with dust that would degrade their power output and bring the mission to a halt. More than once, though, tornado-like dust devils have scoured the panels clean and given the rovers new life.

But that’s the rare bit of good news. Dust storms on Mars will be a serious, continual worry. During Martian summer, daytime highs peak at 68 degrees Fahrenheit, and on these balmy afternoons the planet’s dust devils come alive. These are no little Arizona desert whirlwinds, a few yards across, that pass by in seconds. Martian dust devils are monster columns reaching miles into the sky and nearly half a mile across, 10 times larger than any tornado on Earth. When they pass by, the reddish sand and dust whips around faster than 70 mph, dropping the local visibility to zero for minutes at a time. And they’re everywhere. The Mars rover cameras have filmed them in action, and orbiting spacecraft have spotted their dark tracks all over the planet. “If you were standing next to the Spirit rover midday during Martian summer, you’d see half a dozen [dust devils] at any instant,” says Mark Lemmon, a Mars atmosphere specialist at Texas A & M University in College Station.

The sand in the lower part of a Martian dust devil would be a major hazard. Because the atmospheric pressure on Mars is only one percent of that at sea level on Earth, astronauts won’t feel much wind. But their spacesuits and faceplates would be pinged by high-speed material that would collect in every fold and crevice. Worse, the swirling dust and sand may be electrically charged, to the point of “possibly inducing arcing to a spacesuit or vehicle, and creating electromagnetic interference,” according to William Farrell, one of Stubbs’ colleagues at NASA’s Goddard center.
Farrell has chased dust devils across Arizona deserts and measured their electrical currents. Like levitating lunar dust, the grains of sand and dust become charged as a result of constantly banging into one another. On Mars as well as on Earth, the dust, which can blow in from anywhere, may be quite different from the local sand. When unlike materials rub together (like party balloon and shirt sleeve), one material gives up some of its electrons to the other in what’s known as triboelectric charging (“tribo” means “rubbing”). The smaller dust particles tend to take on a negative charge, having robbed electrons from the larger sand grains.
Triboelectric charging is known to occur on Mars. It came to the attention of NASA engineers building the Sojourner rover for the Mars Pathfinder even before the spacecraft left Earth. When the engineers ran the wheels for a prototype Sojourner over simulated Martian dust in a simulated Martian atmosphere, the model built up a charge of hundreds of volts. That discovery inspired the scientists to add ultrathin half-inch-long tungsten needles at the base of the rover’s radio antennas, to drain any excess charge into the thin Martian air.

Dust devils could even lead to lightning on Mars. All dust devils are powered by a rising central column of hot air, which carries the negatively charged dust upward and leaves the heavier, positively charged sand swirling near the base. The charges become separated, and the separation creates an electric field. In terrestrial dust devils, Farrell has measured electric fields of up to 20,000 volts per meter—peanuts compared to the fields in thunderstorms, where lightning doesn’t flash until the fields become 100 times stronger, enough to break apart air molecules. But 20,000 volts per meter “is very close to the breakdown of the thin Martian atmosphere,” Farrell points out. And because Martian dust devils are so tall, their stored electrical energy can be greater, possibly strong enough to unleash lightning.

Even if lightning doesn’t occur on Mars, the presence of an astronaut or rover might induce local arcing. “You’d have to watch out for corners, where electric fields can get very strong,” Farrell muses. “You might want to make a vehicle or habitat rounded.” And though dust devils can clean off solar panels, astronauts may still find charged dust clinging electrostatically to spacesuits, vehicles, and habitats.

None of these concerns is especially troubling to Apollo 17 astronaut Schmitt. He agrees that dust will be a nuisance for future lunar explorers. But he also expects that engineers will come up with practical solutions like an airlock—which is already being considered for the next-generation lander—that would let astronauts stow their dirty spacesuits before re-entering their living quarters after a moonwalk. “For scientific reasons, the dust is intriguing,” Schmitt says. “It’s a number-one engineering design problem for long-term habitation and settlement [of the moon]. But you protect yourself with a layered defense. The dust is not a problem that should scare us out of going back.”

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