Crackling Planets - Static Electricity on the Moon and Mars

On the Moon and on Mars, conditions are ideal for triboelectric charging. The soil is drier than desert sand on Earth. That makes it an excellent electrical insulator. Moreover, the soil and most materials used in spacesuits and spacecraft (e.g., aluminized mylar, neoprene-coated nylon, Dacron, urethane-coated nylon, tricot, and stainless steel) are completely unlike each other. When astronauts walk or rovers roll across the ground, their boots or wheels gather electrons as they rub through the gravel and dust. Because the soil is insulating, providing no path to ground, a space suit or rover can build up tremendous triboelectric charge, whose magnitude is yet unknown. And when the astronaut or vehicle gets back to base and touches metal - ZAP! The lights in the base may go out, or worse.

Landis and colleagues at NASA Glenn first noticed this problem in the late 1990s before Mars Pathfinder was launched. "When we ran a prototype wheel of the Sojourner rover over simulated Martian dust in a simulated Martian atmosphere, we found it charged up to hundreds of volts," he recalls.

That discovery so concerned the scientists that they modified Pathfinder's rover design, adding needles half an inch long, made of ultrathin (0.0001-inch diameter) tungsten wire sharpened to a point, at the base of antennas. The needles would allow any electric charge that built up on the rover to bleed off into the thin Martian atmosphere, "like a miniature lightning rod operating in reverse," explains Carlos Calle, lead scientist at NASA's Electrostatics and Surface Physics Laboratory at Kennedy Space Center, Florida. Similar protective needles were also installed on the Spirit and Opportunity rovers.

Electrostatic discharge points at the base of Sojourner's antenna.

On the Moon, "Apollo astronauts never reported being zapped by electrostatic discharges," notes Calle. "However, future lunar missions using large excavation equipment to move lots of dry dirt and dust could produce electrostatic fields. Because there's no atmosphere on the Moon, the fields could grow quite strong. Eventually, discharges could occur in vacuum."

"On Mars," he continues, "discharges can happen at no more than a few hundred volts. It's likely that these will take the form of coronal glows rather than lightning bolts. As such, they may not be life threatening for the astronauts, but they could be harmful to electronic equipment."