Lunar dust was seen jumping between the moon’s shadowed and sunlit areas. NASA scientists believe this is being caused by the dust being electrically charged.
“The motion of an individual dust particle is like a pendulum or a swing,” says Michael Collier of NASA’s Goddard Space Flight Center, in a press release. “We predict dust can swarm like bees around a hive over partially shaded regions on the moon and other airless objects in the solar system, such as asteroids. We found that this is a new class of dust motion. It does not escape to space or bounce long distances as predicted by others, but instead stays locally trapped, executing oscillations over a shaded region of 1 to 10 meters (yards) in size. These other trajectories are possible, but we now show a third new motion that is possible.”
Collier is lead author of a paper on this research published October 2012 in Advances in Space Research. He is part of the Dynamic Response of the Environment At the Moon (DREAM) team, which is partnered with the NASA Lunar Science Institute (NLSI), managed at NASA’s Ames Research Center, Moffett Field, Calif.
The researchers believe the jumping dust should be more common during dusk and dawn, as some areas are partially illuminated and the moon’s craters and mountains cast long shadows.
“The dust is an indicator of unusual surface electric fields,” says William Farrell of NASA Goddard, a co-author on the paper and lead of the NLSI DREAM team, in a press release. “In these shaded regions, the surface is negatively charged compared to the sunlit regions. This creates a locally complex, larger electric field with separate positively and negatively charged regions, called a dipole field, over the shaded region. The dust performed its swinging motion under the influence of this dipole. Such a surface process occurring on the moon at the line where night transitions to day, called the terminator, might also occur at small bodies like asteroids. It might be a fundamental process occurring at airless rocky bodies.”
NASA researchers have evidence that suggests that the dust moves this way across the moon’s surface.
“There are hints for this type of dust swarm in Surveyor images. A twilight was observed over the landed platforms during dusk and dawn. This was surprising at first because the moon does not have a dense enough atmosphere to scatter light when the sun is below the horizon. It was long considered to be light scattered from lifted dust. This model suggests the dust is really leaping or swarming overtop a large number of shaded regions that would exist along the lunar dusk/dawn line, called the lunar terminator. It’s a natural fit. Charged lunar dust transport is also believed responsible for the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment’s observation of highly charged dust near the terminator,” adds Collier.
Since the moon has almost no atmosphere, it is directly exposed to solar wind, which is a thin stream of electrnically charged plasma that blows from the surface of the sun at close to a million miles per hour. According to the press release, “On the day-lit side, sunlight knocks negatively charged electrons off the surface, giving it a positive charge. On the night side or in shadow, electrons from the solar wind rush in, giving the surface a negative charge.”
How this works, exactly, is still unkonwn. Researchers know that extremely small meteor impacts can cause small particles to jump. A rough surface has small concentrations of electrical fields that could also lift the dust.
The pendulum motion that has the dust swinging between shadow and light may happen because the sunlit areas have been positively charged and the shaded areas are negatively charged. “If there were no negatively charged area nearby, the dust grain would rise straight up,” states the press release. “However, since opposite charges attract, the positively charged dust gets pulled toward the negatively charged crater floor, bending its path over the crater. Dust launched from the sunlit area with just the right speed will pass over the shaded floor of the crater to the sunlit area on the other side, where the positively charged surface there will reflect it back over the crater again. When many particles do this, the model predicts there should be a swarm or canopy of dust over the crater.”
The dust doesn’t jump indefinitely, however. Due to the moon’s irregular surface and interference from the solar wind, the path of the dust is altered, and the dust eventually falls into craters or gets launched somewhere else.
“This model provides a natural explanation for the observation of dust ponds inside craters on the asteroid Eros,” says Collier.
“Calculating how these complications will affect the path of a dust particle on the moon and around asteroids are good areas for future research,” says Collier. “Additionally, we’re not sure how many particles get charged and move like this – is it something like one in a thousand, one in a million, or one in a billion? We’d like to do more studies to see how likely it is that a particle will behave this way. Since most of the lunar surface is covered in dust, even one in a billion would still be significant.”