March 18, 2013
Solar Wind Helps Lunar Dust Leap Around The Surface
Lee Rannals for redOrbit.com — Your Universe Online
NASA scientists describe using new calculations just how odd the surface of the moon really is, saying dust on the moon can actually leap from the dark side to the light side.
Even though the moon has no atmosphere, solar wind from the sun actually generates some unseen commotion on the surface of our neighboring satellite. Solar wind is a stream of electrically conducting gas known as plasma shot off the surface of the sun at about a million miles per hour. Solar wind and sunlight can combine together to help make the moon's surface more active than it appears.
On the day-lit side of the moon, sunlight knocks negatively charged electrons off the surface, giving it a positive charge. Then, on the night side or in shadowy craters, electrons from the solar wind rush in and give it a negative charge.
Micro-meteoroid impacts can transfer energy to the surface to launch this dust from one side to the other. Because the charges of the different sides of the moon repel each other, a positively charged dust grain in a sunlit area is pushed away from the positively charged surface. If no negatively charged area was close, then a dust grain would jump straight up into space. However, the positively charged dust gets pulled toward the negatively charged crater floor, altering its pathway back towards the surface, where the positively charged surface knocks it back up again.
According to the model created by the NASA scientists, if you walk up to a crater on the moon then you could potentially find a swarm or canopy of dust over the crater. Eventually, the dust will fall into the crater or be launched away due to variables like crater rim height, roughness on the crater floor, and interference from the solar wind weakening the electric field. However, the particle could bounce between sunlit areas on opposite sides of the crater for a while before this takes place.
The scientists said they are still not sure whether this motion of an individual dust particle is one in a thousand, one in a million, or one in a billion. But, even if it was one in a billion, Michael Collier at NASA's Goddard Space Flight Center points out that the moon is covered in dust, so this number could still be pretty significant.
"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," said Collier in a statement.
Collier is lead author of a paper on this research published October 2012 in Advances in Space Research.
The team is going through Apollo-era images to see whether any of these photographs ever snapped some evidence for dust canopies over shadowed craters.
"Calculating how these complications will affect the path of a dust particle on the moon and around asteroids are good areas for future research," said Collier. "We'd like to do more studies to see how likely it is that a particle will behave this way. "
Jumping lunar dust was actually first hypothesized by science fiction writer Hal Clement in the short story called "Dust Rag." In the story, two astronauts descended down to a crater on the Moon to investigate a mysterious haze dimming stars near the lunar horizon. They determined that dust somehow suspended above the ground.
Apollo 17 astronauts orbiting the Moon in 1972 repeatedly said they saw various "bands" for about 10 seconds before lunar sunrise or lunar sunset. NASA labeled this phenomenon as lunar "fountains."