Click image for larger version.
Bright and dark patterns show the distribution of ejecta of a 5 km diameter crater on the moon. Photo Credit: NASA/GSFC/Arizona State University
Dust is a major component of the material on the moon's surface. The moon's intensely cratered terrain tells a story of billions of years of meteoroid impacts. This prolonged bombardment has covered the lunar surface with a fine dust of pulverized rock. However, dust lofted above the moon's surface also may be a significant part of the lunar environment.
Scientists' first clues about a dynamic lunar dust environment were discovered by NASA's Surveyor missions - robotic landers that preceded the crewed Apollo missions. Images from Surveyors 5, 6, and 7 showed a glow that persisted above the western horizon of the moon shortly after lunar sunset. Sunlight was reflecting off of something in the lunar sky that supposedly was empty. Researchers thought this reflected glow might come from dust somehow lofted above the lunar surface, a theory supported by the Apollo missions. While orbiting the moon, astronauts looking out command module windows reported seeing rays and streamers stretching into the lunar sky when the sun was just below the lunar horizon. These streamers reached quite high, perhaps indicating that dust was being lofted up to great altitudes.
How was the dust being lifted into the lunar sky? Scientists' first guess was by energy delivered from meteoroid impacts on the lunar surface. Such impacts are quite common and can even be observed on Earth using amateur telescopes. However, another more intriguing mechanism was proposed: perhaps dust grains are electrostatically levitated above the lunar surface. During the lunar day, the side of the moon in sunlight develops a positive charge, as solar radiation knocks electrons off of atoms on the lunar surface. On the night side of the moon, electrons from the solar wind might deliver a negative charge to the lunar surface. In both cases, the accumulated charge may be enough to repel dust particles to altitudes ranging from feet to miles. (You may remember a classroom demonstration in electrostatic levitation that involves rubbing inflated balloons on your head and lifting the balloon - and your hair - up and away). At the lunar terminator - the line between day and night - there may be lateral transport of dust between these differently charged environments.
Support for electrostatic levitation came from an experiment carried to the moon by the Apollo 17 astronauts. The Lunar Ejecta and Meteoroids (LEAM) experiment was designed to detect and record impacts of micrometeoroids and their ejecta. Surprisingly, LEAM researchers reported that twice each lunar day there were sudden peaks of very small impacts. These peaks corresponded to the times of lunar sunrise and lunar sunset or as the line between day and night crossed over the LEAM instrument.
How much dust is there and how high does it reach into the moon's sky? Although scientists don't have answers at this time, the hope is that NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) mission will provide them. LADEE is equipped to measure the sizes of, and charges upon, lofted lunar dust particles as well as measure the distribution of this dust above the lunar surface.
As researchers look beyond the moon to other worlds with little or no atmosphere, they also see signs of dust transport on those surfaces as well. The studies conducted with LADEE will not only help us to better understand the lunar environment, but also provide insights into processes that may be important throughout the solar system and beyond.