Lunar Soil - Moon Dust Fountains and Electrostatic Levitation

Moon Dust Fountains and Electrostatic Levitation

There is some evidence that the Moon may have a tenuous atmosphere of moving dust particles constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion. The term "Moon fountain" has been used to describe this effect by analogy with the stream of molecules of water in a fountain following a ballistic trajectory while appearing static due to the constancy of the stream. According to the model recently proposed by Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center, this is caused by electrostatic levitation. On the daylit side of the Moon, solar ultraviolet and X-ray radiation is energetic enough to knock electrons out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micrometre and smaller) are repelled from the surface and lofted anywhere from metres to kilometres high, with the smallest particles reaching the highest altitudes. Eventually they fall back toward the surface where the process is repeated over and over again. On the night side, the dust is negatively charged by electrons in the solar wind. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes. This effect could be further enhanced during the portion of the Moon's orbit where it passes through Earth's magnetotail; see Magnetic field of the Moon for more detail. On the terminator there could be significant horizontal electric fields forming between the day and night areas, resulting in horizontal dust transport - a form of "moon storm".

This effect was also predicted in 1956 by science fiction author Hal Clement in his short story "Dust Rag" published in Astounding Science Fiction.

Also in 1956, the American scientist Thomas Townsend Brown appears to have predicted a similar lofting-falling cycle of photoelectrically excited lunar dust (along with controversial and as yet unproven speculations about unusual gravitational properties of this dust, an interest he maintained to the end of his life).

There is some evidence for this effect. In the early 1960s before Apollo 11, Surveyor 7 and several subsequent Surveyor spacecraft that soft-landed on the Moon returned photographs showing an unmistakable twilight glow low over the lunar horizon persisting after the Sun had set. Moreover, the distant horizon between land and sky did not look razor-sharp, as would have been expected in a vacuum where there was no atmospheric haze. Apollo 17 astronauts orbiting the Moon in 1972 repeatedly saw and sketched what they variously called "bands," "streamers" or "twilight rays" for about 10 seconds before lunar sunrise or lunar sunset. Such rays were also reported by astronauts aboard Apollo 8, 10, and 15. These may have been similar to crepuscular rays on Earth.

Apollo 17 also placed an experiment on the Moon's surface called LEAM, short for Lunar Ejecta and Meteorites. It was designed to look for dust kicked up by small meteoroids hitting the Moon's surface. It had three sensors that could record the speed, energy, and direction of tiny particles: one each pointing up, east, and west. LEAM saw a large number of particles every morning, mostly coming from the east or west—rather than above or below—and mostly slower than speeds expected for lunar ejecta. Also, a few hours after every lunar sunrise, the experiment's temperature rocketed so high—near that of boiling water—that LEAM had to be turned off because it was overheating. It is speculated that this could have been a result of electrically-charged moondust sticking to LEAM, darkening its surface so the experiment package absorbed rather than reflected sunlight.

It's even possible that these storms have been spotted from Earth: For centuries, there have been reports of strange glowing lights on the Moon, known as "Transient lunar phenomenon" or TLPs. Some TLPs have been observed as momentary flashes—now generally accepted to be visible evidence of meteoroids impacting the lunar surface. But others have appeared as amorphous reddish or whitish glows or even as dusky hazy regions that change shape or disappear over seconds or minutes. These may have been a result of sunlight reflecting off of suspended lunar dust.