The Surveyor 6 landing location in Sinus Medii will be "in your face" this week as the moon waxes from gibbous to full phase. Photo: Bob King
As the moon waxes toward full phase this week, most of the sky’s fainter stars and galaxies will be snuffed out in its overwhelming light. This is the time many amateur astronomers discover they have nights free to do other things like spend a night on the town with their spouse or just get to bed early for a change. If you happen to step out in the moonlight tonight, fix your eye on the center of its glaring gibbous form. Although too small to see with the naked eye, there’s a small dark, lunar sea there called Sinus Medii (Central Bay). In November 1967 the Surveyor 6 probe soft-landed in the bay and studied the soil, took pictures and obtained other important data characterizing the lunar environment in advance of the manned Apollo missions.
The western horizon photographed by Surveyor 6 in November 1967 reveals the lingering glow of moon dust levitating about a yard above the surface. Credit: NASA
Among its data booty was a curious photograph of a glowing western horizon at sunset. On Earth the horizon glows after sunset because sunlight is still lighting up the atmosphere, but the moon has precious little of what you’d call air so this must have been something else. Further study revealed that the pictures captured the glow of electrostatically-levitating moon dust. The glow was seen by additional Surveyor landers.
The Clementine spacecraft took this photo of the sun’s corona rising over the moon’s edge just before sunrise in 1994. Venus is at top and the moon itself is lit by Earthshine or sunlight reflected off the Earth. Credit: NASA
As Apollo 17 astronauts orbited over the night side of the moon they planned to use the moon to block the brilliant sun so they could make observations of its faint outer atmosphere called the corona as well as a more extended glow of interplanetary dust particles illuminated by sunlight called the zodiacal light. Just before orbital sunrise, they saw what they expected — a hump of zodiacal light emerging from behind the moon’s edge — and something that surprised them, too. The entire curve of the lunar horizon glowed, and just before sunrise, the glow was shoot through with faint rays similar to the rays of light that beam through holes in the clouds in our earthly skies.
"Twilight" rays sketched by Apollo 17 astronauts just before sunrise
as they orbited the moon. Credit: NASA
Again, it was levitating dust at work, but this time, the dust was lofted to a height of 62 miles above the moon’s surface. Other Apollo crews saw similar phenomena while in lunar orbit. Scientists hypothesize that what the astronauts saw and Surveyor photographed was electrically-charged moon dust. Ultraviolet light and X-rays from the sun on the dayside of the moon are powerful enough to knock electrons, negatively charged particles, off lunar dust particles. This leaves the particles with a positive electric charge. Since like charges repel, the dust particles push away from one another and float in the direction of least resistance — up. The smallest particles reach the highest altitudes and then fall back to the surface, pulled down by the moon’s gravity. Others float up to replace them in a repeated cycle or rising and falling like the stream of water in a water fountain.
You can see and feel for yourself what’s happening on the moon if you take an inflated balloon and rub it on your hair (right). As you lift the balloon away from your head, your hair is drawn to the balloon. Rubbing removed electrons from your hair, giving the balloon an excess negative charge and your hair a positive charge. If you take the balloon away completely now, your hair will still stand up because each positively charged hair repels the other. A similar separation of electrical charges neatly explains repelling moon dust.
How about the lunar nightside? It appears that the dust there might be negatively charged because of electrons in the solar wind bombarding the soil. Astronomers speculate that nightside dust could float even higher than the dayside variety. Finally, there’s the interesting situation at the lunar terminator, the border between day and night. The terminator moves to the east as the moon’s phase waxes and to the west as it wanes. It’s a constantly shifting boundary. Timothy J. Stubbs of the Laboratory for Extraterrestrial Physics at NASA’s Goddard Space Flight Center, speculates there could be "significant horizontal electric fields forming between the day and night areas, so there might be horizontal dust transport. Dust would get sucked across the terminator sideways." Can you imagine standing astride the terminator as a lunar dust storm blew by — wow!
Fountains of lunar dust, strange glows. Just when we think the moon is dead, something new pops up like the recent discovery of lunar water, or in this case, levitating dust.
(Balloon photo credit: NASA)