Moon Fountains

Here on Earth we see something similar: crepuscular rays. These are shafts of light and shadow cast by mountain ridges at sunrise or sunset. We see the shafts when they pass through dusty air. Perhaps the Moon's "twilight rays" are caused, likewise, by mountain shadows passing through levitating moondust. Many planetary scientists in the 1970s thought so, and some of them wrote papers to that effect (see the "more information" box at the end of this story for references).

But without an atmosphere, how could dust hover far above the Moon's surface? Even if temporarily kicked up by, say, a meteorite impact, wouldn't dust particles rapidly settle back onto the ground?

Well, no - at least not according to the "dynamic fountain model" for lunar dust 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.

"The Moon seems to have a tenuous atmosphere of moving dust particles," Stubbs explains. "We use the word 'fountain' to evoke the idea of a drinking fountain: the arc of water coming out of the spout looks static, but we know the water molecules are in motion." In the same way, individual bits of moondust are 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.

On the left are lunar "twilight rays" sketched by Apollo 17 astronauts; on the right are terrestrial crepuscular rays photographed by author Trudy E. Bell.

You can get some hands-on experience with the fountain model ... on top of your head.

Rub an inflated balloon on your hair, and then hold the balloon a few inches away. Your hair will rise of its own accord to reach out toward the balloon. Rubbing the balloon removes some of the electrons from your hair, leaving your hair with a net positive charge. Your positively charged hair is attracted to the negatively charged balloon.

Now watch what happens when you hold the balloon far away. This is key: Your individual hairs spread out from one another and do not immediately fall back to lie flat on your head. What's happened? When the balloon was removed, each positively charged hair repels its positively charged neighbor and some of your hair remains suspended - just like dust on the Moon.