Every lunar morning, when the sun first peeks over the dusty soil of the moon after two weeks of frigid lunar night, a strange storm stirs the surface.

The next time you see the moon, trace your finger along the terminator, the dividing line between lunar night and day. That's where the storm is. It's a long and skinny dust storm, stretching all the way from the north pole to the south pole, swirling across the surface, following the terminator as sunrise ceaselessly sweeps around the moon.

Never heard of it? Few have. But scientists are increasingly confident that the storm is real.

Astronauts may have seen the storms, too. While orbiting the Moon, the crews of Apollo 8, 10, 12, and 17 sketched "bands" or "twilight rays" where sunlight was apparently filtering through dust above the moon's surface. This happened before each lunar sunrise and just after each lunar sunset. NASA's Surveyor spacecraft also photographed twilight "horizon glows," much like what the astronauts saw.

What could cause this? Stubbs has an idea: "The dayside of the moon is positively charged; the nightside is negatively charged." At the interface between night and day, he explains, "electrostatically charged dust would be pushed across the terminator sideways," by horizontal electric fields.

In the early 1960s before Apollo 11, several early 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.

But most amazing of all, 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.

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.

There could be "significant horizontal electric fields forming between the day and night areas, so there might be horizontal dust transport," Stubbs speculates. "Dust would get sucked across the terminator sideways." Because the biggest flows would involve microscopic particles too small to see with the naked eye, an astronaut would not notice dust speeding past. Still, if he or she were on the Moon's dark side alert for lunar sunrise, the astronaut "might see a weird, shifting glow extending along the horizon, almost like a dancing curtain of light." Such a display might resemble pale auroras on Earth.

Astronauts need to know, because in the years ahead NASA plans to send people back to the Moon, and deep dark craters are places where they might find pockets of frozen water--a crucial resource for any colony. Will they also encounter swarms of electric dust?

During the Apollo era of exploration
it was discovered that sunlight was scattered at the
terminators giving rise to “horizon glow” and “streamers”
above the lunar surface [1,2] (e.g., Fig. 1). This
was observed from the dark side of the Moon during
sunset and sunrise by both surface landers and astronauts
in orbit. These observations were quite unexpected,
as the Moon was thought to be a pristine environment
with a negligible atmosphere or exosphere.
Subsequent investigations have shown that the sunlight
was most likely scattered by electrostatically charged
dust grains originating from the surface

When humans return to the Moon and travel to Mars, they'll have to be careful of what they inhale.

April 22, 2005: This is a true story.


In 1972, Apollo astronaut Harrison Schmitt sniffed the air in his Lunar Module, the Challenger. "[It] smells like gunpowder in here," he said. His commander Gene Cernan agreed. "Oh, it does, doesn't it?"

The two astronauts had just returned from a long moonwalk around the Taurus-Littrow valley, near the Sea of Serenity. Dusty footprints marked their entry into the spaceship. That dust became airborne--and smelly.

Though the Moon is surrounded by a vacuum higher than is usually created in laboratories on Earth, its atmosphere is extensive and of high scientific interest.

During the two-week daytime period, atoms and molecules are ejected by a variety of processes from the lunar surface, ionized by the solar wind, and then driven by electromagnetic effects as a collisionless plasma. The position of the Moon in its orbit determines the behaviour of the atmosphere. For part of each month, when the Moon is on the sunward side of Earth, atmospheric gases collide with the undisturbed solar wind; in other parts of the orbit, they move into and out of the elongated tail of Earth's magnetosphere, an enormous region of space where the planet's magnetic field dominates the behaviour of electrically charged particles.

In addition, the low temperatures on the Moon's nightside and in permanently shaded polar craters provide cold traps for condensable gases.