An old Apollo experiment is revealing something new
and exciting about the Moon: dust storms that may threaten future
by Trudy E. Bell and Tony Phillips
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.
The Apollo 17 rover wasn't the only thing kicking up dust on the Moon...
Never heard of it? Few have. But scientists are increasingly confident that the storm is real.
The evidence comes from an old Apollo experiment called LEAM, short for Lunar Ejecta and Meteorites. "Apollo 17 astronauts installed LEAM on the Moon in 1972," explains Timothy Stubbs of the Solar System Exploration Division at NASA's Goddard Space Flight Center. "It was designed to look for dust kicked up by small meteoroids hitting the Moon's surface."
Billions of years
ago, meteoroids hit
the Moon almost constantly, pulverizing rocks
and coating the lunar surface with their dusty debris. Indeed,
this is the reason why the Moon is so dusty. Today these impacts
happen less often, but they still occur.
Apollo-era scientists wanted to know, how much dust is ejected by daily impacts? And what are the properties of that dust? LEAM was to answer these questions using three sensors that could record the speed, energy, and direction of tiny particles: one each pointing up, east, and west.
The box in the foreground is the Lunar Ejecta and Meteorites Experiment (LEAM)
LEAM's three-decade-old data are so intriguing, they're now being reexamined by several independent groups of NASA and university scientists. Gary Olhoeft, professor of geophysics at the Colorado School of Mines in Golden, is one of them:
"To everyone's surprise," says Olhoeft, "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."
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.
Even more surprising,
Olhoeft continues, 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."
observations could mean that "electrically-charged moondust was
sticking to LEAM, darkening its surface so the experiment package
absorbed rather than reflected sunlight," speculates Olhoeft.
But nobody knows for sure. LEAM operated for a very short time: only 620 hours of data were gathered during the icy lunar night and a mere 150 hours of data from the blazing lunar day before its sensors were turned off and the Apollo program ended.
Dusty "twilight rays" sketched by Apollo 17 astronauts in 1972
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.
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 "lunar transient phenomena" or LTPs. Some LTPs 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. Early explanations, never satisfactory, ranged from volcanic gases to observers' overactive imaginations (including visiting extraterrestrials).
In 1994, the Clementine Moon-orbiter also saw a dusty glow along the edge of the Moon
Now a new scientific
explanation is gaining traction. "It may be that LTPs are caused
by sunlight reflecting off rising plumes of electrostatically lofted
lunar dust," Olhoeft suggests.
All this matters
to NASA because, by 2018 or so, astronauts are returning to the
Moon. Unlike Apollo astronauts, who never experienced lunar sunrise,
the next explorers are going to establish a permanent outpost. They'll
be there in the morning when the storm sweeps by.
The wall of dust, if it exists, might be diaphanous, invisible, harmless. Or it could be a real problem, clogging spacesuits, coating surfaces and causing hardware to overheat.
Which will it be? Says Stubbs, "we've still got a lot to learn about the Moon."