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.
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 moon's 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
happen.
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.
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."
Those strange
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.
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.
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).
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."
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