An old Apollo experiment is telling researchers something
new and surprising about the moon.
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December 7, 2005: 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."
Right:
The box in the foreground is the Lunar Ejecta and Meteorites
Experiment (LEAM). [More]
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. (Learn more: "Moon
Fountains." )
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.
Above:
Dusty "twilight rays" sketched by Apollo 17 astronauts
in 1972. [More]
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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Authors: Trudy
E. Bell &
Dr. Tony Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
|