Behold the full Moon. Ancient craters and
frozen lava seas lie motionless under an airless sky of profound
quiet. It's a slow-motion world where even a human footprint
may last millions of years. Nothing ever seems to happen there.
Right?
Wrong.
NASA-supported scientists have realized that something does
happen every month when the Moon gets a lashing from Earth's
magnetic tail.
"Earth's
magnetotail extends well beyond the orbit of the Moon and,
once a month, the Moon orbits through it, " says Tim Stubbs,
a University of Maryland scientist working at the Goddard
Space Flight Center. "This can have consequences ranging
from lunar 'dust storms' to electrostatic discharges."
Above: The full Moon inside Earth's magnetic
tail, March 2008.
Yes,
Earth does have a magnetic tail. It is an extension
of the same familiar magnetic field we experience when using
a Boy Scout compass. Our entire planet is enveloped in a bubble
of magnetism, which springs from a molten dynamo in Earth's
core. Out in space, the solar wind presses against this bubble
and stretches it, creating a long "magnetotail"
in the downwind direction: diagram.
Anyone
can tell when the Moon is inside the magnetotail. Just look:
"If the Moon is full, it is inside the magnetotail, "
says Stubbs. "The Moon enters the magnetotail three days
before it is full and takes about six days to cross and exit
on the other side."
It
is during those six days that strange things can happen.
Above:
The Moon's orbit crosses Earth's magnetotail.
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During
the crossing, the Moon comes in contact with a gigantic "plasma
sheet" of hot charged particles trapped in the tail.
The lightest and most mobile of these particles, electrons,
pepper the Moon's surface and give the Moon a negative charge.
On
the Moon's dayside this effect is counteracted to a degree
by sunlight: UV photons knock electrons back off the surface,
keeping the build-up of charge at relatively low levels. But
on the nightside, in the cold lunar dark, electrons accumulate
and voltages can climb to hundreds or thousands of volts.
Walking
across the dusty charged-up lunar terrain, astronauts may
find themselves crackling with electricity like a sock pulled
out of a hot dryer. Touching another astronaut, a doorknob,
a piece of sensitive electronics—any of these simple actions
could produce an unwelcome zap. "Proper grounding
is strongly recommended, " advises Stubbs.
The
ground, meanwhile, may leap into the sky. There is compelling
evidence (see, e.g., the Surveyor 7 image below) that fine particles
of moondust, when sufficiently charged-up, actually float above
the lunar surface. This could create a temporary nighttime atmosphere
of dust ready to blacken spacesuits, clog machinery, scratch
faceplates (moondust is very abrasive) and generally make life
difficult for astronauts.
Stranger
still, moondust might gather itself into a sort of diaphanous
wind. Drawn by differences in global charge accumulation,
floating dust would naturally fly from the strongly-negative
nightside to the weakly-negative dayside. This "dust
storm" effect would be strongest at the Moon's terminator,
the dividing line between day and night.
Much
of this is pure speculation, Stubbs cautions. No one can say
for sure what happens on the Moon when the magnetotail hits,
because no one has been there at the crucial time. "Apollo
astronauts never landed on a full Moon and they never experienced
the magnetotail."
The
best direct evidence comes from NASA's Lunar Prospector spacecraft,
which orbited the Moon in 1998-99 and monitored many magnetotail
crossings. During some crossings, the spacecraft sensed big
changes in the lunar nightside voltage, jumping "typically
from -200 V to -1000 V, " says Jasper Halekas of UC Berkeley
who has been studying the decade-old data.
Above:
In 1968, on many occasions, NASA's Surveyor 7 moon lander
photographed a strange "horizon glow" after dark.
Researchers now believe the glow is sunlight scattered from
electrically-charged moondust floating just above the lunar
surface.
"It
is important to note, " says Halekas, "that the plasma
sheet (where all the electrons come from) is a very dynamic
structure. The plasma sheet is in a constant state of motion,
flapping up and down all the time. So as the Moon orbits through
the magnetotail, the plasma sheet can sweep across it over
and over again. Depending on how dynamic things are, we can
encounter the plasma sheet many times during a single pass
through the magnetotail with encounters lasting anywhere from
minutes to hours or even days."
"As
a result, you can imagine how dynamic the charging environment
on the Moon is. The Moon can be just sitting there in a quiet
region of the magnetotail and then suddenly all this hot plasma
goes sweeping by causing the nightside potential to spike
to a kilovolt. Then it drops back again just as quickly."
The
roller coaster of charge would be at its most dizzying during
solar and geomagnetic storms. "That is a very dynamic
time for the plasma sheet and we need to study what happens
then, " he says.
What
happens then? Next-generation astronauts are going to find
out. NASA is returning to the Moon in the decades ahead and
plans to establish an outpost for long-term lunar exploration.
It turns out they'll be exploring the magnetotail, too.
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Solar
Wind vs. Magnetotail: Earth's magnetotail isn't
the only source of plasma to charge the Moon. Solar
wind can provide charged particles, too; indeed, most
of the time, the solar
wind is the primary source. But when the Moon enters
the magnetotail, the solar wind is pushed back and the
plasma sheet takes over. The plasma sheet is about 10
times hotter than the solar wind and that gives it more
"punch" when it comes to altering the charge
balance of the Moon's surface. Two million degree electrons
in the plasma sheet race around like crazy and many
of them hit the Moon's surface. Solar wind electrons
are relatively cool at only 140 thousand degrees, and
fewer of them zip all the way down to the shadowed surface
of the Moon's nightside.
Above:
Electrostatic forces acting on moondust may cause the
dust to float off the Moon's surface. Credit: Tim Stubbs/U.
Maryland/GSFC.
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Moon
Fountains -- (Science@NASA) When astronauts return
to the Moon, they might encounter electrified fountains
and other strange things.
Moondust
in the Wind -- (Science@NASA) What happens when
moondust is exposed to solar wind? NASA scientists have
found some surprising answers.
Moon
Storms -- (Science@NASA) An old Apollo experiment
is telling researchers something new and surprising
about the moon.
The
Mysterious Smell of Moondust -- (Science@NASA) Decades
after the Apollo program, scientists are still trying
to figure out why moondust smells like gunpowder.
NASA's
Future: US
Space Exploration Policy
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