Full moons
are said to be behind many strange things, but here's one you didn't know
about: At full moon, our favorite satellite is whipped by Earth's magnetotail, causing lunar dust storms and
discharges of static electricity.
This
new finding, announced this week by NASA, is important to future lunar
explorers: Astronauts may find themselves "crackling with electricity like
a sock pulled out of a hot dryer," according to an agency statement.
The
effect on the moon was first noticed in 1968, when NASA's Surveyor 7 lander
photographed a strange glow on the horizon after dark. Nobody knew what it was.
Now scientists think it was sunlight scattered by electrically
charged moon dust floating just above the surface. That fits with data from
NASA's Lunar Prospector, which orbited the moon in 1998-99. During some
crossings of the magnetotail, the spacecraft recorded big changes in the lunar
night-side voltage.
How it works
Our
entire planet is enveloped in a bubble of magnetism generated by the rotating
core. The solar wind, a stream of charged particles, pushes the bubble away
from the sun and creates a long tail of magnetized material downstream.
"Earth's
magnetotail extends well beyond the orbit of the moon and, once a month [at
full moon] the moon orbits through it," said 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."
Here's
what Stubbs and colleagues now think is happening:
At
full moon, the moon passes
through a huge "plasma sheet" — 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, the researchers explained.
On
the moon's dayside this effect is counteracted somewhat by sunlight: Photons
knock electrons back off the surface, lessening the negative charge. But on the
night
side, electrons accumulate and the charge can climb to thousands of volts.
What happens
The
researchers speculate on what happens next.
The
Surveyor 7 images suggest fine dust particles, all charged up, float above the
lunar surface. On the night side, this dust might be intense enough to clog
machinery and scratch an astronaut's faceplate.
The
extreme differences in charge might cause dust to fly from the negative night
side to the less-negative day side, becoming strongest along the regions where
the sun is rising or setting.
Wild place
NASA
has long been concerned about these electrical
charges and moon dust and the overall impacts on astronauts, habitats and
machinery. In fact the
agency is drawing
up plans to probe the secrets of moon dust.
Astronauts
walking on the charged terrain might get electrified like sock from 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," Stubbs advised.
The
plasma sheet is in a constant state of motion, flapping up and down all the
time," said Jasper Halekas of the University of California, Berkeley. "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."
This
makes for a very dramatic environment.
"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 night side potential
to spike to a kilovolt," Halekas said. "Then it drops back again just
as quickly."
Anyone
on the moon would want to know more about how all this works. And it'd be
significantly worse during a solar storm.
"That
is a very dynamic time for the plasma sheet and we need to study what happens
then," he says.
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