Monday, April 21, 2008

The Moon and the Magnetotail

I looked up at a beautiful just-past-full moon tonight and was reminded of something I just learned: The earth has a tail. It's true, because NASA said it does.

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:

Anyone can tell when the Moon is inside the magnetotail. Just look: "If the Moon is full, it is inside the magnetotail," says [Tim Stubbs, a University of Maryland scientist working at the Goddard Space Flight Center]. "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.

NASA's scientists aren't exactly certain what happens, because we never had any astronauts on the moon when it was crossing through the magnetotail, but they've managed to make some very educated guesses.

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.

[O]n 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.

The ground, meanwhile, may leap into the sky. There is compelling evidence (see, e.g., the Surveyor 7 image [in the NASA article]) that fine particles of moondust, when sufficiently charged-up, actually float above the lunar surface.

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.

As NASA makes plans for the next generation of astronauts to establish a lunar outpost, this is the kind of phenomenon they'll need to prepare for, once they figure out exactly what happens.

But knowing this much has given me a new appreciation for our "static" full moon.


Tsofah said...


So much scholarly knowledge is impressive! Methinks I have a headache in my attempts to understand - but it's still impressive! :-)

SkyePuppy said...


The scholarly part is NASA, not me. But it's pretty wild to think about dust storms without any wind, and about high voltages building up just by the moon moving behind the earth.

It'll be interesting to see how the astronauts do when they get a moon base or two up and running...