Joshua Hill  Bio

Email Article

Email Us

Print friendly

Earth’s magnetic tail

The Moon Not so Inactive as First Thought

Our moon has long been our interstellar companion, protecting us from meteor hits and directing the tides. From a glance, it would seem dead and lifeless; a slow-motion world fit for movies about tragic discoveries and losses. 

But this idea that the Moon is just a boring lump of rock has been shifted, thanks to new NASA-supported research.

Tim Stubbs, a University of Maryland scientist working at the Goddard Space Flight Center, is one of the researchers that has found that, every month, around about the time the moon is full, it gets hit by Earth’s magnetic tail.

“Earth’s magnetotail extends well beyond the orbit of the moon and, once a month, the moon orbits through it,” he says. “This can have consequences ranging from lunar ‘dust storms’ to electrostatic discharges.”

A magnetotail is nothing more than an extension of our magnetic field; the field that helps us point our compasses in the right direction. Earth is wrapped in this magnetic field, alike to a bubble, so when the solar winds press against us, our magnetic field stretches, and distorts in the downward direction, creating the magnetotail.

During the period that our moon crosses over the magnetotail, strange things begin to happen on the nightside.

The crossing causes the moon to come in to contact with a large “plasma sheet” of hot charged particles, that are trapped within our magnetic tail. From within the tail, the electrons hit the moon’s surface and give it a negative charge.

And while the lightside of the moon is somewhat counteracted by the UV photons, the nightside gets all very strange and kooky.

With the accumulation of surface voltages climbing, the nightside of the moon can climb to hundreds or even thousands of volts in energy. Subsequently, if an astronaut happens to encounter this while she or he is up there, they too build up an electric charge. Touching another astronaut, electronics then, becomes hazardous; “Proper grounding is strongly recommended,” says Stubbs.

In addition, there’s growing evidence that in such a period, the fine particles of moondust actually begin to float; ejected from the lunar surface by electrostatic repulsion. This too creates hazards for astronauts, who have to deal with their suits getting dusty, machinery getting clogged, and their sensitive faceplates getting scratched.

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.

“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 many times. 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.”

http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html