Photograph courtesy NASA
Published November 18, 2010
You won't get a literal jolt from looking at Sunday's full moon—but you might if you were standing on it. When the moon is full, it develops a strong electric field near the surface as it swings through Earth's magnetic "tail," according to new observations from a Japanese probe.
Earth's magnetic field creates a protective bubble known as the magnetosphere, which surrounds the planet and shields us from solar wind—a rush of charged particles, or plasma, constantly streaming from the sun. (Related: "'Warm Plasma Cloak' Discovered Enveloping Earth.")
As the solar wind pushes on Earth's magnetic bubble, the planet's magnetosphere stretches, forming what's called the magnetotail. This tail reaches beyond the orbit of the moon, and it's always pointed away from the sun. (See NASA illustrations of how the moon and the magnetotail interact.)
Meanwhile, we see a full moon when the lunar orb is on the opposite side of Earth from the sun—and therefore within the magnetotail.
Orbiter Spied Moon's Electric Field
JAXA launched the Kaguya probe in 2007. The craft orbited at a mere 62 miles (100 kilometers) above the moon's surface for 20 months, returning the first high-definition movies of the lunar landscape.
(See some of the first high-res moon pictures from NASA's Lunar Reconnaissance Orbiter.)
The orbiter was intentionally crashed into the moon in June of last year.
Scientists are now mining the data sent back by the craft's imagers and instruments, including the Magnetic field and Plasma experiment (MAP), which recorded that relatively high-energy electrons gyrating in the magnetic field are are being absorbed by the lunar surface when the moon is full.
The result is a strong electric field that develops around the lunar surface around the same time as the full moon, the Kagya team reports in a paper published October 1 in the journal Geophysical Research Letters.
Based on Kaguya's data, the team says this relatively intense electric field can be found when the moon passes through the region of the magnetosphere called the plasma sheet, which runs down the middle of Earth's magnetotail.
Since the moon has no global magnetic field of its own, its surface remains exposed to the trapped charged particles inside the plasma sheet. (See "Mini Magnetic Shield Found on the Moon.")
"We are suggesting the presence of a relatively strong electric field around the moon when it is inside the Earth's magnetosphere, but the origin of this electric field remains a mystery," said study leader Yuki Harada, a graduate student at Kyoto University in Japan.
"We think it may be related to the properties of the plasma and magnetic field in the Earth's magnetotail, and also the interaction between the moon and surrounding plasmas."
The newly seen effect adds to what scientists know about the moon's electric activity. For example, previous observations have shown that the moon gets slightly electrified by sunlight, regardless of whether the moon is in Earth's magnetotail.
The day side of the moon becomes positively charged, as solar radiation knocks electrons from the surface.
Meanwhile, electrons build up on the night side of the moon and give the surface a negative charge, Harada said.
Electric Moon a Hazard for Humans?
The big question now is whether a strongly charged lunar surface poses risks to future robotic and human explorers. (See pictures of award-winning robots designed to excavate the moon.)
"It is quite possible that electric fields induce a charge-up and subsequent discharge around a space vehicle, which could bring about serious damages to the human missions," Harada said.
The biggest hazards of an electrified moon may be that the static charge can transport large amounts of the moon's abrasive dust, which can damage sensitive lenses and electronics. The static buildup can also lead to unexpected electrical discharges.
NASA's Explorer series of probes and the Apollo missions were the first to reveal the perplexing lunar plasma environment, hinting that electrically driven dust may be a concern to robots and humans setting down on the moon.
(Read the original National Geographic magazine coverage of the Apollo 11 moon landing.)
"We've been to the surface before and survived just fine, but we did have a number of problems with dust, among other things. And we happened to be there during very quiet plasma conditions," said Jasper Halekas, a plasma physicist at the University of California, Berkeley.
"Things might be very different during a solar storm, or during a passage through the plasma sheet, the region that was looked at in this study," he said.
"Certainly when you have big electric fields, you start to worry about damage to sensitive electronics, etc. And if those electric fields mobilize dust, that could become an additional problem."
But, he said, "the truth is that we don't really know yet what relevance these kinds of studies may have for exploration. Probably the only way we will ever know for sure is to go back to the surface."
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