Electric fields enveloping magnetic bubbles create lunar swirls

By Meghan Rosen

Web edition: July 11, 2012
Print edition: August 11, 2012; Vol.182 #3 (p. 8)

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Bright white designs called lunar swirls stretch across about 60 kilometers of the moon’s surface.

Credit: NASA

Scientists have charged up an old moon mystery. New research suggests that swirling designs on the dusty lunar surface might be the product of electric fields generated by pockets of magnetic bubbles.

“People have been looking at these strange, mysterious structures since the invention of the telescope,” says physicist Ruth Bamford of the Rutherford Appleton Laboratory in Didcot, England. “Now we know exactly how they are made.”

The milky patterns stand out like pale flesh against darkly tanned skin. It’s as if you used sunblock to paint whorls on your arm and then spent the day outside, says planetary geologist Georgiana Kramer of the Lunar and Planetary Institute in Houston. The sun would color everything but the protected skin, leaving the whorls white.

Scientists have long suspected that weak magnetic fields near the moon’s surface might shape the looping patterns. The moon doesn’t have a dynamo-driven magnetic field like Earth’s, but researchers have found patchy magnetic bubbles scattered across the lunar crust.

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A stream of charged particles (glowing purple) flows around a magnet in a solar wind tunnel experiment.

Credit: Courtesy of R. Bamford

Data from the Apollo missions fed a 1970s theory that the moon’s magnetic bubbles act like a solar wind sunblock. The solar wind — a steady stream of charged particles from the sun — constantly buffets the moon, turning pale lunar dust dark. But magnetic bubbles might protect the moon’s crust, keeping silvery soil fresh and young-looking.

The mystery, Bamford says, was how such puny fields can deflect the raging solar wind. The answer is the bubbles’ electric field, she and her colleagues suggest in an upcoming Physical Review Letters.

Usually, the solar wind’s charged particles travel together. But when the wind smacks into the moon’s magnetic bubbles, flimsy negatively charged particles skirt around the bubble and hefty positive ones try to penetrate it. Splitting apart these oppositely charged particles whips up a heavy-duty electric field.

Bamford’s team created a scaled-down laboratory version to find out if man-made magnetic bubbles could also deflect rushing rivers of particles.

The researchers used a device called a solar wind tunnel to shoot a jet of blazing particles down a tube. The searing stream toasted any object in its path, except, the team discovered, a magnet. The scientists showed that a thin electric field formed around the magnet, shielding it — and anything behind it — from the scorching flow. “It works incredibly well,” Bamford says. Even a marshmallow placed in the magnet’s wake would escape melting, she says.  

And if a tiny magnet — only slightly larger than an eraser tip — could make a protective electric skin, the moon’s much larger magnetic bubbles might also be able to.

“The work ties a bunch of ideas together,” says planetary scientist Ian Garrick-Bethell of the University of California, Santa Cruz. “And the lab model is really cool.”