Saturn Ringed by Electric Doughnut

Saturn Ringed by Electric Doughnut
A series of images from Cassini's MIMI instrument, which shows energetic neutral atom emission from Saturn's ring current. The images indicate the field rotates with Saturn in a counter-clockwise direction.
(Image: © Johns Hopkins Applied Physics Laboratory/Jet Propulsion Laboratory/NASA)

HomerSimpson, meet your match in space: Astronomers have confirmed the existence ofa lopsided "doughnut" of electrified plasma surrounding Saturn.

The giantring current, as the doughnut is called, was confirmed following analysis of recentCassini spacecraft data. But the new information adds a twist to the electricphenomenon, which extends more than 746,000 miles (1.2 million kilometers) intospace: It rotates.

DonMitchell, an astrophysicist at Johns Hopkins University and co-author of a newstudy detailed in the Dec. 13 issue of the journal Nature, revealed histeam's initial findings at a conference earlierthis year, but said the new study now confirms them. He explained that mostof Saturn's ring current plasma comes from its ice-spewing moon Enceladus.

"Earth'sring current is made of upper atmosphere and solar wind particles, so it'smostly hydrogen," Mitchell said. "But Saturn's source is by and largeEnceladus, which shoots out a whole lot of oxygen in the form of water."

Mitchellexplained that sunlight zaps the water and turns it into charged particlescalled ions, which Saturn's magnetic field captures and turns into a tube ofenergized plasma. The pressure of the solar wind, however, smears thenight-side half of the ring into a sheet of plasma that continuously driftsinto space.

UnlikeEarth's relatively stationary ring current, however, Saturn's rotates with theplanet, Mitchell said.

"Earthrotates pretty slow compared to the speed of the ring current particles, so thething is stationary," he said. But Saturn rotates more than twice as fastas Earth, dragging Saturn's heavy oxygen ions around the planet in acounter-clockwise direction.

Although theCassini data resolves the mystery of Saturn's ring—which the existence of,Mitchell said, was only speculative 25 years ago—it has uncovered a new one.

"There'sa point during the rotation where we see an energy spike, so there's somethingspecial about a particular spot on Saturn," he said. "For now,though, we don't know what it could be."

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