Jupiter's faint rings might seem plain-Jane compared with Saturn's spectacular disks, but they have vexed astronomers for years, because the outer ring is lopsided. Now researchers report that the ring was pulled out of round by a tug of war between Jupiter's powerful magnetic field and the energizing effects of sunlight. The discovery should improve understanding of the forces that shape the rings around Saturn and other planets.
The jovian rings are difficult to spot from Earth. Astronomers didn't even know they existed until the twin Voyager probes visited the planet in 1979 and photographed them, backlit by the sun. Together, the rings are about 130,000 kilometers wide, or a bit less than half the width of Saturn's famous feature. Another difference is in their shape. Small shepherd moons keep Saturn’s rings in check. But part of the gossamer ring, Jupiter's farthest, extends beyond the orbit of the outermost shepherd moon, called Thebe.
Two astronomers think they've found the reason. Douglas Hamilton of the University of Maryland, College Park, and Harald Krüger of the Max Planck Institute of Nuclear Physics in Heidelberg, Germany, analyzed data collected by NASA's now-defunct Galileo spacecraft, which trawled the rings for a while before it plunged into the big planet's atmosphere in 2003. They report in the 1 May issue of Nature that as the ring particles slowly orbit Jupiter, they gain an electrical charge from the energy they receive from the sunlight. Then, when they fall into Jupiter's shadow, those charged particles get tugged in several directions by the planet's powerful magnetic field. The net effect is to warp their nightside orbit away from Jupiter and past Thebe.
So why don't Saturn's rings bulge? That's because Jupiter's magnetic field is more than 10 times as strong as Saturn's. The sunlight is more intense, too. Together, these effects make the shadow more important at Jupiter, Hamilton explains.
"It's great to see this simplest of the planetary rings finally understood," says astronomer Joseph Burns of Cornell University, part of NASA's Cassini spacecraft science team. Cassini is currently orbiting Saturn, and Burns hopes it will reveal similar--if subtler--features in those rings.
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