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The Little Satellite That Could

A wee moonlet appears to be the main source of the 260,000-kilometer-wide ring circling Jupiter. At a press conference today at Cornell University in Ithaca, New York, scientists unveiled new images of the Jovian ring system suggesting that erosion of tiny Adrastea--just 20 kilometers in diameter--is feeding Jupiter's brightest dust ring. "It's an American success story--the little guy wins," says planetary dynamicist Joseph Burns of Cornell.

Soon after the Voyager 1 spacecraft discovered the Jovian ring system in 1979, researchers realized that on a cosmic timescale the ring is ephemeral. Fresh dust must be continually feeding into the ring system to replace the dust that drifts into Jupiter. Some researchers, Burns among them, suggested that the continual rain of hypervelocity comet and asteroid debris pelting the small moons near the ring could be blasting dust off them into the rings.

The new images from the Galileo spacecraft, now orbiting the planet, also show that two sizable moons contribute to the ring's faint outer reaches. The pictures reveal that the faint outer "gossamer" ring is actually a ring within a ring. The 247-kilometer moon Almathea sweeps along the sharp outer edge of one ring and 116-kilometer Thebe orbits along the edge of the other. Those orbital correlations and gossamer ring thicknesses that match the motions of the moons have convinced Galileo team members that the gossamer ring dust is indeed coming from these two moons. "Here's an example of beautiful new data that proves an existing hypothesis" for a change, says ring specialist Jeffrey Cuzzi of NASA's Ames Research Center in Mountain View, California.

That little Adrastea could give rise to the far brighter main ring had seemed unlikely following the Voyager observations, but Burns now envisions a scenario in which impact erosion works at Adrastea far more efficiently than previously assumed. The moon presents a fair-sized target to speeding impactors, but it isn't so large that its gravity can hold onto much of the resulting debris, he says. In fact, Adrastea is so close to the massive planet that tidal forces would sweep parts of the moon clean. "Even though it's small," says Burns, "it's the perfect source."