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'Shepherd' satellite for Neptune's ring-arcs.

The narrow, diffuse rings encircling Neptune display several singular features. In particular, the outermost ring contains a cluster of bright acrs, strung out like elongated beads along a dusty thread. Since their discovery in 1984, these ring-arcs, which correspond to concentrations of rocks and particles, have challenged the ingenuity of theorists who seek to explain why such features form and appear stable.

An analysis of data gathered by the Voyager spacecraft when it passed by Neptune in 1989 now suggests that a single satellite circling Neptune may supply the pattern of gravitational forces necessary to keep orbiting material confined to arcs. In the Aug. 30 SCIENCE, planetary scientist Carolyn C. Porco of the University of Arizona in Tucson proposes that the small satellite Galatea, which circles Neptune just 1,000 kilometers inside the arc-containing ring, has the right characteristics to act as a "shepherd," keeping trapped material from spreading out along the ring.

Neptune's ring-arcs lie about 60,000 kilometers away from the planet's center and cover about 10 percent of the ring's circumference. Individual arcs have a width of roughly 15 kilometers.

Porco's analysis of the Voyager data has yielded a number of clues crucial to solving the ring-arc mystery. She discovered that ring-arcs wiggle, periodically undergoing a distortion that shifts their radial positions by as much as 30 kilometers. This gravitational disturbance travels through the arcs at the right speed to be associated with Galatea, she says. Other observational evidence points to a second important influence, also associated with Galatea, that prevents the particles from spreading out along the ring. Together, the two perturbations keep the particles confined to certain ring positions, Porco suggests.

The shepherding influences of a single satellite provide "the most plausible explanation for the confinement of Neptune's ring arcs," she says. "But it may not be the whole story."

One difficulty is that individual arc particles would have to travel in orbits that cross each other to account for the width and radius of the arcs. "That's the weakness in this picture," says Peter Goldreich of the California Institute of Technology in Pasadena. "There would be frequent collisions, and that would tend to destroy the arcs." Before Voyager's arrival at Neptune and the discovery of Galatea, Goldreich and his collaborators had raised the possibility that a single satellite might be responsible for the ring-arcs.

The origin of the ring-arcs poses another puzzle. Porco's model indicates which ring sites could serve as particle traps, but it doesn't specify which ones are actually filled. "The observation that Neptune's arcs are so few in number and clustered so closely ... suggests that they may have had their origin in the collisional disruption of a small moon," she says.

Her data analysis has also turned up additional arcs. "To the eye, it looks like there are three main arcs, but there are really five," Porco says. "And I wouldn't be surprised if there are more."

The Voyager data may yet contain other important clues concerning the origin and stability of Neptune's ring-arcs. "We're not finished," Porco says. "There's still much more work to be done."
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Title Annotation:planet Neptune
Author:Peterson, Ivars
Publication:Science News
Date:Aug 31, 1991
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