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

The narrow, diffuse rings encircling encircling (en·serˑ·k  Neptune display several singular features. In particular, the outermost out·er·most  
Most distant from the center or inside; outmost.


furthest from the centre or middle

Adj. 1.
 ring contains a cluster of bright acrs, strung out like elongated e·lon·gate  
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.

adj. or elongated
1. Made longer; extended.

2. Having more length than width; slender.
 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 grav·i·ta·tion  
1. Physics
a. The natural phenomenon of attraction between physical objects with mass or energy.

b. The act or process of moving under the influence of this attraction.

 forces necessary to keep orbiting material confined con·fine  
v. con·fined, con·fin·ing, con·fines
1. To keep within bounds; restrict: Please confine your remarks to the issues at hand. See Synonyms at limit.
 to arcs. In the Aug. 30 SCIENCE, planetary scientist Carolyn C. Porco of the University of Arizona (body, education) University of Arizona - The University was founded in 1885 as a Land Grant institution with a three-fold mission of teaching, research and public service.  in Tucson proposes that the small satellite Galatea Galatea, in Greek mythology
Galatea (gălətē`ə), in Greek mythology.

1 Sea nymph, daughter of Nereus and Doris.
, 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 Peter Goldreich (born July 14, 1939) is an American astrophysicist whose research focuses on planetary rings, helioseismology and neutron stars.[1] He is currently the Lee DuBridge Professor of Astrophysics and Planetary Physics at California Institute of Technology.  of the California Institute of Technology California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20.  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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