Envisioning arcs of moondust at Neptune.

Envisioning arcs of moondust at Neptune

As Voyager 2 approaches its Aug. 24 rendezvous with Neptune, scientists wait eagerly to see whether the spacecraft will confirm that the planet possesses the strangest set of rings known in the solar system.

About a decade ago, in studying the way a star's light blinked off and on as Neptune got in the way, Peter Goldreich and Scott Tremaine of the California Institute of Technology in Pasadena concluded that the planet has rings--but that unlike the rings of Jupiter, Saturn and Uranus, Neptune's might consist of short, unconnected arcs. Now Goldreich heads a research group proposing that if the arcs actually exist, as Voyager's photos should show, they may consist of debris left over from the destruction of some of Neptune's moons.

The source of this moon wreckage would be Neptune's big satellite Triton, say Goldreich and Caltech colleagues Norman Murray, Pierre-Yves Longaretti and Donald Banfield in the Aug. 4 SCIENCE. They propose that Triton originally formed in an orbit around the sun and was later gravitationally captured by Neptune as the result of a collision with another of the planet's moons. This left Triton in an orbit that gradually became more steeply tilted. In such an orbit, Triton circled Neptune and collided with a number of the planet's smaller satellites.

Triton could have gone around this path as many as 100 million times, picking off lesser moons all the while, says Goldreich. As a result, the researchers calculate, Triton "cannibalized" most of the satellites then orbiting Neptune at distances between about five and 200 times the planet's radius, which spans approximately 15,000 miles.

"Close to the planet, in the region where Triton never penetrated, regular satellites might still survive," the group concludes. "If comparison with the satellite systems of other giant planets is an accurate guide, several satellites must have formed in this region."

By Aug. 1, Voyager 2 scientists had identified only one moon other than the pair (Triton and Nereid) detected from Earth. But the craft had discovered no more moons at the other giant planets while at a similar distance from them.

The debris resulting from Triton's collisions would be confined in individual arcs, rather than a continuous ring, by "resonances" located at the balance points of the gravitational attractions of Neptune, Triton and perhaps several other satellites, say Goldreich and his colleagues. They base this whole idea on observations of stellar light blockages, or occultations, during only two occasions. Nonetheless, the group estimates from the limited data that Neptune may possess 10 to 100 arcs. The arcs would measure from a few tens of miles to thousands of miles long, laid out like segments of what would otherwise be a continuous ring. Holding them in place might require "a large number" of additional satellites, the researchers suggest.

On the other hand, the group also suggests that all the arcs might be held in position by resonances associated with a single moon, which they call Satellite X. If one Satellite X is indeed responsible, they speculate, its orbit is most likely tilted or elliptical. Tides caused on Triton by Neptune could have made Triton almost entirely molten during most of its unusual orbital evolution, they conclude.