Voyager sees Neptunian ring-arcs at last.

Voyager sees Neptunian ring-arcs at last

It's an unconventional idea, a "ring" structure unlike any known in the solar system, but since the mid-1980s scientists have wondered whether Voyager 2's flyby of Neptune might show the planet surrounded by short arcs rather than whole rings like those encircling Jupiter, Saturn and Uranus. Early on Aug. 11, with the spacecraft about 13 million miles out from Neptune and less than two weeks from its closest approach to the planet, Voyager photographed two "ring-arcs."

The pictures show the arcs, one of them about 30,000 miles long and wrapping about 45[deg.] around the planet. That arc, visible in the photo detail at right, sits a few hundred miles outside 1989 N4, one of four recently discovered Neptunian moons (SN: 8/12/89, p. 193). N4 orbits about 38,500 miles out from Neptune's center.

The other arc stretches about 6,000 miles and lies some 32,300 miles from the planet's core. Barely visible in the Voyager pictures, it appears to lag behind 1988 N3, another of the newly found moons, trailing the moon by about 90[deg.].

Scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif.--the Voyager control center--say they expect to discover more arcs and moons. The spacecraft detected these first arcs early enough to allow controllers to re-aim some of the remaining photos for a more detailed look. Both arcs show brightness variations along their lengths, possibly resulting from differences in density, particle size or rock type. Similar brightness variations appear in the F-ring of Saturn and a pair of narrow rings in Saturn's Encke division. Some planetary scientists suggest the arcs consist of fragments of former moons ground up by collissions with other moons (SN: 8/5/89, p.87).