When Voyager 2 flew by Neptune, it confirmed what ground-based astronomers had recently surmised from studies of stars being eclipsed by Neptune as the planet passed in front of them in its journey around the Sun. Neptune had rings of dust and very low reflectivity rock in orbit around it, but in the outermost ring system, the rings had arc-like features superimposed on a fainter ring system where much of the material seemed to be collected into 30 degree segments. The best guess we have about why Neptune has these ring-arcs is that there are small orbiting moons that 'shepard' the material into these orbits because of gravitational resonance effects. The net result is that some components to the rings of Neptune make it look like a lop-sided ring system as seen from Earth.
Here is a summary of the ring system from the NASA-Ames Research Center
Evidence for incomplete arcs around Neptune first arose in the mid-1980's, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberty, Equality and Fraternity. The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs. Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the center of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.
Copyright 1997 Dr. Sten Odenwald
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