Neptune has a faint ring system of unknown composition
and curiously clumpy structure for which there is yet no satisfactory explanation.
Evidence for incomplete arcs around Neptune first arose in the mid-1980s,
when stellar occultation experiments
were found to occasionally show an extra blink just before or after the
planet occulted the star. Images sent back 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
difficult to understand because the laws of motion suggest that arcs should
spread out into a uniform ring over very short timescales. Several other
rings were detected by Voyager. In addition to the narrow Adams Ring, 61,000
km from the center of Neptune, there is 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.
|This Voyager image, FDS 11412.51, shows the three
main arcs in Neptune's Adams Ring. The Liberty arc is near the lower
edge of the image, followed by Equality and Fraternity above it. A
fourth, fainter arc, called Courage, would be just ahead of Liberty,
outside the field of view
New Earth-based observations announced in 2005 appeared to show that Neptune's
rings are much more unstable than previously thought. In particular, it
seems that the Liberté ring might disappear in as little as one century.