Neptune

Voyager 2 confirmed the suspected existence of a ring system around Neptune, consisting of four rings with imbedded ring arcs within some of them. A ring is a continuous band of particles in a narrow orbital belt around the planet. A ring arc is part of a ring wherein particles are more heavily concentrated and the ring is thicker. The leading edge of a ring arc is rather vague and the trailing edge is sharp. There are names for the rings, in order from nearest the planet, they are: Galle (26,100 mi [42,000 km] from Neptune); Lassell (30.500 mi [49,000 km]); Arago (39,032 mi [53,000 km]); and Adams (39,150 mi [63,000 km]). The Lassell ring contains a named ring arc (Leverrier) at 31,690 mi (51,000 km) from Neptune's surface. The rings revolve directly around Neptune, are optically thinner and dustier (with much smaller average particle size) than Uranus's rings, are very dark with albedos (ratios of reflected to incident Figure 1. The Voyager 2 trajectory through the Neptune system. (A) is a large view of the system showing the projected orbits of Nereid and Triton. Tick marks on the trajectory show Voyager 2's position at one-day intervals. (B) is an enlarged view covering a 10-hour period that includes closest approaches to Neptune and Triton and passage through Earth and solar shadows (occultation zones). Neptune's ring system is also indicated. Here tick marks along the trajectory are at one-hour intervals. Illustration by Hans & Cassidy. Courtesy of Gale Group.

light) less than 0.07, and lie in or near the plane of Neptune's equator. Neptune's outer rings are closely associated with some of the six small satellites of Neptune discovered by Voyager 2 (see below). The cause of the clumping of many of the particles in the rings into compact ring arcs is still not definitely known. Gravitational perturbations by nearby satellites, for example Galatea affecting the Adams ring, are suspected of producing gravitational resonances that produce the arcs. The perturbed motions of ring particles can lead to collisions of Figure 2. Diagram of the offset and tilted magnetic field of Neptune. Illustration by Hans & Cassidy. Courtesy of Gale Group.

them that may be the main source of the copious and extensive fine (micron-sized) dust observed in Neptune's rings. The plasma wave instruments on Voyager 2 have detected a "halo" of dust around Neptune that extends far from the ring (equatorial) plane; it was detected even above Neptune's north polar region.

Like the case for Uranus's rings, the origin and evolution of Neptune's rings are unknown. Are they the result of earlier tidal disruption of other nearby satellites? Are they a transitory phenomenon, or will they persist for millions or billions of years? Comparison of the positions of the arcs in the Adams ring observed by Voyager 2 in 1989 with their positions extrapolated back in time to 1984 and 1985 shows that they match the positions of three occultations of stars observed in those years. This indicates that the arcs in the Adams ring are stable over time intervals of at least five years.