Rings of Neptune
Neptune's rings are named after astronomers who contributed important work on the planet:[2] Galle, Le Verrier, Lassell, Arago, and Adams.
[6] Uniquely, the Adams ring includes five distinct arcs, named Fraternité, Égalité 1 and 2, Liberté, and Courage.
The arcs occupy a narrow range of orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980.
[2] Soon after the Uranus discovery, a team from Villanova University led by Harold J. Reitsema began searching for rings around Neptune.
Later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa, a highly unusual event.
[2] In the 1980s, significant occultations were much rarer for Neptune than for Uranus, which lay near the Milky Way at the time and was thus moving against a denser field of stars.
[8] Something (probably incomplete arcs) definitely existed around Neptune, but the features of the ring system remained a mystery.
[2] The Voyager 2 spacecraft made the definitive discovery of the Neptunian rings during its fly-by of Neptune in 1989, passing by as close as 4,950 km (3,080 mi) above the planet's atmosphere on 25 August.
[10] They are visible, slightly above background noise levels, at methane-absorbed wavelengths in which the glare from Neptune is significantly reduced.
[12][13] Neptune possesses five distinct rings[5] named, in order of increasing distance from the planet, Galle, Le Verrier, Lassell, Arago and Adams.
[4][7] The particles in Neptune's rings are made from a dark material; probably a mixture of ice with radiation-processed organics.
[7][18] The small moon Despina, which orbits just inside of it at 52,526 km, may play a role in the ring's confinement by acting as a shepherd.
[18] There is a small peak of brightness near the outer edge of the Lassell ring, located at 57,200 km from Neptune and less than 100 km wide,[4] which some planetary scientists call the Arago ring after François Arago, a French mathematician, physicist, astronomer and politician.
[23] The highest resolution Voyager 2 images revealed a pronounced clumpiness in the arcs, with a typical separation between visible clumps of 0.1° to 0.2°, which corresponds to 100–200 km along the ring.
Because the clumps were not resolved, they may or may not include larger bodies, but are certainly associated with concentrations of microscopic dust as evidenced by their enhanced brightness when backlit by the Sun.
[11] Courage, a very faint arc found during the Voyager flyby, was seen to flare in brightness in 1998; it was back to its usual dimness by June 2005.
[4] Their existence is a puzzle because basic orbital dynamics imply that they should spread out into a uniform ring over a matter of years.
[26] A third hypothesis proposed in 1986 requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its stable Lagrangian points.
[5] Some other more complicated hypotheses hold that a number of moonlets are trapped in co-rotational resonances with Galatea, providing confinement of the arcs and simultaneously serving as sources of the dust.
[6] The spaceprobe observed the rings in different geometries relative to the Sun, producing images of back-scattered, forward-scattered and side-scattered light.
[5] Analysis of Voyager's images also led to discovery of six inner moons of Neptune, including the Adams ring shepherd Galatea.
