remain unexplained. Their existence is a puzzle because basic orbital dynamics imply that they should spread out into a uniform ring over a matter of years. Several theories about the arcs' confinement have been suggested, the most widely publicized of which holds that Galatea confines the arcs via its 42:43 co-rotational inclination resonance (CIR). The resonance creates 84 stable sites along the ring's orbit, each 4° long, with arcs residing in the adjacent sites. However measurements of the rings' mean motion with Hubble and Keck telescopes in 1998 led to the conclusion that the rings are not in CIR with Galatea. A later model suggested that confinement resulted from a co-rotational eccentricity resonance (CER). The model takes into account the finite mass of the Adams ring, which is necessary to move the resonance closer to the ring. A byproduct of this theory is a mass estimate for the Adams ring—about 0.002 of the mass of Galatea. A third theory proposed in 1986 requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its stable Lagrangian points. However Voyager 2'''s observations placed strict constraints on the size and mass of any undiscovered moons, making such a theory unlikely. Some other more complicated theories 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. Exploration The rings were investigated in detail during the Voyager 2 spacecraft's flyby of Neptune in August 1989. They were studied with optical imaging, and through observations of occultations in ultraviolet and visible light. The spaceprobe observed the rings in different geometries relative to the Sun, producing images of back-scattered, forward-scattered and side-scattered light. Analysis of these images allowed derivation of the phase function (dependence of the ring's reflectivity on the angle between the observer and Sun), and geometrical and Bond albedo of ring particles. Analysis of Voyager's images also led to discovery of six inner moons of Neptune, including the Adams ring shepherd Galatea. Properties *A question mark means that the parameter is not known.'' Notes References External links Neptune's Rings by NASA's Solar System Exploration Gazetteer of Planetary Nomenclature – Ring and Ring Gap Nomenclature (Neptune), USGS Neptune Neptune 198908??