Normally, any clumps would quickly spread and be erased within a year, but somehow these structures persist. First detected from earth in 1986, they are as mysterious as when Voyager 2 imaged them in 1989. Tidal resonance with Galatea, one of the inner moons, is involved somehow. 
Earlier theories invoked corotational inclination resonance (CIR), but this cannot explain the main arc. Porco proposes a corotational eccentricity resonance (CER), that works if the ring arcs have sufficient mass and Galatea has a certain eccentricity. Galatea’s orbit, however, should have been circularized (no eccentricity) within 108 years, one fiftieth the assumed age of the solar system. Perhaps the arcs’ mass causes a residual eccentricity in Galatea’s orbit. The authors point out several variables that don’t fit neatly into any model, and say perhaps CIR and CER are both needed in concert to explain all the features. Some values need to be further constrained before their latest model can be evaluated.
"This was an odd and unexpected discovery. Ring particles are subject to continuous disruption and bombardment by
micrometeorites, subatomic particles, collisions, gas drag and gravitational perturbations by nearby moons. That rings exist at all around the four gas giants is puzzling, if the solar system is 4.5 billion years old, but that some of Neptune’s should be confined to clumps and arcs was startling and totally unexpected. Planetary scientists may someday figure out how to keep them going for long time periods, but for 4.5 billion years? It would seem very difficult to explain how something so tenuous could be that persistent, because the moons like Galatea are themselves undergoing orbital changes. That’s why most planetary scientists feel that rings formed recently, and we are just lucky to have evolved at the same time too see them. Another possibility never considered is that maybe the solar system is not that old. When Cassini gets to Saturn in 2004, we can probably expect more mysteries of the rings to be unveiled."
CEH