The collisionless rings of Uranus

In light of the Voyager 2 mission, the ringlets of Uranus seem unlikely to be accompanied by the swarm of 10 to 18 "shepherding" satellites generally supposed. Consequently, coherent motion (i.e., bulk circulation) in the ringlets again deserves serious consideration. Moreover the radio occultation data show the rings to be composed of larger boulders but few of the small particles one would expect if these boulders bumped and ground against one another as they would have to if the motions were incoherent. Recent evidence that Neptune's ringlets may be incomplete arcs further points in this direction. Dense (‘liquid’) rings have been proposed by Borderies, Goldreich and Tremaine. We point out here that such rings would be essentially viscosity free if executing coherent circulation and hence endure for geologically long time scales.