Charged particle depletion surrounding Saturn's F ring: Evidence for a moonlet belt?

Abstract

Pioneer 11 detected five abrupt depletions in the flux of trapped magnetospheric electrons within a 2000-km-wide band surrounding Saturn's F ring. We reconsider these observations using an improved description of the magnetosphere and of the locations of known rings and moons. We show that two of the five observed abrupt decreases in electron flux are probably due to absorption by the known F ring. However, none of the other three are likely to be caused by the F ring, its “shepherds” (Pandora and Prometheus), or by material sharing their orbits. We infer that the observed depletions of charged particles are caused by clumps of material with low optical depth ( τ ∼ 10 −4−10 −3); we hypothesize that these clumps are composed of regolith ejecta and result from collisions within a belt of unseen objects of moonlet scale and smaller (0.1–10 km radius) which occupies the entire radial region lying between Pandora and Prometheus. We have developed a self-consistent scenario in which these debris clouds are created, spread longitudinally, and are swept up onto surfaces of the belt objects only to be thrown off again in a subsequent collision. Within this framework, the microsignature absorbers are the outcomes of “typical” collisions in such a belt of objects, while the stranded, braided F ring revealed in Voyager images may be the outcome of a much less common collision between some of its largest, least numerous members. The hypothetical belt objects have a steep powerlaw distribution and a total optical depth of about 10 −4 to 10 −3. We expect that some narrow rings and ring arcs in the ring systems of Uranus and Neptune may be similarly indicative of ongoing collisional/accretional processes taking place within moonlet belts at the edge of the Roche zone of their parent planets.