A model for the azimuthal brightness variations in Saturn's rings

Abstract

The precise nature of the rings of Saturn is poorly understood. There is controversy, for example, over whether the rings have relaxed into essentially a monolayer, or whether the observed sharp brightening near opposition confirms the classical model of mutual shadowing in a layer many particles thick1–3. Likewise, the observations of azimuthal brightness variations in the rings (variations as a function of orbital phase of the ring particles)4–6 have been interpreted as implying the presence of large, synchronously rotating particles with either elongated shapes or systematic leading–trailing–edge albedo differences7 (although such synchronous rotation seems very unlikely)8; or alternatively the presence of density waves analogous to those postulated for spiral galaxies9 or ‘density wakes’ resulting from mutual gravitation between a relatively large ring particle and nearby smaller particles. Here we present a model to explain azimuthal brightness variations in the rings. Assuming the existence of density irregularities as a result of self-gravitation, we outline the probable photomeric effects. The results amplify the purely geometric discussion given by Franklin and Colombo10.