The formation of the Cassini division in Saturn's rings

Abstract

The satellite Mimas excites a trailing spiral density wave in Saturn's rings at the position of the 2:1 resonance. The density wave carries negative angular momentum and propagates outward. The wave is damped by a combination of nonlinear and viscous effects, and its negative angular momentum is transferred to the ring particles. Consequently, the particles just outside the 2:1 resonance spiral inward, opening a gap. The inner edge of the gap is close to the resonance position in agreement with the location of the inner edge of the Cassini division. Despite its tiny mass, Minas is able to clear a gap as wide as the Cassini division. We estimate the ability of Saturn's satellites to open other gaps in the rings. The upper limit to the width of Encke's division implies that the velocity dispersion of the ring particles is <10^(-2) cm sec^(-1).