Dynamics of saturn's dense rings

Abstract

The Cassini mission to Saturn opened a new era in the research of planetary rings, bringing data in unprecedented detail, monitoring the structure and properties of Saturn's ring system. The question of ring dynamics is to identify and understand underlying physical processes and to connect them to the observations in terms of mathematical models and computer simulations. For Saturn's dense rings important physical processes are dissipative collisions between ring particles, their motion in Saturn's gravity field, their mutual self-gravity, and the gravitational interaction with Saturn's moons, exterior to or embedded in the rings. The importance of the rings' self-gravity became strikingly clear from the identification of gravitational wakes in Cassini data nearly everywhere in the A and B rings. Self-gravity wakes imply that the rings are in a flat, dynamically cold state, ring particles colliding very dissipa-tively, being densely packed in the ring plane, continuously forming transient gravitationally bound opaque clumps, that are disrupted again by shear on orbital timescales. Current mathematical dynamical models usually treat self-gravity in an approximate manner, which does not lead to a wake state. The dense packing of ring particles, in turn, strongly influences the collisional dynamics, since the mean free path of the particles is then comparable to or smaller than the particle size. This leads to a strong nonlocal component of pressure and momentum transport, which determines the viscous evolution of the rings, the damping of density waves, as well as the stability properties of the ring's flow. A strong nonlocal contribution to viscosity is, for instance, favorable for viscous overstability, leading to axisymmetric waves of about 100 m wavelength. Such wavelike perturbations in the ring's opacity, consistent with overstability, are seen in Cassini stellar and radio occultations. A classical topic of ring dynamics is the interaction of moons and rings. On the one hand, there are exterior moons with resonances in the rings, creating numerous density and bending waves. With the large sets of Cassini occultation and imaging data, improved estimates of the ring surface mass density and viscosity are obtained from fits of the observations to dynamical models. On the other hand, the embedded moons Pan and Daphnis open the Encke and Keeler gaps, respectively, and moonlets in the rings, too small to open a circumferential gap, are found to produce a characteristic propeller structure. Comparison between theoretical studies and Cassini observations of thermal emission from the rings provides constraints on spin rates of ring particles, which are otherwise not directly observable. The size distribution of particles and small moonlets embedded in the rings, together with the observed shapes and internal densities of small moons just exterior to the rings, underline the importance of accretion and fragmentation for the dynamical evolution of Saturn's ring system. © 2009 Springer Science+Business Media B.V.