Saturn's Rings

Abstract

The rings are changing before our eyes; structure varies on all timescales and unexpected things have been discovered. Many questions have been answered, but some answers remain elusive. Here we highlight the major ring science progress over the mission to date, and describe new observations planned for Cassini’s final three years. Ring Composition and particle sizes: The rings are nearly all water ice with no other ices – so why are they reddish? The C Ring and Cassini Division are “dirtier” than the more massive B and A Rings, as shown by near- IR and, recently, microwave observations. Particle sizes, from stellar and radio occultation's, vary from place to place. Ring structure, micro and macro: numerous spiral density waves and ubiquitous “self-gravity wakes” reveal processes which fostered planet formation in the solar system and elsewhere. However, big puzzles remain regarding the main ring divisions, the C Ring plateau structures, and the B Ring irregular structure. Moonlets, inside and out, seen and unseen: Two gaps contain sizeable moonlets, but more gaps seem to contain none; even smaller embedded “propeller” objects wander, systematically or randomly, through the A ring. Rubble pile ring moons just outside the rings may escaped from the rings, and the recently discovered “Peggy” may be trying this as we watch. Impact bombardment of the rings: Comet fragments set the rings to rippling on century-timescales, and boulders crash through hourly; meanwhile, the constant hail of in falling Kuiper belt material has a lower mass flux than previously thought. Origin and Age of the Rings: The ring mass and bombardment play key roles. The ring mass is well known everywhere but in the B Ring (where most of it is). New models suggest how tidal breakup of evolving moons may have formed massive ancient rings, of which the current ring is just a shadow. During its last three years, the Cassini tour profile will allow entirely new observations: direct measurement of the still-unknown ring mass; direct in-situ sampling of ring particle composition (targeting the iron- or carbon based red non-icy component); and radar backscattering observations.