Collisional Features in Saturn's F Ring.

Abstract

The role of physical collisions in shaping Saturn's F ring is explored using a mixture of dynamical theory, image analysis and computer simulations. The F ring is highly dynamic, being perturbed by the nearby moons, Prometheus and Pandora, and by a population of small bodies, whose presence is inferred by their influence on the ring, charged particle data and, occasionally, direct detection. Small-scale features, termed `mini-jets', are catalogued from images taken by the Imaging Science Subsystem of the Cassini spacecraft. More than 1000 are recorded, implying a population of 100 objects on nearby orbits, colliding with the ring at velocities of a few msð€€€1. Many are seen to collide several times, forming repeated structures, and must have enough physical strength, or self-gravity, to survive multiple passages through the core. Larger features, called `jets', share a similar morphology. They are likely caused by a more distant population which collide at higher velocities ( 10 msð€€€1) and are roughly an order of magnitude less common. Differential orbital motion causes jets to shear out over time, giving the ring its multi-stranded appearance. Jets have different orbital properties to mini-jets, probably because they result from multiple, overlapping collisions. Simulations using an N-body code show that the shape of collisional features depends heavily on the coefficient of restitution, particularly the tangential component. When both components are < 1 large objects merely sweep up small particles. Features like jets and mini-jets require large particles in both the target and impactor, as is the case for two similarly-sized aggregates colliding. A single population of aggregates is proposed, ranging from large, unconsolidated clumps, embedded in the core, through mini-jet-forming objects to the more distant, jet-forming colliders. Prometheus may be ultimately responsible for all of these features as its gravity can trigger clump formation as well as perturb particles.