The physics of unconstrained edges in planetary rings.

摘要

The edges of planetary rings can provide important constraints on the overall behavior of the ring, especially by setting the timescale for the ring to spread. Previous studies of ring edges have mainly focused on edges constrained by rnoons, leaving the question of unconstrained edges unresolved. This thesis focuses on such ring edges where no moons provide constraints.; Chapter 3 presents the results of simulations of narrow planet rings with low velocity dispersions. Such rings are unstable and quickly collapse into a collisionally dense compact ring. In this state, the ring particles are phase-aligned so that they are all near periapse or apoapse. The compact ring is subject to a new instability that causes sections to break off radially away from the ring. This behavior can be understood in terms of the particle's epicyclic motion. Angular momentum is efficiently transported across the narrow ring during the collapse and instability. This process occurs on small scales inside broad planetary rings in the wake-like self-gravitating clumps seen by many modelers.; To model the behavior of planetary ring edges, I have developed a simulation code which incorporates the extra gravitational acceleration on the simulation particles due to the rest of the ring and generates a boundary which mimics the ring interior. I present the findings from investigations based on this code in Chapter 4. Rings spread almost exclusively in a boundary layer a few tens of meters wide at their edges. Interior to this boundary layer, the rings behave as if there were no edge nearby. This is contrary to previous models of ring spreading and edge behaviors.; Chapter 5 examines how the behavior of the ring edges with varying properties of the ring. As the angular momentum transport rate into the boundary layers increases, the edges respond by widening the boundary layer rather than spreading faster, suggesting that the spreading rate is limited. This limit is affected by how strongly gravitational clumps form near the edge. When the particles are of different sizes, smaller particles move radially outward significantly more rapidly than the large particles causing a differentiation by size which should be observable.