ORIGINS OF THE RINGS OF URANUS AND NEPTUNE .2. INITIAL CONDITIONS AND RING MOON POPULATIONS

The smallest moons of the Jovian planets are unlikely to survive intact the flux of cometary impactors in the outer solar system for billions of years. In paper 1 (Colwell and Esposito, 1992) we showed that the small moons of Uranus and Neptune are fragments or rubble pile agglomerations left over from some older, larger population of satellites. Catastrophic fragmentation occurs in approximately 10(8) years for these ring moons. The fate of the debris following a fragmenting impact is central to understanding the evolution of these satellites and the hypothesized origin of rings from their debris. In this extension of our earlier work we examine the possible effects of the velocity distribution of fragments following a catastrophic fragmentation on satellite diminution via a collisional cascade. We compare our results with those presented in paper 1. Fragment velocities are critical in the evolution of the collisional cascade because of the possibility of reaccretion following disruption. Using our simulations of the collisional cascade including the effects of the fragment velocity distribution we estimate an unseen population of moons in the 1 to 10 km size range of approximately 1000 at Uranus and at Neptune. Using our model fragment velocity distribution we calculate the initial phase space distribution of the new ring particles. This provides a physically realistic initial condition for simulations of the collisional evolution of planetary rings. We find that a narrow ring with a characteristic width of approximately 50 km is a natural outcome of the catastrophic disruption of satellites.