**History of Neptune's Ring Arcs**

**L.W. Esposito, J.E. Colwell, R.M. Canup (LASP)**

The recent dynamical calculations for Neptune's Adams ring arcs by Foryta and Sicardy (1996) and Hanninen and Porco (1997) determine the basic evolutionary parameters for this system. The ring evolution is dominated by stochastic events, particularly chaotic motion that causes a migration between the corotation sites (FS96) and collisions near quadrature (HP97). A basic problem is that the high velocity collisions that produce the dusty arcs at the Galatea corotation resonances rapidly depopulate these sites (Colwell and Esposito 1990). With the new results in hand for the evolution of the ring particles over periods of less than a century, we can now calculate the long-term stochastic evolution of the Adams ring.

Using a finite Markov chain as a model for this stochastic process, we
follow the suggestion by FS96 that corotation sites provide preferential
locations for accretion. A more general conclusion is that the
longitudinal concentration of material in a few nearby sites (and that
the majority of the Adams ring material is residing there) requires
either an exceedingly recent event (EC92) or that the corotation sites be
absorbing states of the Markov chain.In the latter case, the competing
processes of chaotic diffusion and frustrated accretion can provide the
arc and clump features as recurrent transient events near the Roche
limit. Similar phenomena would be expected for Saturn's F and G rings.