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K. Ohtsuki (U. Colorado), S. Ida (Tokyo Inst. Tech.), L.W. Esposito (U. Colorado)
Voyager data on Saturn's rings have greatly advanced our knowledge of the particle size distribution. Understanding the evolution of the size distribution would constrain the origin and evolution of the rings, and it can also let us estimate abundance of unseen large moonlets, which would be responsible for many observed features in ring systems, such as those observed in Saturn's F ring (Barbara and Esposito, submitted to Icarus). In the Roche zone, where the effects of tidal forces are important, particles which differ greatly in mass can gravitationally accrete more easily (Ohtsuki 1993, Icarus 106, 228; Canup and Esposito 1995, Icarus 113, 331). We study the evolution of particle size distribution due to accretion in the Roche zone using direct N-body simulations. We study two cases: in one case, particles are assumed to accrete whenever they collide, while we use the three-body capture criteria in the Roche zone (Ohtsuki 1993) in the other case. In both cases, initially equal-sized particles first evolve toward a power-law size distribution. In the case of perfect accretion, the growth of particles proceeds as an orderly growth afterwards. However, with the three-body capture criteria, we find development of a bimodal size distribution and slow growth of large bodies, as it becomes difficult for small particles with high random velocities to be accreted by other bodies once large bodies are formed and start perturbing the system. This result suggests the coexistence of unseen large bodies and small particles in the Roche zone (Canup and Esposito 1995).