Accretion in the Roche Zone: Coexistence of Rings and Ringmoons

Abstract

Traditional accretion simulations predict rapid accumulation of ring debris into single satellites, while most theories of ring formation dismiss any accretion within the classical Roche limit. The former contradicts the continued presence of planetary rings, while the latter fails to adequately account for the many small satellites observed within ring systems. The coexistence of rings and small satellites thus challenges the premise of a strict boundary between accreting and nonaccreting regions. We have developed an accretion model designed to better examine accumulation processes in the dynamically transitional regime of outer planetary rings. We utilize "three-body" capture criteria, motivated by the work of Ohtsuki (1993 Icarus 106, 228-246), to account for the effects of strong tidal forces on accretion. Our findings indicate that tidally modified accretion occurs in a relatively broad range of orbital radii surrounding the classical Roche limit. Tidally modified accretion has a very unique character: for a given particle density, only bodies which differ greatly in mass can remain gravitationally bound, as like-sized bodies overflow their mutual Hill sphere. We find that this constraint greatly limits the degree of accretional growth and prevents runaway accretion near the Roche limit. Numerical simulations show that through the course of tidally modified accretion, a fragmentation-produced debris distribution evolves into a bimodal population, with one element consisting of a swarm of small, high-velocity bodies and the other composed of a small number of large "moonlets" on fairly circular orbits. The latter are precluded from accreting with one another due to the tidal influences of the planet. Tidally modified accretion thus offers a natural explanation for the formation of systems of coexisting rings and ringmoons from disrupted parent bodies.