Compositional evolution of Saturn's rings due to meteoroid bombardment

In this paper we address the question of compositional evolution in planetary ring systems subsequent to meteoroid bombardment. The huge surface area to mass ratio of planetary rings ensures that this is an important process, even with current uncertainties on the meteoroid Aux. We develop a new model which includes both direct deposition of extrinsic meteoritic "pollutants" and ballistic transport of the increasingly polluted ring material as impact ejecta. Our study includes detailed radiative transfer modeling of ring particle spectral reflectivities based on refractive indices of realistic constituents. Voyager data have shown that the lower optical depth regions in Saturn's rings (the C ring and Cassini division) have darker and less red particles than the optically thicker A and B rings. These coupled structural-compositional groupings have never been explained; we present and explore the hypothesis that global scale color and compositional differences in the main rings of Saturn arise naturally from extrinsic meteoroid bombardment of a ring system which was initially composed primarily, but not entirely, of water ice. We find that the regional color and albedo differences can be understood if all ring material was initially water ice colored by tiny amounts of intrinsic reddish, plausibly organic, absorber, which then evolved entirely by addition and redistribution of extrinsic, nearly neutrally colored, plausibly carbonaceous material. The regional compositional differences result from different susceptibilities to pollution of regions with very different surface mass density. We further demonstrate that the detailed radial profile of color across the abrupt B ring-C ring boundary can constrain key unknown parameters in the model. We carefully reanalyze and revise meteoroid flux estimates by Cuzzi and Durisen (1990, Icarus 84, 467-501) and estimate the duration of the exposure to extrinsic meteoroid flux of this part of the rings, at least, to be on the order of 10(8) years. This conclusion is easily extended by inference to the Cassini division and its surroundings as well. This geologically young "exposure age" is compatible with time scales estimated elsewhere based on the evolution of ring structure due to ballistic transport and also with other "short time scales" estimated on the grounds of gravitational torques. However, uncertainty in the flux of interplanetary debris and in the ejecta yield may preclude ruling out a ring age as old as the Solar System at this time. (C) 1998 Academic Press.