COMPOSITIONAL EVOLUTION OF SATURNS RINGS DUE TO METEOROID BOMBARDMENT

In this paper we address the question of compositional evolution in pl anetary ring systems subsequent to meteoroid bombardment. The huge sur face area to mass ratio of planetary rings ensures that this is an imp ortant process, even with current uncertainties on the meteoroid Aux. We develop a new model which includes both direct deposition of extrin sic meteoritic ''pollutants'' and ballistic transport of the increasin gly polluted ring material as impact ejecta. Our study includes detail ed radiative transfer modeling of ring particle spectral reflectivitie s 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-composit ional groupings have never been explained; we present and explore the hypothesis that global scale color and compositional differences in th e main rings of Saturn arise naturally from extrinsic meteoroid bombar dment of a ring system which was initially composed primarily, but not entirely, of water ice. We find that the regional color and albedo di fferences can be understood if all ring material was initially water i ce colored by tiny amounts of intrinsic reddish, plausibly organic, ab sorber, which then evolved entirely by addition and redistribution of extrinsic, nearly neutrally colored, plausibly carbonaceous material. The regional compositional differences result from different susceptib ilities to pollution of regions with very different surface mass densi ty. We further demonstrate that the detailed radial profile of color a cross the abrupt B ring-C ring boundary can constrain key unknown para meters 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 concl usion is easily extended by inference to the Cassini division and its surroundings as well. This geologically young ''exposure age'' is comp atible 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. Howev er, 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 a t this time. (C) 1998 Academic Press.