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.