  The origin and evolution of Saturn's rings is critical to understanding the
Saturnian system as a whole. Here, we discuss the physical and chemical
composition of the rings, as a foundation for evolutionary models described in
subsequent chapters. We review the physical characteristics of the main rings,
and summarize current constraints on their chemical composition. Radial trends
are observed in temperature and to a limited extent in particle size
distribution, with the C ring exhibiting higher temperatures and a larger
population of small particles. The C ring also shows evidence for the greatest
abundance of silicate material, perhaps indicative of formation from a rocky
body. The C ring and Cassini Division have lower optical depths than the A and
B rings, which contributes to the higher abundance of the exogenous neutral
absorber in these regions. Overall, the main ring composition is strongly
dominated by water ice, with minor silicate, UV absorber, and neutral absorber
components. Sampling of the innermost D ring during Cassini's Grand Finale
provides a new set of in situ constraints on the ring composition, and we
explore ongoing work to understand the linkages between the main rings and the
D ring. The D ring material is organic- and silicate-rich and water-poor
relative to the main rings, with a large population of small grains. This
composition may be explained in part by volatile losses in the D ring, and
current constraints suggest some degree of fractionation rather than sampling
of the bulk D ring material.
