Description: Publication date: 15 July 2018 Source: Icarus, Volume 309 Author(s): Jeffrey N. Cuzzi, Richard G. French, Amanda R. Hendrix, Daniel M. Olson, Ted Roush, Sanaz Vahidinia We have observed the main rings of Saturn with the Space Telescope Imaging Spectrometer (STIS) on the Hubble Space Telescope (HST), covering the spectral region from 180–570 nm  (including for the first time the critical near-UV range 190–340 nm) with very good signal to noise ratio and a radial resolution of approximately 160–330 km. After correcting for an unexpected grating scatter problem associated with the bright, red, extended planet-ring target, we obtained complete I / F spectra for each major ring region. We have interpreted the spectra in terms of the ring particle material composition using a combination of traditional “Hapke” theory and a new correction for shadowing on the rough, re-entrant ring particle surfaces, along with a correction for the nonclassical scattering of the ring layer itself. We tested a variety of UV absorbers: iron (including nano-iron) grains, hematite, “planetary silicates”, organic carbon-ring tholins of varying aromaticity, and amorphous carbon. The A and B rings can contain no more NH 3 than about 10 − 4 by volume. We conclude that the best spectral fit for the well-known, unusually red color of the A and B rings is provided by a sub-percent mass fraction of organic tholins. It appears that the most likely regolith configuration for the A and B Rings is a heterogeneous “intimate mixture”, dominated by relatively pure water ice, with some 2–40% of the grains containing roughly 5–10% tholin by volume (the amount depending on whether silicates are present), but it is hard to allow much amorphous carbon to be present in the B Ring material at least. These predictions of compositional heterogeneity can be tested by Cassini direct compositional measurements. There is some suggestion that the tholin properties differ slightly between the A and B rings. We show that tholins of this type, in the abundance we predict, would be difficult to detect at near-IR wavelengths. The C Ring particles have lower albedos, and the best fit models require a significantly higher abundance of silicates and (more importantly) “neutral” absorber which we model as amorphous carbon, plausibly representing meteoritic infall. Because of our new treatment of shadowing, our estimates of the abundance of amorphous carbon in the C Ring particles are lower (1–5% in the particle regoliths) than previously obtained. The relative abundance of silicate and carbonaceous materials in the C Ring remains uncertain due to uncertainties in how to model the C ring particle phase function.