Cassini CIRS Observations of a Roll-Off in Saturn Ring Spectra at Submillimeter Wavelengths

Abstract

The Cassini Composite Infrared Spectrometer (CIRS) spatially resolved Saturn's main rings in the far-infrared, measuring the spectrum from 20 to 400 wavenumbers (cm^‑1) (tens of microns to submillimeter wavelengths). We find a spectral roll-off below 50 cm^‑1 (200 μm) for each of the A, B and C rings. From these data we derive temperatures and emissivities for each ring. Mie calculations of individual water ice particles show a natural variation in the optical properties of the rings similar to the roll-off we observe in our data. A simple radiative transfer model placing a distribution of water ice particles randomly in a layer provides a good fit to the data and illustrates one possible interpretation of the results. This is most likely only part of the explanation for the roll-off effect as the impact of shape, surface, and composition variations have been left for future analysis.