Cassini CIRS Measurements of Thermal Phase Curves in Saturn's Main Rings

After more than three years in orbit around Saturn, the Cassini Composite Infrared Spectrometer (CIRS) has acquired a broad set of thermal measurements of Saturn's main rings (A, B, C and Cassini Division) for a number of different viewing geometries that include solar phase angle, spacecraft elevation, solar elevation and local hour angle. The thermal characteristics of each main ring vary with changing viewing geometry. Ring temperatures decrease with increasing solar phase angle on both the lit and unlit sides of the rings. To first order, the largest temperature changes on the lit face of the rings are driven by variations in phase angle while differences in temperature with changing spacecraft elevation are a secondary effect. We find that the characteristic shapes of the phase curves vary with changing solar elevation angle, We present a preliminary report on the thermal phase curves obtained thus far. For a solar elevation of 14 degrees the outer B ring exhibits a thermal surge in temperature greater than 10 K at low phase angles. For a solar elevation of 21 degrees the thermal profile with phase angle is more nearly linear. Our first attempt at modeling the thermal phase curves is presented, using parameterized shadowing functions as described in the standard Hapke model. We discuss the significance of the retrieved parameters as potential input for forward- modeling of Saturn's ring thermal properties. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA and at CEA Saclay supported by the "Programme National de Planetologie".