Implications for Particle Rotation and Vertical Mixing from Cassini CIRS Thermal Measurements of Saturn's Main Rings

The Cassini Composite Infrared Spectrometer (CIRS) obtained spatially resolved thermal infrared scans of Saturn's main rings (A, B and C, and Cassini Division) for both the lit and unlit faces of the rings over a variety of phase angles. The scans show ring temperatures decreasing with increasing solar phase angle. The largest ring temperatures are at a phase angle of zero degrees. These temperature differences with phase suggest that Saturn's main rings include a population of ring particles that spin slowly, possibly with a spin period greater than a few hours, given their low thermal inertia. For the B and A rings, the temperature is correlated with optical depth when viewed from the lit face, and anti- correlated when viewed from the unlit face. On the unlit face of the B ring, not only do the lowest temperatures correlate with the largest optical depths, but these temperatures are the same at both low and high phase angles, suggesting that little sunlight is penetrating these regions. The temperature differential from the lit to the unlit side of the rings is a strong, nearly linear, function of optical depth. This is consistent with the expectation that little sunlight penetrates to the dark side of the densest rings, but also suggests that little vertical mixing of ring particles is taking place in the A and B rings. 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".