The Phase Dependence of Temperatures Measured in Saturn's Main Rings Indicates Slowly Rotating Ring Particles

Temperatures mapped with Cassini's Composite Infrared Spectrometer (CIRS) throughout Saturn's main rings (A, B and C) and Cassini Division show that ring temperatures decrease with increasing phase angle on both the lit and unlit sides of the rings. These changes indicate that the ring particles spin relatively slowly, i.e. less than one to a few rotations per orbit, compared to their orbital periods of 6 to 14 hours. The ring particles that dominate our measurements also have low thermal inertia and are larger than a few centimeters, since particles that rotate quickly, or have small size (less than about one centimeter), or have high thermal inertia would possess essentially the same temperature at all phase angles. The thermal characteristics of each main ring vary noticeably with phase angle. The C ring and Cassini Division exhibit the largest change in temperature. The temperatures of the lit C ring and Cassini Division decreases by about 15 K between relatively low (< 60 deg.) and high (> 130 deg.) phase angles; a similar contrast is present for the unlit sides of the C ring and Cassini Division. The lit B ring shows a temperature contrast with phase angle of approximately 7 K while the unlit B ring shows very little thermal contrast because little sunlight penetrates the thick B ring to its unlit side, and vertical transport of ring particles from one side of the rings to the other may be impeded by high B ring optical depths. The lit A ring is particularly interesting because the magnitude of the thermal contrast decreases with increasing radial distance from Saturn. The outer A ring shows only a small temperature change with phase angle, possibly because it contains a larger number of smaller, and/or more rapidly rotating ring particles which have more uniform temperatures with phase angle. Modeling of these and future Cassini observations at additional phase angles may provide further constraints on particle rotation rates, spin direction and spin axis within each ring. 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 Planetology".