Searching for Seasonal Changes in Saturn's A Ring

An intriguing result from Cassini's first observations of Saturn's A ring from Saturn Orbit Insertion (SOI), was the lack of a temperature contrast between the ring's illuminated and unilluminated sides [1]. Determined from observations taken by Cassini's Composite Infrared Spectrometer (CIRS), this lack of contrast was unexpected, as direct solar heating is the largest factor in maintaining the rings' heat budget. Observations taken with the Infrared Radiometer and Spectrometer (IRIS) onboard Voyager 1, however, yield a temperature difference of ~ 5 - 10 K . CIRS observations would easily have revealed such a difference. Additionally, Voyager 1 temperatures are some 15 - 25 K colder than those measured at SOI. One explanation for this is seasonal changes in solar insolation. In 1980 Voyager 1 flew past Saturn after northern vernal equinox, when the solar inclination angle was just 4° . At SOI the Sun was 24° above the ring plane. Since then the Sun has dropped ~ 3° lower. Temperature contrasts have appeared in data taken since April 2005. However, whereas unlit ring temperatures are lower, lit side ring temperatures are now higher, suggesting that solar inclination angle is not the only factor involved. Indeed, observations taken since SOI have clearly shown that the observed temperature profile of the rings is a strong function of observing geometry. The temperature profile changes with phase angle, emission angle and local hour angle. Yet, the large temperature differences between SOI and the Voyager flybys seem most likely due to the change in Saturnian season. We will interpret CIRS A ring observations in the context of Voyager IRIS observations. And, we will attempt to determine to what extent the observed ring temperatures have been driven by the changing solar inclination angle, using ring thermal models and recent Cassini observations to sort out the effects of observation geometry. [1] F.M. Flasar, et al. 2005. ``Science'', vol. 307, pgs. 1247-1251.