Incomplete cooling down of Saturn’s A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles
Abstract
At the solar equinox in August 2009, the Composite Infrared Spectrometer (CIRS) onboard Cassini showed the lowest temperatures of Saturn’s rings ever observed. The equinox temperature of Saturn’s A ring is found to be much higher than model predictions regardless of ring structure assumed as long as only the flux from Saturn is taken into account. This temperature anomaly is likely to indicate incomplete cooling down of ring particles at the equinox, and this fact allows us to estimate the ring particle size and the seasonal thermal inertia. We first assume that the internal density and the thermal inertia of a ring particle are uniform in depth. In the model, we use the analytic expression of Froidevaux (1981) for the seasonal solar flux variation and the time dependence is taken into account by solving the thermal diffusion equation. Model fits show that the particle size is about 1 m. The seasonal thermal inertia is found to be as high as 30-40 in MKS units in the middle A ring whereas it is as low as 10 in MKS units or as the diurnal thermal inertia in the inner A ring and outermost A ring. An additional model, in which a particle has a high-density core covered by a fluffy regolith layer, showed that the size of core radius is about 90 % of the particle radius for the middle A ring and is much less in the outer and inner A ring. This indicates that the radial gradient of the internal mean density of particles exits across the A ring. We discuss possible origins of the density gradient.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #46
- Pub Date:
- November 2014
- Bibcode:
- 2014DPS....4641702M