Thermal transport in Saturn's B Ring inferred from Cassini CIRS observations
Monday, 15 December 2014
We examine the heat budget of Saturn's B ring using all of the quality data from Cassini's Composite Infra Red Spectrometer (CIRS) from one Saturn season, together with a detailed numerical calculation for the incident flux. At all times and locations, 30\% to 40\% of the energy incident on the sunlit side appears to be transported through the ring and emitted on the unlit side nearly immediately. The specific fraction of heat that is throughput varies inversely with the normal optical thickness as f∼0.41−0.024τ.
The total emitted flux from the B ring varies linearly with sinB′ as expected, and appears to be dominated by isotropic emission both on the lit and unlit sides. There is no indication of non-isotropic emission from the unlit side, while on the lit side there is an additional emission from a low-phase hot spot with a width of 50∘, which accounts for approximately 2-10\% of the total emission. Accounting for wake orientation affects only the inner B Ring, decreasing the absorbed incident radiation by 5-10\% there.
Using a simple model for thermal transport, we derive high effective conductivity of the ring of 0.5Wm−1K−1, together with a heat flux through the midplane of the rings that is about 1.5Wm−2 at high solar elevations. This conductivity is at the high end of plausible values derived using only radiative and conductive heat flux, but can easily be explained if particle diffusion carries heat across the midplane of the rings. We briefly examine the manner in which radiative conduction enhances the conductivity of a static medium, and present a study of the effective emissivity of the B ring that might help to resolve whether mechanical diffusion is necessary to explain the data.