Temperatures and optical depths of Saturn's rings and a brightness temperature for Titan

Abstract

The Pioneer Saturn infrared radiometer viewed Saturn's rings at 20‐ and 45‐µm wavelength under several conditions of illumination. The data are analyzed to infer radial locations of major ring boundaries, temperatures and temperature gradients, and normal optical depths. Error bounds on the above inferred quantities are given. Most ring boundaries are defined to ±0.01 Rs(1 Rs≡6 × 104 km) and are in good agreement with those inferred from the imaging photopolarimeter experiment. Temperatures generally decrease with radial distance from the planet. A significant temperature gradient exists from the colder north (unilluminated) side of the rings to the warmer south side. The gradient appears to be steepest on the south side. Ring optical depths are greater than some previously published values and are approximately 0.1 for the Cassini division and the C ring. In addition, the C ring optical depth decreases towards the planet. The temperature drop during eclipse is ≳10 K, implying low thermal inertia for the ring particles. Titan's 45‐µm brightness temperature is 75±5 K, in good agreement with earth‐based observations.