Temporal Albedo Variations in Saturn's Atmosphere.

Temporal variations in the reflectivity of Saturn's atmosphere were investigated through the analysis of photographic images of Saturn. Pairs of plates obtained eight years apart but with similar illumination and viewing geometries were digitized with a microphotometer, photometrically calibrated, and the results correlated with seasonal heating calculations. It was found that the south temperate regions show a brightening in blue light during the spring while the South Equatorial Belt remains more constant in reflectivity. Voyager images which have been analyzed show that the southern hemisphere, now entering its autumn, is considerably brighter than the northern hemisphere. From our modelling of the solar insolation we have found that, because of the eccentricity of Saturn's orbit, the southern hemisphere undergoes rapid heating just after its vernal equinox while the northern hemisphere is subjected to much more constant illumination during its warm season. We interpret the brightening of the south temperate regions as a seasonal process involving a reduction in the amount of ultraviolet absorbing aerosol present. The constancy of the deeper clouds of the South Equatorial Belt is consistent with a longer thermal time constant and lower amplitude of thermal response. The Equatorial Zone has shown some rapid variations in its reflectivity possibly due to dynamical effects while the very high aerosols of the polar regions may be influenced by magnetosopheric interactions.