Short-term changes in the belt/zone structure of Saturn's Southern Hemisphere (1996–2004)

Aims.The main goal of this paper is to study the reflectivity changes that affect Saturn's global belt/zone pattern on time-scales from weeks to a few months, and to determine their frequency and intensity, as well as the variations of the vertical cloud structure that cause such changes. Methods.We have used 177 images from Hubble Space Telescope observations covering the 336–814 nm spectral range taken between 1996 and 2001, and the 255–890 nm range for observations from 2002 to 2004. These HST images were photometrically calibrated and geometrically corrected in order to quantify real reflectivity changes. We also used radiative transfer codes to retrieve vertical cloud structure models able to account for the observed changes, fitting the variation of absolute reflectivity from the center of the disk to the limb at the selected latitude at all wavelengths simultaneously. Results.The results show that wavelength-dependent reflectivity changes are not in general related to localized disturbances (larger than the HST resolution of about 260 km/pixel), affecting instead an entire latitude band simultaneously. Those changes are most frequently found at equatorial and polar latitudes, brightening or darkening by 5% to 10% a latitude band typically less than $3^{\circ}$ wide. We find that reflectivity changes can be explained as a variation of the single scattering albedo of the tropospheric particles. Conclusions.Our results confirm the kind of rapid changes observed during the Voyager mission in the belt/zone pattern of Saturn. They occur in narrow latitude bands due to a change in the cloud particle properties. This provides a framework for Cassini spacecraft observations, which will give the temporal and spatial resolution needed to understand Saturn cloud variability.