Saturn's Zonal Winds at Cloud Level

We present an analysis of new cloud motion measurements in Saturn's atmosphere from Voyager 1 and 2 images and define a nominal zonal wind velocity vs latitude profile for the cloud top level that can be used as reference for the upcoming Cassini mission. Improvements over previous studies include: (i) twice the number of wind vectors used, (ii) independent measurements of cloud tracers, (iii) extended latitudinal coverage, (iv) strict error control and evaluation, and (v) a new zonal averaging methodology. Mean zonal wind velocities are given for the latitude range 81° to -71° (planetographic) in steps of 0.5° with a gap between 0° and -10° due to ring shadowing. This research also includes the study of the morphology and motions of cloud systems in the equator and in the southern hemisphere which have not been described previously. We give an ephemeris (motion equation) for several long-lived features that could be observable if they survive until Cassini arrives on Saturn. The long-term behavior of the zonal wind profile is studied by comparing it with a complete set of wind velocities compiled from historical and modern ground-based and HST observations. Significant changes are noted in the equatorial jet between this set and the nominal Voyager profile. We discuss in detail the possible origin for such differences.