Structure and dynamics of Saturn's atmosphere

Abstract

The atmosphere of Saturn exhibits dynamical structures (jets, bands, spots, eddies) with horizontal scales ranging from one scale height (60 km) up to the planetary radius (60,000 km). Although the kinds of structures are the same as those on Jupiter, there are quantitative differences. First, Saturn's equatorial jet speed is four times greater and the jet width is two times wider than on Jupiter. Eastward jets at higher latitudes are also speedier and wider, but westward jets are slower compared with Jupiter. There are fewer spots in all size ranges, and no spot comparable in size to Jupiter's Great Red Spot. The rms eddy wind speed is lower, as is the conversion of eddy kinetic energy to zonal mean kinetic energy (the measured rate of conversion is positive but not significantly so). Saturn's convective regions, where eddy lifetimes are a few days or less, are more isolated and cover less area than on Jupiter. The basic causes of these differences have not been quantitatively identified. Basic differences between the two planets include Saturn's lower heat source, lower gravity, and consequently greater depth of atmosphere and deeper nonconducting fluid layers. Observed temperatures imply that the zonal winds decrease from cloud top upwards, but change more gradually below the cloud tops. One possibility is that zonal winds are constant on concentric cylinders that extend throughout the planet. However the observations, which are limited to cloud-top altitudes and above, do not preclude other configurations. It is remotely possible that Saturn's internal rate of rotation is different from that of its radio emissions. Dynamical models differ in their assumption about the interior and its dynamical interaction with the atmosphere. The time-dependent behavior of the large-scale structures offers a promising way of testing the models and choosing among the various assumptions.

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