Cloud Features and Zonal Wind Measurements of Saturn's Atmosphere as Observed by Cassini/VIMS

TL;DR

This study analyzes Saturn's atmospheric features and zonal wind profiles using Cassini VIMS data, revealing consistent deep jet speeds over time and complex vortex structures in the planet's atmosphere.

Contribution

It provides new zonal wind measurements from VIMS data, showing minimal vertical shear and deep jet speeds similar to Voyager era, and suggests dark features are vortices observed at different pressure levels.

Findings

Deep jet speeds exceed 450 m/s, consistent with Voyager data.

Most dark features are smaller than 1000 km and are likely vortices.

Vertical shear in zonal winds is minimal, with complex vortex structures present.

Abstract

We present an analysis of data about Saturn's atmosphere from Cassini's Visual and Infrared Mapping Spectrometer (VIMS), focusing on the meteorology of the features seen in the 5-micron spectral window. We present VIMS mosaics and discuss the morphology and general characteristics of the features backlit by Saturn's thermal emission. We have also constructed a zonal wind profile from VIMS feature tracking observation sequences using an automated cloud feature tracker. Comparison with previously constructed profiles from Voyager and Cassini imaging data reveals broad similarities, suggesting minimal vertical shear of the zonal wind. However, areas of apparent wind shear are present in the VIMS zonal wind profile at jet stream cores. In particular, our analysis shows that the equatorial jet reaches speeds exceeding 450 m/s, similar to speeds obtained during the Voyager era. This suggests that recent inferences of relatively slower jet speeds of ~275-375 m/s are confined to the upper troposphere and that the deep (>1 bar) jet has not experienced a significant slowdown. Our measurements of the numerous dark, spotted features seen in the VIMS mosaics reveals that most of these features have diameters less than 1000 km and reside in confined zonal bands between jet stream cores. We propose that these spot features are vortices and that VIMS and ISS are sensing the same vortices at two different pressure levels. The local structure at the zonal jet streams remains complex, as VIMS may be sensing cloud features that are deeper than the NH3 cloud deck.