Clouds and aerosols in saturn's atmosphere

Abstract

In this chapter we review the photochemical and thermochemical equilibrium theories for the formation of condensate clouds and photochemical haze in Saturn's upper troposphere and stratosphere and show the relevant observations from ground-based and spacecraft instruments. Based on thermochemical equilibrium models we expect ammonia ice crystals to dominate in the high troposphere. There is very little spectral evidence to confirm this idea. Thanks to a stellar occultation observed by the Cassini VIMS instrument we now have spectral evidence for a hydrocarbon stratospheric haze component, and we still seek evidence for an expected diphosphine stratospheric haze component. The vertical distributions of stratospheric and upper tropospheric hazes have been mapped well with ground-based and Hubble Space telescope data, and Cassini data are beginning to add to this picture. Polar stratospheric aerosols are dark at UV wavelengths and exhibit strong Rayleigh-like polarization which suggests that auroral processes are important for their formation as is the case for the jovian polar stratospheric haze. The cloud and haze structure exhibits a variety of temporal variation, including seasonal change, long-term secular change near the equator, and short-term changes with a complicated latitudinal structure, and still not understood. Cassini instruments, especially the VIMS instrument, show an abundance of small-scale structure (convective clouds) at a pressure near 2 bar. © 2009 Springer Science+Business Media B.V.