Probing Saturn's tropospheric cloud with Cassini/VIMS

TL;DR

This study uses Cassini VIMS data to analyze Saturn's tropospheric cloud structure, revealing cloud base at ~2.3 bar, NH3 abundance variations, and a photochemically derived haze with seasonal differences.

Contribution

It provides a detailed deep cloud model for Saturn's atmosphere based on spectral data, incorporating new insights into cloud composition and seasonal haze variations.

Findings

Cloud base at approximately 2.3 bar

NH3 abundance increases at the equator

Haze optical depth decreases in the winter hemisphere

Abstract

In its decade of operation the Cassini mission has allowed us to look deep into Saturn's atmosphere and investigate the processes occurring below its enshrouding haze. We use Visual and Infrared Mapping Spectrometer (VIMS) 4.6-5.2 micron data from early in the mission to investigate the location and properties of Saturn's cloud structure between 0.6 and 5 bars. We average nightside spectra from 2006 over latitude circles and model the spectral limb darkening using the NEMESIS radiative transfer and retrieval tool. We present our best-fit deep cloud model for latitudes between -40 and 50 degrees, along with retrieved abundances for NH3, PH3 and AsH3. We find an increase in NH3 abundance at the equator, a cloud base at ~2.3 bar and no evidence for cloud particles with strong absorption features in the 4.6-5.2 micron wavelength range, all of which are consistent with previous work. Non-scattering cloud models assuming a composition of either NH3 or NH4SH, with a scattering haze overlying, fit limb darkening curves and spectra at all latitudes well; the retrieved optical depth for the tropospheric haze is decreased in the northern (winter) hemisphere, implying that the haze has a photochemical origin. Our ability to test this hypothesis by examining spectra at different seasons is restricted by the varying geometry of VIMS observations over the life of the mission, and the appearance of the Saturn storm towards the end of 2010.