The Vertical Structure and Phosphine Distribution on Saturn as Determined by Cassini/VIMS

We present the preliminary results of our study on the variations of (1) the vertical aerosol layer structure and (2) the phosphine (PH3) distribution on Saturn, utilizing Cassini/VIMS spectral imagery. From the Cassini/VIMS infrared channel spectral data of Saturn acquired in 2004, we sample a dozen pixels at each of the following four latitudes (-10, -27, -47, and -73 degree) to create a set of multi-spectral center-limb profiles for our radiative transfer analysis. We first fit the NIR methane band spectra to determine the altitude of the upper tropospheric haze at the four latitudes, using the newest CH4 near-infrared correlated-k coefficients (Irwin et al. 2006, Icarus, 181, 309-319) and a multi-layer model. This latitudinal variation in the haze altitude is an important clue to unveil Saturn's global climate system. On that basis, we fit the phosphine band spectrum near 3 micron to constrain the phosphine column abundance, using recently published PH3 absorption data sets (Temma et al. 2006, JGR, accepted). Since phosphine is a disequilibrium species in Saturn's upper atmosphere, its increase suggests enhanced convective activity from the deep interior. We thus map the latitudinal phosphine distribution on Saturn, including the analysis of the south polar region where an anomalously strong phosphine absorption was reported (Baines et al. 2005, Earth, Moon, and Planets, 96, 119--147). This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The first author of this presentation is supported by the NASA Postdoctoral Fellowship at the Jet Propulsion Laboratory.