Saturn's Upper Atmospheric Structure: Forward Modeling and Comparison with a Reanalysis of the Voyager UVS Occultation Data

Knowledge of the thermal structure and composition of the upper atmospheres of the giant planets is critical to a better understanding of chemical-radiative-dynamical balance in these planets. Using a combination of forward modeling and data inversion, we have reanalyzed the Voyager UVS occultation data to better constrain the structure and chemistry of Saturn's thermosphere and mesosphere. For the forward modeling part of the investigation, we use the Caltech/JPL KINETICS code (Allen et al. 1981, JGR 86, 3617-3627) to develop 1-D photochemical models that are consistent, to the highest extent possible, with all available Earth-based and spacecraft observations concerning the temperature structure and altitude variation of H2, H, He, and hydrocarbon densities. Synthetic light curves and spectra generated from the photochemical models are compared with the Vervack (2006) reanalysis of the Voyager UVS occultation data. We make several conclusions from our model-data comparisons in combination with other recent datasets: (1) the location of the base of the thermosphere (mesopause) cannot be constrained from the Voyager UVS occultation data, (2) Saturn's exospheric temperature at the time of the Voyager encounters approaches 400-500 K, (3) Saturn's upper stratosphere/mesosphere in the spring/summer/fall is likely warmer than pre-Galileo-era analyses have indicated, (4) some differences in atmospheric structure are apparent among the different occultations, suggesting that the methane homopause level (and related thermal structure) varies with time and/or location on the planet. Implications with regard to chemistry and transport will be discussed.

We gratefully acknowledge support from the NASA Outer Planet Research Program.