The Structure of the Upper Atmosphere of Saturn

Introduction: Knowledge of the structure and composition of the upper atmosphere of the giant planets is critical to a better understanding of thermospheric heat sources, radiative transport, methane photochemistry, and chemical-dynamical balance in the atmospheres of these planets, as well as being critical for planning future missions that involve deep atmospheric probes or aerobraking. Although the middle and lower stratosphere of Saturn is probed relatively easily at ultraviolet, visible, infrared, and microwave wavelengths, the upper stratosphere and thermosphere are more difficult regions to access from remotesensing methods. Two techniques, ground-based observations of stars being occulted by Saturn and spacecraft ultraviolet observations of stars or the Sun being occulted by the planet, have provided our only constraints on Saturn’s upper atmospheric thermal structure and chemistry. However, deriving temperatures and densities from ground-based stellar occultations [e.g., 1] requires several assumptions about atmospheric properties that are not well known. Furthermore, the analyses of the Voyager 1 and 2 Ultraviolet Spectrometer (UVS) occultations published to date [2,3] differ greatly in their derived density and thermal structures. Therefore, we have combined new photochemical modeling [e.g., 4] with a careful and thorough reanalysis of the UVS occultation dataset [5] to better constrain Saturn’s upper atmospheric thermal structure, composition, and chemistry. We have developed one-dimensional photochemical models for Saturn using the Caltech/JPL KINETICS code [6] 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 hydrocarbons. We then compare synthetic light curves generated from the photochemical model results with the UVS occultation light curves from [5]. To set up the background atmospheric structure for the photochemical models, we solve the hydrostatic equilibrium equation for a rapidly rotating, nonspherical planet with strong zonal winds, as in [1]. The tropospheric helium abundance and temperature profile are assumed to be consistent with that derived by [7]; note that this reanalysis of the Voyager IRIS data [7] indicates that the helium abundance on Saturn is higher than was previously believed. The higher He abundance will cause the stratospheric temperatures derived by [1] to be shifted to higher temperatures. We assume a stratospheric temperature profile such that the equatorial refractivity-radius profile derived from the 28 Sgr stellar occultation [1] is reproduced; the T profile so derived is also in line with recent thermal infrared observations of the ν4 methane band [e.g., 8,9]. In the upper stratosphere and thermosphere, the temperatures are iterated such that the H2 density profile from [5] is reproduced. When solving the hydrostatic equilibrium equation, we also allow the atmospheric mean molecular mass to vary with altitude, using the photochemical/diffusion model to help prescribe this variation.