Saturn upper atmospheric structure from Cassini EUV and FUV occultations

Stellar occultations of the Saturn atmosphere using the Cassini ultraviolet imaging spectrograph (UVIS) experiment have provided vertical structure at a range of latitudes. The transmission spectra in the extreme-far ultraviolet (EUV-FUV) range allow extraction of vertical profiles of H₂ and hydrocarbon abundances from the top of the atmosphere to about 300 km above the 1 bar (1 bar D 100 kPa) pressure level. A reanalysis of the Voyager 2 δSco occultation in 1981 is consistent with the original report. The hydrocarbon homopause is near a pressure of 0.2 μbar in the UVIS analysis, compared to ∼0.01 μbar obtained from the Voyager occultation. Measured hydrocarbon abundances are obtained in the pressure range 600-0.1 μbar in the Cassini UVIS experiment. The combined UVIS results provide evidence for significant latitudinal dependence of vertical temperature profile. The confinement of the hydrocarbons in the current observations compared to published models and the Voyager ultraviolet spectrograph (UVS) results at solar maximum, infer smaller eddy diffusion coefficients in this epoch. Model calculations indicate that the latitudinal dependence of H₂ vertical displacements is caused primarily by the combined effects of gravitational potential and evident differences in electron energy deposition at the top of the atmosphere affecting the temperature profile. The derived H₂ density profiles from ∼40^o latitude and others close to the equator, are found to be nearly identical on a pressure scale below the exobase. The inference is that that the pressure profile of H₂ density at Saturn is unchanged over a broad range of latitudes. (author)