Saturn upper atmosphere structure D. Shemansky & X. Liu SET/PSSD 06/26/11

Occultation results The fuv spectrum exclusively provides hydrocarbon vertical structure. The euv spectrum exclusively obtains the properties of H2 and HI. At the altitude of the hydrocarbon homopause the H2 opacity removes measureable flux from the stellar or solar source.

Hydrocarbons and H2O Vertical distribution

Mixing ratios on a pressure scale

Summary H2 vertical structure at low latitude in 2009 shows top of atmosphere temperature of 450 K. H2 X is non LTE with evidence of non LTE in rotation. Proximal S/C tumble density altitude has been provided to project. On basis of -3.6o latitude observation. There is disagreement with CIRS hydrocarbon results partly stemming from their dependence on the Voyager CH4 UVS profile. Moses theory also uses the Voyager profile. The UVIS result shows a much more vertically confined CH4 distribution indicating probable seasonal change in vertical dynamics. The UVIS upper limit on H2O falls below Moore etal 2006, and at the level of the Moses calculachtion which is based on the Fru chtgruber etal 1997 observation.

H2O issues at Saturn The Moore etal calculations assume precipitating H2O. This results in a constant vertical mixing ratio. Their nominal rate is ~4.5 X 1027 s-1. The UVIS result limits to ~1026 s-1, within the range of The Fruchtgruber etal observation. Enceladus appears to be an unlikely source at a level of ~1026 s-1. Moses etal argue for meteoritic dust.

Ionospheric Reaction Chemistry Electron Sink: Development of Activated H2 X (v): Photoelectron excitation, energetic electron excitation, solar fluorescence, electron recombination with H3+, three-body recombination, proton charge-exchange with H2 X

Summary H3+ dominant ionospheres predicted for both forcing conditions Energy deposition from Stevens et al. (1993) predicted for heterogeneous electron forcing, solar forcing falls short by 3 orders of magnitude Observed H2 EUV band emissions predicted with heterogeneous electron forcing Requirement: H* multi-scattering model to predict excitation of H2 X (v) Requirement: H3+ fine-structure to predict Trafton et al. (1999) emissions