The First Cassini UVIS Stellar Occultation of the Saturn Atmosphere

Details The First Cassini UVIS Stellar Occultation of the Saturn Atmosphere The First Cassini UVIS Stellar Occultation of the Saturn Atmosphere

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p23d..04s&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P23D-04

Physics

0355 Thermosphere: Composition And Chemistry, 0358 Thermosphere: Energy Deposition (3369), 5704 Atmospheres (0343, 1060), 5709 Composition (1060), 6275 Saturn

Scientific paper

Cassini UVIS obtained the first of a series of stellar occultations of the Saturnian atmosphere on April 13, 2005. Egress occultation measurements were made with both UVIS FUV and EUV spectrographs, allowing for the analysis of absorption spectra over the range ~900 Å to 1900 Å. The latitude of the occultation varied from 40o S to 38o S on the sunlit side of the southern hemisphere. A rotational-level H2 absorption model is applied to fit the spectra from 1600 km to 795 km above 1bar, producing line-of-sight H2 abundances and temperatures. Analysis of deeper absorption data containing hydrocarbon components is reserved for a later report. A hydrostatic atmosphere model is applied in a forward-modeling process to derive the final H2 density and temperature profiles. The results are not in good agreement with the Voyager UVS occultation results of Smith et al. (1983) or Festou & Atreya (1982). The temperature at the top of the atmosphere is 345 K ± 15 K, with a peak temperature of 366 K at 1206 km. Moderate systematic effects may be present because we obtain no measure of atomic hydrogen mixing in the upper thermosphere. The analysis ends at 795 km, with a temperature of 158 K. We infer the temperature profile to the 1 bar level using the results from the Voyager 2 radio science (Lindal et al., 1985) experiment (RSS) (altitude range 0 to 400 km), finding the resulting projected H2 density distribution in agreement with Voyager RSS at and below 400km. The temperature profile contains a tropopause (82.5 K, 100 km), stratopause (142 K, 335 km) and a mesopause (125 K, 613 km). The complex behavior of the temperature profile between ~1400 km and ~1000 km indicates the presence of a non-solar heating mechanism in the thermosphere. The atmospheric profiles and details of calculation are presented.

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