The far ultraviolet reflection spectrum of Uranus: Results from the Voyager encounter

Roger V. Yelle, John C. McConnell, Darrell F. Strobel, Lyn R. Doose

Research output: Contribution to journalArticle

23 Scopus citations

Abstract

The spectrum of Uranus in the 1250- to 1700-Å region, as measured by the Voyager ultraviolet spectrometer (UVS), is analyzed as primarily solar reflected light from an H2 Rayleigh and Raman scattering atmosphere with small but measurable absorption by hydrocarbons. CH4 and C2H2 are expected to be the primary absorbers in the 1250- to 1700-Å region. The UVS spectra definitely show a C2H2 absorption signature and the effects of CH4 are evident as well. By comparison of the observed subsolar spectra to synthetic spectra based on Rayleigh-Raman scattering in some simple three-layer model atmospheres we infer a CH4 mixing ratio of ∼1-3 × 10-7 and C2H2 mixing ratio of ∼0.6-1.2 × 10-8 above 3 mbar. Between 3 and 5 mbar these respective mixing ratios are ∼3 × 10-7and 0.6-2 × 10-8. Previously, the UVS spectra in the 1250- to 1700-Å region were interpreted as evidence for excitation of H2 by very low energy electrons (A. L. Broadfoot et al. 1986, Science 233, 74), but it is possible to explain the observations without any severe anomalies in the H2 emission spectrum. The hydrocarbon abundances determined in our analysis are far below the abundances at comparable levels in the atmospheres of Jupiter or Saturn. We suggest that, in a 1-D view, this is due to a combination of diffusive separation and photochemical depletion caused by a very low eddy diffusion coefficient, on the order of 100 cm2 sec-1 or less. For these low values of the eddy coefficient the hydrocarbon mixing ratios should decrease rapidly with height; however, analysis of the UVS solar occultation experiment by F. Herbert, B. R. Sandel, R. V. Yelle, J. B. Holberg, A. L. Broadfoot, D. E. Shemansky, S. K. Atreya, and P. N. Romani (1987, J. Geophys. Res. 92, 15093) which occurred near the terminator, suggest a C2H2 mixing ratio of ∼10-8 near 100 μbar. In addition, the UV albedo in the 1338- to 1523-Å range is enhanced over the pole relative to low-latitude regions. Therefore, all available evidence suggests strong latitudinal variations in the hydrocarbon abundances, with substantial depletions in the subsolar, polar stratosphere. We discuss the possibility that the meridional circulation of the Uranian stratosphere inferred by F. M. Flasar, B. J. Conrath, P. J. Gierash, and J. A. Pirraglia (1987, J. Geophys. Res. 92, 15011) could cause the inferred latitudinal variations.

Original languageEnglish (US)
Pages (from-to)439-456
Number of pages18
JournalIcarus
Volume77
Issue number2
DOIs
StatePublished - Feb 1989

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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