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Y. L. Yung, M. C. Liang, G. Tinetti (Caltech), D. Shemansky (USC), R. A. West (JPL), D. Tice (Caltech)
Photolysis of methane results in the synthesis of more complex hydrocarbons. The hydrocarbon chemistry inevitably leads to the formation of high molecular weight species, and eventually gives rise to the production of aerosols. We carried out a photochemical study in a one-dimensional model. The temperature profile is based on that recently acquired by the Cassini UVIS experiment. Only solar UV radiation was used to drive the chemistry. The modeled concentrations of hydrocarbons species (C2H2, C2H6, C2H4, and C4H2) are in general agreement with the Cassini measurements in the region below ~800 km. Above this level, the observed concentrations of these hydrocarbon compounds show a much larger density scale height than that in the model. This implies (1) there is an additional energy source (e.g., precipitating electrons) driving the production of organic compounds at higher altitudes, and/or (2) large scale transport of photochemical products from regions that have been in the sun for longer duration. We also found that the most probable molecule for the production of the detached haze layer observed at ~500 km is C4H2. This compound and related polyynes would condense near the temperature minimum at ~600 km. Other plausible precursor molecules under investigation for producing the detached aerosol layer include C6H6 and PAH, and nitrile species. Estimates are made to relate the photochemical production rates to the size distribution and optical properties of the aerosol particles.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.