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[26.03] Implications for Atmospheric Vertical Mixing from ISO Measurements of C₂H₂ on Uranus and CH₃ on Saturn

S. K. Atreya (U.Michigan), Th. Encrenaz (Obs. Paris-Meudon), H. Feuchtgruber (MPI-Garching), S.G. Edgington (U.Michigan)

ISO/SWS measured acetylene (C₂H₂) in the stratosphere of Uranus in the \nu₅ band centered at 13.7 micron (Encrenaz et al, A&A, 333, L43, 1998) and the methyl radical (CH₃) in Saturn's upper atmosphere in the \nu₂ band of the Q-branch at 16.5 micron (Bézard et al, A&A, 334, L41, 1998). Interpretation of the C₂H₂ density profile with a hydrocarbon photochemical model---in which the eddy diffusion coefficient, K, is a free parameter---leads to a value of (0.5-1)\times10⁴ cm²s^-1 for K\rm_h at the Uranus homopause, which for these values of K\rm_h corresponds respectively to 354 km (0.037 mb) and 390 km (0.02 mb) above the 1-bar level. Despite the drastically different observation geometries and solar illumination, the eddy coefficient derived from the ISO data is consistent with the values obtained by the Voyager ultraviolet spectrometer ten years earlier, indicating that the dynamical behavior of the upper atmosphere is fairly uniform over the planet. On the other hand, using similar photochemical models, the best fit to the ISO Saturn CH₃ data requires that the value of eddy diffusion coefficient be reduced by orders of magnitude below the homopause value of ~!\!10⁸ cm²s^-1 measured by Voyager sixteen years earlier (Atreya, PSS, 30, 849, 1982; Sandel etal, GRL, 9, 1077, 1982; Parkinson etal, Icarus, 133, 210, 1998). This we find to be unlikely. Instead, it is suggested that speeding up the loss of CH₃ via its self reaction in a 3-body recombination reaction can solve the problem. This may be reasonable, considering the available lab data on the kinetics of this reaction are at temperatures significantly above those relevant for Saturn's CH₃ production and loss region.