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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\rmh at the Uranus homopause, which for these values of K\rmh 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.