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A. Y. T. Lee, Y. L. Yung (Caltech), B. M. Cheng, E. P. Chew, C. P. Liu (SRRC), M. Bahou, Y. P. Lee (NTHU)
The deuterated compounds in the upper atmospheres of giant planets have not been observed or modeled, except for HD and CH3D. We report a new photochemical model for the deuterated hydrocarbons up to C2 in the upper atmosphere of Jupiter. We include in the model laboratory measurements of the ultraviolet cross sections of CH3D and C2H5D. The fast reactions for exchanging H and D atoms between H2 and CH4 have been incorporated in the model: H + HD \longleftrightarrow D + H2, CH3 + D \longleftrightarrow CH2D + H. Since there is no reliable kinetics measurement for the reaction, CH2D + H \longrightarrow CH3 + D, we use Yung et al.'s (1988) estimate for the rate constant. The strong temperature dependence for this reaction leads to large isotopic fractionation for CH3D and C2H5D, whose production rate depends largely on the deuterated methyl radical abundance. The model predicts that the D/H in deuterated ethane is about 15 times that of the D/H of the bulk atmosphere. The successful confirmation of this result would provide a sensitive test of the photochemistry of hydrocarbons in the atmosphere of Jupiter.