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A. S. Wong, C. G. Morgan, Y. L. Yung (Caltech), T. Owen (Hawaii)
There are theoretical and observational reasons for believing that the atmosphere of Titan has evolved extensively. We expect the heavy isotopomers of CO, 13CO and C18O, to be enriched relative to CO. However, the isotopic signature resulting from atmospheric evolution may be altered by photochemical reactions in the atmosphere of Titan. We investigate the possibility of isotopic fractionation and dilution using the Caltech/JPL one-dimensional kinetics model of Titan. In the upper atmosphere of Titan there are energetic processes initiated by solar EUV radiation, precipitation of magnetospheric particles and photoelectrons. As a result of the interaction with the energetic particles, CO is readily dissociated into C and O. Some of the energetic C and O can escape to space. The fates of C and O in CO that remain in the atmosphere are quite different with respect to exchange with other reservoirs. The most important reaction for O is insertion into CH3 and C2H4 to form H2CO, but all eventually recycle back to CO. The C atom in the reformed CO comes from CH3 and C2H4, which in turn are derived from CH4 photolysis. However, the most rapid exchange for C atoms between the CH4 and CO reservoirs is mediated by the exchange reaction 1CH2 + C*O arrow 1C*H2 + CO, where C* is an isotopically heavy carbon atom. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 2.3 \times 10-11 cm3 s-1 at the temperature appropriate for the upper atmosphere of Titan. The time constants for the isotopic exchange processes will be estimated by our model.