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[22.02] Isotopic Fractionation and Dilution of CO on Titan

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.