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A. S. Wong, Y. L. Yung, C. G. Morgan (California Institute of Technology), T. Owen (University of Hawaii)
The recent observations of the heavy isotopomers of CO, 13CO and C18O, may be used to place constraints on the evolution of the atmosphere of Titan. However, the original isotopic signatures may be altered by photochemical reactions. We explain why there is no isotopic enrichment in C in Titan's atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange for C atoms between the CH4 and CO reservoirs, mediated by the reaction 1CH2 + *CO = 1*CH2 + CO, where *C is 13C. Based on recent laboratory measurements, we estimate the rate coefficient for this reaction to be 3.2 \times 10-12 cm3 s-1 at the temperature appropriate for the upper atmosphere of Titan. We investigate the isotopic dilution of CO using the Caltech/JPL one-dimensional photochemical model of Titan. Our model suggests that the time constant for isotopic exchange through the above reaction to be about 800 Myr, which is significantly shorter than the age of Titan, and therefore any original isotopic enhancement of 13C in CO may have been diluted by the exchange process. No such isotopic dilution exists for 18O in CO. A plausible model for the evolution history of Titan's atmosphere is proposed.
The author(s) of this abstract have provided an email address for comments about the abstract: asw@gps.caltech.edu