Evolution of CO on Titan

The early evolution of Titanʹs atmosphere is expected to produce enrichment in the heavy isotopomers of CO, 13CO and C18O, relative to 12C16O. However, the original isotopic signatures may be altered by photochemical reactions. This paper explains why there is no isotopic enrichment in C in Titanʹs atmosphere, despite significant enrichment of heavy H, N, and O isotopes. We show that there is a rapid exchange of 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 3.2×10−12 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 is 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. In addition, a plausible model for the evolution history of CO on Titan after the initial escape is proposed.