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Monika E. Kress (NASA Ames Research Center), Kelly McMillen (U. Colorado), Kevin J. Zahnle, Christopher P. McKay (NASA Ames Research Center)
The volatiles present in Titan's atmosphere and on the surface of other satellites in the outer solar system may have been delivered by cometary impacts. Because the elemental abundances in comets are roughly solar, except for hydrogen which is greatly depleted, the thermodynamically favored chemical composition of the atmosphere at 1 bar for this mixing ratio of the elements is H2O, CO2, CH4, and N2. Initially, the high-temperature shocked mixture consists of H2, H2O, CO, and N2. As the atmosphere cools below about 1000 K, CO2 and CH4 become the thermodynamically favored forms of C, while nitrogen remains as N2, but reactions converting CO to CO2 and CH4 cannot proceed in the gas phase at lower temperatures due to high activation energies, suggesting that the atmosphere would remain CO/N2-rich. However, metallic nickel, which is expected to condense out of this comet mixture, is an excellent catalyst of the reactions which drive CO and H2O to CO2 and CH4. These catalyzed reactions proceed rapidly at temperatures down to about 400 K. We present model results which suggest that, depending on the size of the impactor and thus the cooling rate, there may be sufficient time after a large impact to allow the atmosphere to reach chemical equilibrium when these catalyzed reactions are considered, providing a possible explanation for the existence of CH4 with N2 in Titan's atmosphere and on other outer solar system satellites.