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E.H. Wilson, S.K. Atreya (U. of Michigan)
The photodissociation of CH4 (methane) at Lyman \alpha (1216 {Å}) has been the object of much scrutiny over the past few years. Methane photolysis leads to the formation of H2 molecules as well as H, CH, 1CH2, 3CH2, and CH3 radicals which promote the propagation of hydrocarbon chemistry. However, laboratory studies and their interpretation [1,2,3] have not fully resolved the issue of the magnitudes of these product yields at this wavelength. We use a one-dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titan's atmosphere, where Lyman \alpha radiation accounts for 75% of the methane absorption longward of 1000 {Å}. Sensitivity studies [4] show that only the C3H4 isomers (methylacetylene, allene) and C3H6 (propylene) display major variation in atmospheric abundance under the implementation of these schemes, with a variation approximating a factor of five in C3H4 abundance and a factor of four for C3H6. In these cases, our nominal scheme, recommended by Romani [2], offers an intermediate result in comparison with the other schemes. Furthermore, simple hydrocarbons like C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance. We also find that the choice of pathway for non-Lyman \alpha methane absorption does affect subsequent chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C2H6 (ethane) abundance, a value within observational uncertainty, is the largest divergence found. These results will have significance in future modeling and interpretation of observations of the atmosphere of Titan.
REFERENCES: [1] D. H. Mordaunt et al. (1993) J. Chem. Phys., 98, 2054-2065. [2] P. N. Romani (1996) Icarus, 122, 233-241. [3] N. S. Smith and F. Raulin (1999) J. Geophys. Res., 104, 1873-1876. [4] E. H. Wilson and S. K. Atreya (1999), submitted for publication.