Vertical distribution of Titan's atmospheric neutral constituents

The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 k.m. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N-2, of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH4 or (preferably) C2H6. The observed CO2 profile can be modeled hy assuming an external source of water of similar to 6 x 10(6) cm(-2) s(-1). For a nominal CO mixing ratio of 5 x 10(-5), the chemical loss of CO exceeds its production by similar to 15%, and equilibrium is achieved for CO = 1 x 10(-5).