The photochemistry of hydrocarbons in Titan's atmosphere

Results of the far uv photolysis of methane diluted 1000 times with argon are presented for temperatures of 300 and 200°K. It was found that ethylene and acetylene reached a temperature-dependent steady state, while ethane and propane continued to rise linearly with time. The experimental ethylene column abundance in a photochemical steady state was extrapolated to the temperatures expected in the methane photolysis layer on Titan. This, together with Gillett's [(1975) Astrophys. J. 201] observations, make it possible to set a lower limit to the altitude of the top of Titan's aerosol layer, below which the ethylene is protected from photolysis. We find this to be 4.8 or 7.2 scale heights above the surface, for mean temperatures in the photolysis layer of 130 and 160°K, respectively. For 130°K, the ethylene abundance above the inversion base agrees well with Gillett's observations, and leads to an aerosol cloud top coincident with the inversion base. In order to transport ethane to the cold (78°K) "surface" with a flux sufficiently large so that the rate of condensation equals the rate of production, an eddy diffusion coefficient at the surface of at least 10² cm² sec^-1 is required. The experimental results lead to an acetylene abundance in a photochemical steady state whicch is ∼2 orders of magnitude smaller than the one suggested by Strobel [(1974) Icarus 21]. Since propane is one of the major products, the possibility of observing it at 13.4 μm is suggested.