Photochemical models of Pluto's atmosphere

A one-dimensional photochemical steady state model is used to study the vertical distribution of different gas species in the neutral atmosphere of Pluto. The major species is supposed to be N-2 with CH4 as the leading minor species with a mixing ratio between 3.7 x 10(-3) and 7.4 x 10(-3), depending on the pressure and temperature on the surface. As a result of photolytic reactions, a number of hydrocarbons and nitriles are mainly produced in the 50-300 km region. Because of the uncertainty in the atmospheric thermal structure of Pluto, photochemical models with three different T(p) models have been developed. The turbulent processes have been parameterized, as in previous atmospheric models, with an eddy diffusion coefficient K = K-0(n(0)/n)(0.5) placing the homopause at an assumed altitude of similar to 150 km. For the case we call nominal (T-surf = 37 K and p(surf) = 10 mu bar) parent molecules such as N-2, CH4, and CO must be supplied from the surface with rates of 1.85 x 10(7), 2.87 x 10(8), and 6.83 x 10(3) cm(-2) sec(-1), respectively. As a consequence of photolysis, H and H-2 escaping fluxes, scaled to the surface, are 3.50 x 10(8) and 2.12 x 10(8) cm(-2) sec(-1), respectively. Condensation of light hydrocarbons and nitriles occur between 3 and 5 km, depending on the compound and the surface temperature. Scaling the downward flux of these compounds to the amount of ice that would be deposited on the surface, we have deduced that the most abundant surface ices would be C2H6, C2H2, HCN, and C2H4, in that order. (C) 1997 Academic Press.