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 N₂with CH₄as the leading minor species with a mixing ratio between 3.7 × 10^-3and 7.4 × 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 differentT(p) models have been developed. The turbulent processes have been parameterized, as in previous atmospheric models, with an eddy diffusion coefficientK=K_o(n_o/n)^0.5placing the homopause at an assumed altitude of ~150 km. For the case we call nominal (T_surf= 37 K andp_surf= 10 μbar) parent molecules such as N₂, CH₄, and CO must be supplied from the surface with rates of 1.85 × 10⁷, 2.87 × 10⁸, and 6.83 × 10³cm^-2sec^-1, respectively. As a consequence of photolysis, H and H₂escaping fluxes, scaled to the surface, are 3.50 × 10⁸and 2.12 × 10⁸cm^-2sec^-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.