The photochemistry of Pluto's atmosphere as illuminated by New Horizons

New Horizons has granted us an unprecedented glimpse at the structure and composition of Pluto's atmosphere, which is comprised mostly of N2 with trace amounts of CH4, CO, and the photochemical products thereof. Through photochemistry, higher-order hydrocarbons are generated, coagulating into aerosols and resulting in global haze layers. Here we present a state-of-the-art photochemical model for Pluto's atmosphere to explain the abundance profiles of CH4, C2H2, C2H4, and C2H6, the total column density of HCN, and to predict the abundance profiles of oxygen-bearing species. The CH4 profile can be best matched by taking a constant-with-altitude eddy diffusion coefficient Kzz profile of 1 × 103 cm2 s-1 and a fixed CH4 surface mixing ratio of 4 × 10-3. Condensation is key to fitting the C2 hydrocarbon profiles. We find that C2H4 must have a much lower saturation vapor pressure than predicted by extrapolations of laboratory measurements to Pluto temperatures. We also find best-fit values for the sticking coefficients of C2H2, C2H4, C2H6, and HCN. The top three precipitating species are C2H2, C2H4, and C2H6, with precipitation rates of 179, 95, and 62 g cm-2 s-1, respectively.