Photochemistry on Pluto - I. Hydrocarbons and aerosols

In light of the recent New Horizons flyby measurements, we present a coupled ion-neutral-photochemistry model developed for simulating the atmosphere of Pluto. Our model results closely match the observed density profiles of CH₄, N₂ and the C₂ hydrocarbons in the altitude range where available New Horizons measurements are most accurate (above ~ 100-200 km). We found a high eddy coefficient of 10⁶ cm² s^-1 from the surface to an altitude of 150 km, and 3 × 10⁶ cm² s^-1 above 150 km for Pluto's atmosphere. Our results demonstrate that C₂ hydrocarbons must stick to and be removed by aerosol particles in order to reproduce the C2 profiles observed by New Horizons. Incorporation into aerosols in Pluto's atmosphere is a significantly more effective process than condensation, and we found that condensation alone cannot account for the observed shape of the vertical profiles. We empirically determined the sticking efficiency of C2 hydrocarbons to aerosol particles as a function of altitude, and found that the sticking efficiency of C₂ hydrocarbons is inversely related to the aerosol surface area. Aerosols must harden and become less sticky as they age in Pluto's atmosphere. Such hardening with ageing is both necessary and sufficient to explain the vertical profiles of C₂ hydrocarbons in Pluto's atmosphere. This result is in agreement with the fundamental idea of aerosols hardening as they age, as proposed for Titan's aerosols.