Hydrocarbon and HCN Condensation in the Atmosphere of Pluto

Observations by the New Horizons spacecraft revealed the presence of haze in the Pluto atmosphere, which have been shown by microphysical models to be likely composed of fractal aggregates originating from CH4 photolysis and subsequent polymerization of higher hydrocarbons. However, temperatures in the Pluto atmosphere are such that higher hydrocarbons, such as C2H2, C2H4, C2H6, as we well as HCN, should condense, possibly onto the fractal aggregate haze particles. This process can change their shape, as well as their optical properties. We use a modified microphysical model to investigate the characteristics of haze particles as C2 hydrocarbons and HCN condense on them during their sedimentation through the atmosphere. The composition of the particles as a function of altitude can in turn inform the interpretation of New Horizons observations. In addition, we use the condensation rates from the microphysics model to augment the photochemical model that calculates the concentrations of C2 hydrocarbons and HCN to ensure self-consistency.