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Published May 1, 2017 | public
Journal Article

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 N_2 with trace amounts of CH_4, 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 CH_4, C_2H_2, C_2H_4, and C_2H_6, the total column density of HCN, and to predict the abundance profiles of oxygen-bearing species. The CH_4 profile can be best matched by taking a constant-with-altitude eddy diffusion coefficient K_(zz) profile of 1 × 10^3 cm^2 s^(–1) and a fixed CH_4 surface mixing ratio of 4 × 10^(–3). Condensation is key to fitting the C_2 hydrocarbon profiles. We find that C_2H_4 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 C_2H_2, C_2H_4, C_2H_6, and HCN. The top three precipitating species are C_2H_2, C_2H_4, and C_2H_6, with precipitation rates of 179, 95, and 62 g cm^(–2) s^(–1), respectively.

Additional Information

© 2016 Elsevier Inc. Received 16 May 2016, Revised 27 August 2016, Accepted 17 September 2016, Available online 29 September 2016. This research was supported in part by a grant from the New Horizons mission. YLY and RLS were supported in part by the Cassini UVIS program via NASA Grant JPL.1459109, NASA NNX09AB72G grant to the California Institute of Technology. PG was supported in part by an RTD grant from JPL. MLW is grateful to Theater Arts at Caltech as a source of personal motivation by giving him the chance to portray Clyde Tombaugh in Planet Between the Stars during the course of this project.

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