A photochemical model of Pluto's atmosphere and ionosphere

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2020-01, Vol.335, p.113374, Article 113374
• Main Author: Krasnopolsky, Vladimir A.
• Format: Article
• Language: English
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2019.07.008

The model for Titan by Krasnopolsky (2014, Icarus 236, 83-91) has been adjusted to Pluto's conditions during the New Horizons flyby. The model includes 419 reactions of 83 neutrals and 33 positive ions plus 10 reactions of CO+ and HCO+ that were missing for Titan because of the low CO abundance. The model has 289 altitude steps up to the exobase at 1600 km with thermal escape for neutrals and diffusion velocities for ions as the upper boundary conditions. The model involves condensation on the haze and the surface with sticking coefficient γ = 0.002 for hydrocarbons, H2O, and CO2, γ = 0.01 for nitriles except HC3N with γ = 0.1. The photochemistry is driven by the solar EUV and UV radiation, the interplanetary Lyman-alpha emission, and the galactic cosmic rays. The adopted eddy diffusion K = 3 × 104 cm2 s−1 facilitates transport of C2H4 and C2H6, their condensation on the surface, and does not require the revision of the laboratory data on saturated vapor densities of C2H4 and C2H6 by orders of magnitude to fit the New Horizons observations. The CH4 homopause is at 110 km for this K, and the CH4 vertical profile is mostly controlled by molecular diffusion and agrees with the NH observations. Productions and losses of major hydrocarbons, nitriles, oxygen species, and hydrogen are briefly discussed. The daytime ionosphere is predicted with a maximum electron density of 800 cm−3 at 750 km. The most abundant ions are HCNH+ and C9H11+ above and below 600 km, respectively. Chemical effects of the ion reactions on some neutral species are significant. The predicted ion densities are measurable by an analog of the Cassini ion-neutral mass spectrometer. Evolution of the atmosphere includes thermal escape of H2 + H, CH4 + CH3, and N2 + N (92, 203, and 1 g cm−2 Byr−1, respectively), condensation of hydrocarbons and nitriles (306 and 101 g cm−2 Byr−1, respectively), and polymerization (30 g cm−2 Byr−1). (Annually mean values are halves of those.) The composition and photochemistry of Pluto during the New Horizons flyby are very different from those of Triton during the Voyager 2 flyby and controlled by the methane mole fraction near the surface. Seasonal variations of the atmospheric methane should cover both states of the atmospheres on both Pluto and Triton. •Photochemical model of Pluto's atmosphere and ionosphere includes 429 reactions of 83 neutrals and 35 ions•The model has 289 altitude steps up to exobase at 1600 km•Condensation on the haze and the surface with