Constraints on Pluto’s H and CH4 profiles from New Horizons Alice Lyα observations

Abstract The Alice spectrograph on New Horizons performed several far-ultraviolet (FUV) airglow observations during the July 2015 flyby of Pluto. One of these observations, named PColor2, was a short (226 s) scan across the dayside disk of Pluto from a range of ∼ 34 , 000  km, at about 40 minutes prior to closest approach. The brightest observed FUV airglow signal at Pluto is the Lyman alpha (Ly α ) emission line of atomic hydrogen, which arises primarily through the resonant scattering of solar Ly α by H atoms in the upper atmosphere, with a brightness of about 30 Rayleighs. Pluto appears dark against the much brighter ( ∼ 100  Rayleigh) sky background; this sky background is likewise the result of resonantly scattered solar Ly α , in this case by H atoms in the interplanetary medium (IPM). Here we use an updated photochemical model and a resonance line radiative transfer model to perform detailed simulations of the Ly α emissions observed in the Alice PColor2 scan. The photochemical models show that H and CH 4 abundances in Pluto’s upper atmosphere are a very strong function of the near-surface mixing ratio of CH 4 , and could provide a useful way to remotely monitor seasonal climate variations in Pluto’s lower atmosphere. The morphology of the PColor2 Ly α emissions provides constraints on the current abundance profiles of H atoms and CH 4 molecules in Pluto’s atmosphere, and indicate that the globally averaged near-surface mixing ratio of CH 4 is currently close to 0 . 4 % . This new result thus provides independent confirmation of one of the primary results from the solar occultation, also observed with the New Horizons Alice ultraviolet spectrograph.