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

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 similar to 34,000 km, at about 40 minutes prior to closest approach. The brightest observed FUV airglow signal at Pluto is the Lyman alpha (Ly alpha) emission line of atomic hydrogen, which arises primarily through the resonant scattering of solar Ly alpha by H atoms in the upper atmosphere, with a brightness of about 30 Rayleigh. Pluto appears dark against the much brighter (similar to 100 Rayleigh) sky background; this sky background is likewise the result of resonantly scattered solar Ly alpha, 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 alpha emissions observed in the Alice PColor2 scan. The photochemical models show that H and CH4 abundances in Pluto's upper atmosphere are a very strong function of the near-surface mixing ratio of CH4, and could provide a useful way to remotely monitor seasonal climate variations in Pluto's lower atmosphere. The morphology of the PColor2 Ly alpha emissions provides constraints on the current abundance profiles of H atoms and CH4 molecules in Pluto's atmosphere, and indicate that the globally averaged near-surface mixing ratio of CH4 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.

Automated summary

The Alice spectrograph on New Horizons observed far-ultraviolet airglow emissions on Pluto during a flyby in July 2015. One particular observation, PColor2, provided insight into the emission lines of atomic hydrogen and the abundance profiles of H atoms and CH4 molecules in Pluto's atmosphere, which can be used to monitor seasonal climate variations remotely. The brightness of Lyman alpha emissions and the near-surface mixing ratio of CH4 on Pluto were highlighted in the study.