Constraints on Pluto’s H and CH<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e528" altimg="si75.svg"><mml:msub><mml:mrow /><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:math> profiles from New Horizons Alice Ly<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e536" altimg="si43.svg"><mml:mi>α</mml:mi></mml:math> observations

The Alice spectrograph on New Horizons performed several far-ultraviolet (FUV) airglow observations during the July 2015 flyby of Pluto. One these observations, named PColor2, was a short (226 s) scan across dayside disk Pluto from range ∼34,000 km, at about 40 minutes prior to closest approach. brightest observed FUV signal is Lyman alpha (Lyα) emission line atomic hydrogen, which arises primarily through resonant scattering solar Lyα by H atoms in upper atmosphere, with brightness 30 Rayleigh. appears dark against much brighter (∼100 Rayleigh) sky background; this background likewise result resonantly scattered Lyα, case interplanetary medium (IPM). Here we use an updated photochemical model and resonance radiative transfer perform detailed simulations emissions PColor2 scan. models show that CH4 abundances Pluto’s atmosphere are very strong function near-surface mixing ratio CH4, could provide useful way remotely monitor seasonal climate variations lower atmosphere. morphology provides constraints current abundance profiles molecules indicate globally averaged currently close 0.4%. This new thus independent confirmation one primary results occultation, also ultraviolet spectrograph.