Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations

Abstract

<i>Context. <i/>Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported.<i>Aims. <i/>The goal is to constrain essential but so far unknown parameters of Pluto's atmosphere, such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio.<i>Methods. <i/>We use high-resolution spectroscopic observations of gaseous methane and a novel analysis of occultation lightcurves.<i>Results. <i/>We show that (i) Pluto's surface pressure is currently in the 6.5–24 <i>μ<i/>bar range, (ii) the methane mixing ratio is 0.5<i>±<i/>0.1%, adequate to explain Pluto's inverted thermal structure and ~100 K upper atmosphere temperature, and (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundances of methane are strikingly similar, a possible consequence of a CH<sub>4<sub/>-rich top surface layer.