Letter to the Editor

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

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France e-mail: emmanuel.lellouch@obspm.fr
² Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
³ European Space Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁴ Laboratoire de Spectrométrie Physique, Université Joseph Fourier, BP 87, 38402 St-Martin d'Hères Cedex, France

Received: 9 January 2009
Accepted: 29 January 2009

Abstract

Context. Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported.

Aims. 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.

Methods. We use high-resolution spectroscopic observations of gaseous methane and a novel analysis of occultation lightcurves.

Results. We show that (i) Pluto's surface pressure is currently in the 6.5–24 μbar range, (ii) the methane mixing ratio is 0.5±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 CH₄-rich top surface layer.