Letters to the Editor

High resolution spectroscopy of Pluto’s atmosphere: detection of the 2.3 μm CH₄ bands and evidence for carbon monoxide

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 European Space Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Chile

Received: 24 March 2011
Accepted: 21 April 2011

Abstract

Aims. The goal is to determine the composition of Pluto’s atmosphere and to constrain the nature of surface-atmosphere interactions.

Methods. We perform high-resolution spectroscopic observations in the 2.33–2.36 μm range, using CRIRES at the VLT.

Results. We obtain (i) the first detection of gaseous methane in this spectral range, through lines of the ν₃ + ν₄ and ν₁ + ν₄ bands (ii) strong evidence (6-σ confidence) for gaseous CO in Pluto. For an isothermal atmosphere at 90 K, the CH₄ and CO column densities are 0.75 and 0.07 cm-am, within factors of 2 and 3, respectively. Using a physically-based thermal structure model of Pluto’s atmosphere also satisfying constraints from stellar occultations, we infer CH₄ and CO mixing ratios ${\mathit{q}}_{\mathrm{C}{\mathrm{H}}_{\mathrm{4}}}\mathrm{=}\mathrm{0.}{\mathrm{6}}_{-0.3}^{\mathrm{+}\mathrm{0.6}}$% (consistent with results from the 1.66 μm range) and ${\mathit{q}}_{\mathrm{CO}}\mathrm{=}\mathrm{0.}{\mathrm{5}}_{-0.25}^{\mathrm{+}\mathrm{1}}\hspace{0.17em}\mathrm{\times }\hspace{0.17em}{\mathrm{10}}^{-3}$. The CO atmospheric abundance is consistent with its surface abundance. As for Triton, it is probably controlled by a thin, CO-rich, detailed balancing layer resulting from seasonal transport and/or atmospheric escape.