High resolution spectroscopy of Pluto's atmosphere: detection of the 2.3 $\mu$m CH$_4$ bands and evidence for carbon monoxide

TL;DR

This study used high-resolution spectroscopy to detect methane and carbon monoxide in Pluto's atmosphere, providing new insights into its composition and surface-atmosphere interactions.

Contribution

First detection of gaseous methane in the 2.3 μm range and strong evidence for gaseous CO in Pluto's atmosphere, with derived mixing ratios and comparison to surface abundance.

Findings

Detection of methane and CO in Pluto's atmosphere.

Methane and CO mixing ratios quantified.

CO abundance consistent with surface levels.

Abstract

The goal is to determine the composition of Pluto's atmosphere and to constrain the nature of surface-atmosphere interactions. We perform high--resolution spectroscopic observations in the 2.33--2.36 $\mu$m range, using CRIRES at the VLT. We obtain (i) the first detection of gaseous methane in this spectral range, through lines of the $\nu_3$ + $\nu_4$ and $\nu_1$ + $\nu_4$ bands (ii) strong evidence (6-$\sigma$ confidence) for gaseous CO in Pluto. For an isothermal atmosphere at 90 K, the CH$_4$ 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 CH$_4$ and CO mixing ratios q$_{CH_4}$= 0.6$^{+0.6}_{-0.3}$% (consistent with results from the 1.66 $\mu$m range) and q$_{CO}$ = 0.5$^{+1}_{-0.25}$$\times10^{-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.