Exploring the spatial, temporal, and vertical distribution of methane in Pluto’s atmosphere

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2015-01, Vol.246, p.268-278
• Main Authors: Lellouch, E., de Bergh, C., Sicardy, B., Forget, F., Vangvichith, M., Käufl, H.-U.
• Format: Article
• Language: English
• Subjects: Astrophysics; Columns (structural); Infrared observations; Latitude; Mathematical models; Methane; Mixing ratios; Physics; Pluto; Pluto, atmosphere; Spectra; Spectroscopy; Surface pressure
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2014.03.027

•A study of temporal, rotational, and vertical variability of Pluto’s atmospheric methane is performed.•No variations of the methane column larger than 20% with Pluto’s phase are present.•At most marginal temporal variations over 2008–2012 are found.•Large departures from uniform mixing are excluded.•The source of methane gas is probably the large scale CH4 ice deposits, especially at high Northern latitudes. High-resolution spectra of Pluto in the 1.66μm region, recorded with the VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to constrain the spatial and vertical distribution of methane in Pluto’s atmosphere and to search for mid-term (4year) variability. A sensitivity study to model assumptions (temperature structure, surface pressure, Pluto’s radius) is performed. Results indicate that (i) no variation of the CH4 atmospheric content (column-density or mixing ratio) with Pluto rotational phase is present in excess of 20%, (ii) CH4 column densities show at most marginal variations between 2008 and 2012, with a best guess estimate of a ∼20% decrease over this time frame. As stellar occultations indicate that Pluto’s surface pressure has continued to increase over this period, this implies a concomitant decrease of the methane mixing ratio (iii) the data do not show evidence for an altitude-varying methane distribution; in particular, they imply a roughly uniform mixing ratio in at least the first 22–27km of the atmosphere, and high concentrations of low-temperature methane near the surface can be ruled out. Our results are also best consistent with a relatively large (>1180km) Pluto radius. Comparison with predictions from a recently developed global climate model indicates that these features are best explained if the source of methane occurs in regional-scale CH4 ice deposits, including both low latitudes and high Northern latitudes, evidence for which is present from the rotational and secular evolution of the near-IR features due to CH4 ice. Our “best guess” predictions for the New Horizons encounter in 2015 are: a 1184km radius, a 17μbar surface pressure, and a 0.44% CH4 mixing ratio with negligible longitudinal variations.