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

Abstract

High-resolution spectra of Pluto in the 1.66 um 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 (4 year) 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 22-27 km 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 (> 1180 km) Pluto radius. Comparison with predictions from a recently developed global climate model GCM 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 1184 km radius, a 17 ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible longitudinal variations.

21 pages, 6 figures

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• Detection of CO in Triton's atmosphere and the nature of surface-atmosphere interactions
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• Pluto's atmosphere from stellar occultations in 2012 and 2013
• The CH4 cycles on Pluto over seasonal and astronomical timescales
• Pluto's lower atmosphere and pressure evolution from ground-based stellar occultations, 1988-2016
• Detection and characterization of Io's atmosphere from high-resolution 4-μm spectroscopy
• The long-wavelength thermal emission of the Pluto-Charon system from Herschel observations. Evidence for emissivity effects
• Solar Wind at 33 AU: Setting Bounds on the Pluto Interaction for New Horizons
• The Spectrum of Pluto, 0.40 - 0.93 $\mu$m I. Secular and longitudinal distribution of ices and complex organics
• On the escape of CH4 from Pluto's atmosphere
• Evidence for longitudinal variability of ethane ice on the surface of Pluto
• Modeling the formation of N2 and CH4 frost on Pluto's slopes