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High-resolution spectra of Pluto in the 1.66 um region, recorded with theVLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed toconstrain the spatial and vertical distribution of methane in Pluto'satmosphere and to search for mid-term (4 year) variability. A sensitivity studyto model assumptions (temperature structure, surface pressure, Pluto's radius)is performed. Results indicate that (i) no variation of the CH4 atmosphericcontent (column-density or mixing ratio) with Pluto rotational phase is presentin excess of 20 % (ii) CH4 column densities show at most marginal variationsbetween 2008 and 2012, with a best guess estimate of a ~20 % decrease over thistime frame. As stellar occultations indicate that Pluto's surface pressure hascontinued to increase over this period, this implies a concomitant decrease ofthe methane mixing ratio (iii) the data do not show evidence for analtitude-varying methane distribution; in particular, they imply a roughlyuniform mixing ratio in at least the first 22-27 km of the atmosphere, and highconcentrations of low-temperature methane near the surface can be ruled out.Our results are also best consistent with a relatively large (> 1180 km) Plutoradius. Comparison with predictions from a recently developed global climatemodel GCM indicates that these features are best explained if the source ofmethane occurs in regional-scale CH4 ice deposits, including both low latitudesand high Northern latitudes, evidence for which is present from the rotationaland 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 17ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligiblelongitudinal variations.

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