  Context: Pluto possesses a thin atmosphere, primarily composed of nitrogen,
in which the detection of methane has been reported.
  Aims: The goal is to constrain essential but so far unknown parameters of
Pluto's atmosphere such as the surface pressure, lower atmosphere thermal
stucture, and methane mixing ratio.
  Methods: We use high-resolution spectroscopic observations of gaseous
methane, and a novel analysis of occultation light-curves.
  Results: We show that (i) Pluto's surface pressure is currently in the 6.5-24
microbar range (ii) the methane mixing ratio is 0.5+/-0.1 %, adequate to
explain Pluto's inverted thermal structure and ~100 K upper atmosphere
temperature (iii) a troposphere is not required by our data, but if present, it
has a depth of at most 17 km, i.e. less than one pressure scale height; in this
case methane is supersaturated in most of it. The atmospheric and bulk surface
abundance of methane are strikingly similar, a possible consequence of the
presence of a CH4-rich top surface layer.
