Context :

Aims : The goal is to determine the composition of Pluto ’ s atmosphere and to constrain the nature of surface-atmosphere interactions .

Methods : We perform high–resolution spectroscopic observations in the 2.33–2.36 \mu m range , using CRIRES at the VLT .

Results : 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 } \times 10 ^ { -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 .

Conclusions :