Near-Infrared Spectral Monitoring of Pluto's Ices II: Recent Decline of   CO and N$_2$ Ice Absorptions
  IRTF/SpeX observations of Pluto's near-infrared reflectance spectrum during 2013 show vibrational absorption features of CO and N$_2$ ices at 1.58 and 2.15 {\mu}m, respectively, that are weaker than had been observed during the preceding decade. To reconcile declining volatile ice absorptions with a lack of decline in Pluto's atmospheric pressure, we suggest these ices could be getting harder to see because of increasing scattering by small CH$_4$ crystals, rather than because they are disappearing from the observed hemisphere. 
W.M. Grundy, C.B. Olkin, L.A. Young, and B.J. Holler
astro-ph.EP