Near-infrared spectral monitoring of Pluto's ices II: Recent decline of CO and N2 ice absorptions

IRTF/SpeX observations of Pluto's near-infrared reflectance spectrum during 2013 show vibrational absorption features of CO and N2 ices at 1.58 and 2.15 μm, 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 CH4 crystals, rather than because they are disappearing from the observed hemisphere.