IRTF/SpeX observations of Plutos 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 rec
oncile declining volatile ice absorptions with a lack of decline in Plutos 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.
We report results from monitoring Plutos 0.8 to 2.4 {mu}m reflectance spectrum with IRTF/SpeX on 65 nights over the dozen years from 2001 to 2012. The spectra show vibrational absorption features of simple molecules CH4, CO, and N2 condensed as ices
on Plutos surface. These absorptions are modulated by the planets 6.39 day rotation period, enabling us to constrain the longitudinal distributions of the three ices. Absorptions of CO and N2 are concentrated on Plutos anti-Charon hemisphere, unlike absorptions of less volatile CH4 ice that are offset by roughly 90{deg} from the longitude of maximum CO and N2 absorption. In addition to the diurnal variations, the spectra show longer term trends. On decadal timescales, Plutos stronger CH4 absorption bands have been getting deeper, while the amplitude of their diurnal variation is diminishing, consistent with additional CH4 absorption at high northern latitudes rotating into view as the sub-Earth latitude moves north (as defined by the systems angular momentum vector). Unlike the CH4 absorptions, Plutos CO and N2 absorptions appear to be declining over time, suggesting more equatorial or southerly distributions of those species. Comparisons of geometrically-matched pairs of observations favor geometric explanations for the observed secular changes in CO and N2 absorption, although seasonal volatile transport could be at least partly responsible. The case for a volatile transport contribution to the secular evolution looks strongest for CH4 ice, despite it being the least volatile of the three ices.
