Search for nonmethane hydrocarbons on Pluto

We have carried out infrared high-resolution spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope. The spectrum is dominated by the strong and broad absorption features of methane but includes some additional features. Comparing the spectrum with model calculations, we suggest that absorption features around 3.1, 3.2, and 3.35 μm could be an indication of nonmethane hydrocarbons on Pluto's uppermost surface. Implications of the estimated mass ratio between hydrocarbons for the formation and evolution of Pluto are discussed.