Pluto ’ s Spectrum from 1 . 0 to 4 . 2 μ m : Implications for Surface Properties

We present spectra of Pluto’s anti-Charon hemisphere obtained from the Keck and Subaru telescopes from 2.8 to 4.2 μm. Combined with 1 2.5 μm spectra from the IRTF telescope, this collective data set lets us constrain several surface frost properties. The surface area of pure nitrogen frost (as opposed to nitrogen with dissolved methane) is constrained to be 6% or less. The ratio of pure to dilute (in nitrogen) methane frost is best fit by a ratio of 3 to 4. The grain size of pure methane is constrained to be near 200 μm. We were not able to constrain the temperature of pure methane. An additional surface component with spectral properties similar to ‘Titan tholin’ was necessary to fit the entire 1 to 4.2 μm spectrum; our best-fit model requires 21% of Pluto’s anti-Charon hemisphere (by area) to be this Titan-tholin component. Contrary to Sasaki et al.’s (2005) spectra of Pluto’s sub-Charon hemisphere, we find no evidence for other hydrocarbons in the 3-3.3 μm region. Subject headings: planets and satellites: Pluto