New Horizons/LESIA observations of Charon on approach.

On July 14, 2015, New Horizons made its closest approach to the Pluto system. Among its many tasks were spectroscopic observations of Charon’s surface using LEISA (Linear Etalon Imaging Spectral Array), the near-infrared spectroscopic component of the Ralph instrument. The LEISA component obtains spectra in the 1.25 to 2.5 μm range at a spectral resolution (λ/Δλ) ~240 and the 2.1-2.25 μm range at λ/Δλ ~560. LEISA observations in the days leading up to the closest approach range in spatial scale from when Charon was less than a pixel (but still spatially resolved from Pluto) to filling the field of view. We have analyzed each LEISA scan of Charon as a disk-integrated spectrum. These spectra show absorption bands due to crystalline H₂O-ice at 1.3, 1.5, 1.65, and 2.0 μm. In addition, we also see absorption around 2.21 μm, attributed to NH₃-hydrates, other ammoniated species (e.g., NH₄Cl), or their mixtures. We examine the size and shape of the 2.21 μm band in detail by normalizing each spectrum with a best-fit Hapke model in the 1.9-2.5 μm wavelength range. This procedure isolates the 2.21 μm band from the other H₂O-ice features in the spectrum. We will present our analysis of these spectra, including an examination of the band size and shape. We will compare the 2.21 μm band observed on Charon with a similar band seen on Pluto’s small satellites, Nix and Hydra. Finally, we will also compare the New Horizons/LEISA observations of Charon with ones obtained from Earth.