Hubble Space Telescope NICMOS spectroscopy of Charon's leading and trailing hemispheres

We used the near-infrared camera and multiobject spectrometer NICMOS on the Hubble Space Telescope to obtain 1.1-2.4 km, low-resolution (R similar to 200) slitless grism spectrophotometry of the individual members of the Pluto-Charon system. Water ice is present in its crystalline state on both the leading and trailing hemispheres of Charon. A 2.21 km absorption band, possibly due to the presence of ammonia hydrate in surface, is detected in the reflectance spectrum of its leading side only. Geological activity on Charon or implantation of ions escaped from Pluto's atmosphere could account for the formation of species such as NH3.H2O locally on the satellite. We also measured a slightly higher geometric albedo for Charon than reported from the mid-1980s observations of the mutual events.