Spectral variability of Charon’s 2.21-μm feature

The clear angular separation of Pluto and Charon from ground-based telescopes has been enabled by improved technology, particularly adaptive optics systems. Near-infrared spectral data have revealed Charon’s surface to be rich in crystalline water ice and ammonia hydrates. In this work, we search for spectral differences across Charon’s surface with new near-infrared spectral data taken in the K-band (2.0–2.4 μm) with SINFONI on the VLT and NIRI on Gemini North as well as with previously published spectral data. The strength of the absorption band of ammonia hydrate is dependent on the state of the ice, concentration in H2O, grain size, temperature and exposure to radiation. We find variability of the band center and band depth among spectra. This could indicate variability of the distribution of ammonia hydrate across Charon’s surface. If the spectral variation is due to physical properties of Charon, the New Horizons flyby could find the concentration of ammonia hydrate heterogeneously distributed across the surface. Comparison between this work and New Horizons results will test the limits of ground-based reconnaissance.