Analysis of Charon’s spectrum at 2.21-μm from New Horizons/LEISA and Earth-based observations

We examine the 2.21-μm band from 19 disk-integrated Charon spectra measured by New Horizons/LEISA in the week leading up to its closest encounter with the Pluto system. These observations cover one Charon rotation period. Additionally, we analyze Charon’s 2.21-μm band from 22 Earth-based spectra obtained over the last two decades. We measure the equivalent width of the 2.21-μm band from all observations and study it as a function of sub-observer longitude. We find no significant variation in the 2.21-μm band as Charon rotates. Compared to the same band seen on Nix and Hydra, Charon’s 2.21-μm band is several times weaker. We attribute the 2.21-μm band to NH4Cl based on the appearance of a weaker band at 2.24-μm. Furthermore, we see two never-before-reported absorption features in Charon’s spectrum at 1.60 and 1.63-μm, which may also be due to NH4Cl. If NH3-H2O-ice mixtures are present on Charon, they must be a small fraction of the disk-average composition to be consistent with the spectrum at 1.99-μm.