The Distribution of Water Frost on Charon

We present high-spatial-resolution imaging observations of the Pluto-Charon system taken with ProtoCAM on the IRTF. Our dataset consists of measurements from eight nights at widely separated rotational longitudes and covering five wavelengths—standard J, H, and K, plus two special narrow band filters at 1.5 and 1.75 μm. The relative flux contributions of Pluto and Charon were extracted, when possible, by fitting a two-source Gaussian image model to the observed images. At K, we find the Charon-Pluto magnitude difference to be on average 1.8 mag, somewhat less than the value of 2.2 mag found by Bosh et al. (1992, Icarus 95, 319-324). The average differential magnitude at 1.5 and 1.75 μm is 2.0 and 1.6, respectively. The larger magnitude difference at 1.5 μ is due to a water-frost absorption band on the surface of Charon. Our observations are consistent with a surface of Charon dominated by water frost at all longitudes.