THE DISTRIBUTION OF WATER FROST ON CHARON

We present high-spatial-resolution imaging observations of the Pluto-C haron system taken with ProtoCAM on the IRTF. Our dataset consists of measurements from eight nights at widely separated rotational longitud es and covering five wavelengths-standard J, H, and K, plus two specia l narrow band filters at 1.5 and 1.75 mum. The relative flux contribut ions of Pluto and Charon were extracted, when possible, by fitting a t wo-source Gaussian image model to the observed images. At K, we find t he Charon-Pluto magnitude difference to be on average 1.8 mag, somewha t 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 mum is 2.0 and 1.6, respectively. The larger magnitude difference at 1.5 mum is due to a water-frost absorption band on the surface of Charon. Our observations are consistent with a surface of Charon dominated by wate r frost at all longitudes. (C) 1994 Academic Press, Inc.