The Distribution and Physical State of H ₂O on Charon

Abstract

We present new 1.4- to 2.5-μm geometric albedo spectra of Charon taken with HST/NICMOS in 1998. These new data provide global coverage of the surface with four spectra at evenly spaced longitudes. The surface of Charon is seen to be globally dominated by H ₂O ice. The data indicate that the ice is in the crystalline phase at a temperature consistent with its heliocentric distance. The spectrum of Charon has only weak variations with longitude. There is an indication of a slightly stronger H ₂O ice absorption on the leading hemisphere. No variations in the spectrum are seen in response to differing solar phase angles. From model fits there is no indication of any of the volatile species that are seen on Pluto, i.e., CO, CH ₄, or N ₂. There is spectroscopic evidence for a contaminant with an absorption coefficient which increases with wavelength past ∼2 μm. This contaminant is unidentified but is similar to what is seen on other icy satellites in the outer Solar System. We also present a standard spectrophotometric model for Charon that can be used to subtract Charon light from ground-based spectra of the combined Pluto-Charon system.