Charon: More than Water Ice?

Abstract

A significant non-H 2O ice component may be present on the surface of Pluto's satellite Charon and yet remain undetected by existing observations. This suggestion arises from a comparison of calculated reflectance spectra with Charon's 1.5- to 2.5-μm reflectance spectrum. The calculated spectra rely upon descriptions of the interaction of light scattered from particulate surfaces and the optical constants of H 2O, CH 4, and CO 2 ices. Calculated spectra of mixtures composed of H 2O and CO 2 ice remain consistent with the observed spectrum of Charon for high abundances of CO 2 (≈50% relative mass fraction) in intimate mixtures, and for areal coverages of about 40% CO 2 in spatial mixtures. Calculations for mixtures of H 2O and CH 4 ice indicate that ≥5% relative mass fraction of CH 4 in intimate mixtures and ≥5-10% areal coverage of CH 4 in spatial mixtures result in spectra that cannot reproduce the observed Charon spectrum. Calculated spectra of three-component intimate mixtures of H 2O, CH 4, and CO 2 ices with similar grain sizes can fit the observed spectrum of Charon only for low abundances of CH 4 (≤ 5%). If the CH 4 ice grain size is much greater than the other components, then the spectrum of Charon can be modeled by calculated spectra containing up to ≈30% CH 4 in the intimate mixtures. Calculated spectra for spatial mixtures of H 2O, CH 4, and CO 2 ices indicate that ≤5-10% areal coverage of CH 4 can be incorporated and remain consistent with the observational data. The suggestion of significant amounts of non-H 2O components on Charon can be tested as Earth-based telescopic instrumentation improves. This suggestion should be considered during instrumental design for spacecraft destined for the Pluto-Charon system.