CHARON - MORE THAN WATER ICE

A significant non-H2O 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-mum reflectance spectrum. The calculated spectra rely upon descriptions of the interaction of light scattered from particulate surfaces and the optical constants of H2O, CH4, and CO2 ices. Calculated spectra of mixtures composed of H2O and CO2 ice remain consistent with the observed spectrum of Charon for high abundances Of CO2 (almost-equal-to 50% relative mass fraction) in intimate mixtures, and for areal coverages of about 40% CO2 in spatial mixtures. Calculations for mixtures of H2O and CH4 ice indicate that greater-than-or-equal-to 5% relative mass fraction of CH4 in intimate mixtures and greater-than-or-equal-to 5-10% areal coverage of CH4 in spatial mixtures result in spectra that cannot reproduce the observed Charon spectrum. Calculated spectra of three-component intimate mixtures of H2O, CH4, and CO2 ices with similar grain sizes can fit the observed spectrum of Charon only for low abundances of CH4 (less-than-or-equal-to 5%). If the CH4 ice grain size is much greater than the other components, then the spectrum of Charon can be modeled by calculated spectra containing up to almost-equal-to 30% CH4 in the intimate mixtures. Calculated spectra for spatial mixtures of H2O, CH4, and CO2 ices indicate that less-than-or-equal-to 5-10% areal coverage of CH4 can be incorporated and remain consistent with the observational data. The suggestion of significant amounts of non-H2O 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. (C) 1994 Academic Press, Inc.