CHARON - MORE THAN WATER ICE

A significant non-H2O ice component may be present on the surface of P luto's satellite Charon and yet remain undetected by existing observat ions. This suggestion arises from a comparison of calculated reflectan ce spectra with Charon's 1.5- to 2.5-mum reflectance spectrum. The cal culated spectra rely upon descriptions of the interaction of light sca ttered 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 a bundances Of CO2 (almost-equal-to 50% relative mass fraction) in intim ate mixtures, and for areal coverages of about 40% CO2 in spatial mixt ures. Calculations for mixtures of H2O and CH4 ice indicate that great er-than-or-equal-to 5% relative mass fraction of CH4 in intimate mixtu res and greater-than-or-equal-to 5-10% areal coverage of CH4 in spatia l 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 s pectrum of Charon only for low abundances of CH4 (less-than-or-equal-t o 5%). If the CH4 ice grain size is much greater than the other compon ents, then the spectrum of Charon can be modeled by calculated spectra containing up to almost-equal-to 30% CH4 in the intimate mixtures. Ca lculated spectra for spatial mixtures of H2O, CH4, and CO2 ices indica te that less-than-or-equal-to 5-10% areal coverage of CH4 can be incor porated and remain consistent with the observational data. The suggest ion of significant amounts of non-H2O components on Charon can be test ed as Earth-based telescopic instrumentation improves. This suggestion should be considered during instrumental design for spacecraft destin ed for the Pluto-Charon system. (C) 1994 Academic Press, Inc.