Ice and minerals on Callisto: A reassessment of the reflectance spectra

Abstract

Laboratory spectral reflectance measurements, including the 3 μm H 2O fundamental region, of particulate mixtures of both hydrated silicates and palagonite with water ice are presented. In all cases, the reflectances of these mixtures, at and beyond 3 μm, are dominated by absorptions due to water ice until ≥50 weight percent of the non-ice component is present. The laboratory measurements are compared to two previously unpublished reflectance spectra of Callisto. This comparison provides direct support for previous suggestions that the measured reflectance of Callisto contains a significant spectral contribution from a non-ice component, especially at and beyond 3 μm. Theoretical modeling, using Hapke's (1981, J. Geophys. Res. 86, 3039–3054; 1986, Icarus, 67, 264–280) equations, is used to more thoroughly analyze the telescopic data. The modeling considers homogeneous (intimate) and heterogeneous (areal) mixtures of both three components (ice, serpentine, and magnetite) and two components (ice and serpentine or magnetite) as a function of the relative particle size and abundance of the individual components. Comparison of the resultant calculations to the measured reflectance of Callisto imply that (1) three component mixtures of ice-magnetite-serpentine provide a better comparison to the telescopic data than any two component mixtures of ice and serpentine or magnetite; (2) the ice-magnetite-serpentine intimate mixtures which provide the best comparisons to the telescopic spectra indicate that the water ice contributes more to the spectrum of the leading hemisphere than the trailing hemisphere of Callisto; (3) all ice-magnetite mixtures fail to provide the spectral shape exhibited by the telescopic spectra of Callisto at and beyond 3 μm, indicating that the non-ice component possesses its own characteristic spectral features in this region.