Organics and Other Molecules in the Surfaces of Callisto and Ganymede

Five absorption features are reported at wavelengths of 3.4, 3.88, 4.05, 4.25, and 4.57 micrometers in the surface materials of the Galilean satellites Callisto and Ganymede from analysis of reflectance spectra returned by the Galileo mission near-infrared mapping spectrometer. Candidate materials include CO$_2$, organic materials (such as tholins containing C≡N and C-H), SO$_2$, and compounds containing an SH-functional group; CO$_2$, SO$_2$, and perhaps cyanogen [(CN)$_2$] may be present within the surface material itself as collections of a few molecules each. The spectra indicate that the primary surface constituents are water ice and hydrated minerals.