Carbon dioxide on Ganymede

Abstract

CO2 is inferred on the surface of Ganymede by the presence of an absorption band at ∼4.26 μm in reflectance spectra returned by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft. Detailed studies of NIMS observations of Ganymede show that the CO2 absorption band on Ganymede is symmetric about 4.257 μm ± 0.004 μm and is negligibly different from the CO2 absorption band on Callisto. In general, bright terrains (sulci) contain less CO2 than dark terrains (regiones), little or no CO2 is detected at the poles, and, unlike for Callisto, there does not appear to be any leading/trailing hemisphere asymmetry in the distribution Of CO2 nor do impact craters tend to be CO2 rich. High spatial resolution observations show that CO2 is occasionally enriched in terrain containing larger-grained ice in comparison with adjacent terrain of similar morphology and ice abundance, that the dark ejecta of Kittu is depleted in CO2 with respect to adjacent terrain, and that only one observed impact crater (Mir) is enriched in CO2. The CO2 that is detected by NIMS is in the nonice material(s) present, not in the fraction that is ice. The ice in all terrain types where CO2 is detected is always sufficiently large-grained that its low reflectance in the region of the CO2 band prevents discrimination of the CO2 absorption band from the continuum. Areas with relatively fine-grained ice that are sufficiently bright at 4 μm for CO2 to be observed, such as the polar regions, do not contain detectable CO2.