Carbon dioxide on Ganymede

CO 2 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 CO 2 absorption band on Ganymede is symmetric about 4.257 μm ± 0.004 μm and is negligibly different from the CO 2 absorption band on Callisto. In general, bright terrains (sulci) contain less CO 2 than dark terrains (regiones), little or no CO 2 is detected at the poles, and, unlike for Callisto, there does not appear to be any leading/trailing hemisphere asymmetry in the distribution of CO 2 nor do impact craters tend to be CO 2 rich. High spatial resolution observations show that CO 2 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 CO 2 with respect to adjacent terrain, and that only one observed impact crater (Mir) is enriched in CO 2 . The CO 2 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 CO 2 is detected is always sufficiently large‐grained that its low reflectance in the region of the CO 2 band prevents discrimination of the CO 2 absorption band from the continuum. Areas with relatively fine‐grained ice that are sufficiently bright at 4 μm for CO 2 to be observed, such as the polar regions, do not contain detectable CO 2 .