CO2-rich impact craters on Callisto

Carbon dioxide is present over most of Callisto's surface and is often concentrated in and around morphologically fresh and bright impact craters. The depths of the 4.26-micron absorption band in reflectance spectra returned by the Near-Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft imply a CO2 abundance in the top 1 mm of the surface that is not greater than 0.3 wt percent. This absorption band is due to CO2 that is contained in the nonice material. Ice on Callisto may also contain CO2, but the ice appears to be large-grained and therefore is not sufficiently reflective for an absorption band at 4.26 microns to be visible. The depth of the CO2 absorption band has at most only a very slight photometric dependence. The distribution of CO2 around morphologically fresher impact craters suggests its origin there is related to the impact process, but there is no indication that impactors are the source of the impact-related CO2, (e.g., known comet chains are not CO2-rich). These distributions of CO2 also suggest that the enrichment of CO2 in and around impact craters is a transient phenomenon. The increased CO2 abundance around impact craters on Callisto is associated with (1) the dark, less icy material within and nearby the craters, (2) occasionally the icy material inside the craters and their icy rims, (3) their continuous ejecta blankets, and, if existing, (4) their icy ejecta. (Author)