Ice Shell Structure of Ganymede and Callisto Based on Impact Crater Morphology

Understanding the thermal structure of the ice shells around Ganymede and Callisto remains a critical step in unraveling the geologic histories of the moons. The depth-diameter (d-D) trends of pristine craters on each surface have an inflection point in crater morphology at approximately 26 km diameter, at which point observed crater depths transition from craters deepening with increasing crater diameter to craters shallowing with increasing crater diameter. In this work, we use iSALE-2D to simulate impact crater formation in ice shells. We test conductive thermal gradients between 5 and 15 K/km and convective ice temperatures between 240 and 260 K in a modeled ice shell to determine if these observed d-D trends correlate to specific thermal parameters. We find that the conductive thermal gradient has a more pronounced effect than the temperature of the underlying convective ice on reproducing the d-D trends of craters up to 100 km in diameter, and that the inflection point in the observed crater depth trend on both Ganymede and Callisto is replicated in ice shells with a conductive thermal gradient of similar to 10 K/km. With this conductive thermal gradient, the conductive ice was approximately 12-14 km thick on each body at the time they accrued these unmodified craters. The similar thermal constraints for both moons suggest that there are other differences between Ganymede and Callisto responsible for their divergent evolutionary paths, such as an increased proportion of strong impurities in Callisto's ice shell.