Differences in the energy delivered by impacts to Jupiter's large moons Callisto and Ganymede during the period of late heavy bombardment, about 3,800 to 4,100 million years ago, can explain their divergent characteristics, according to a study published online this week in Nature Geoscience. The findings explain why Ganymede has a large core of rock and metal, whereas the separation of ice and rock within Callisto is incomplete, even though the moons are very similar in size and composition.

Amy Barr and Robin Canup developed a model of melting and core formation in the presence of planetary impacts. They found that, if sufficient energy is released during a series of impacts, the process of ice-rock separation and core formation can become self-sustaining and will drive itself to completion. Ganymede experienced such self-sustaining separation, they suggest, whereas Callisto did not, because Jupiter's gravity field directed more impact energy towards Ganymede.