Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment

Abstract

Jupiter’s large moons Ganymede and Callisto are similar in size and composition, but different in surface and interior characteristics. Simulations with geophysical models of core formation indicate that the difference in impact energy received by the two satellites during the period of late heavy bombardment can explain the dichotomy. Jupiter’s large moons Ganymede1,2 and Callisto2,3 are similar in size and composition. However, Ganymede has a tectonically evolved surface1 and a large rock/metal core2, whereas Callisto’s surface shows no sign of resurfacing3 and the separation of ice and rock in its interior seems incomplete2. These differences have been difficult to explain4,5,6,7,8,9,10,11. Here we present geophysical models of impact-induced core formation to show that the Ganymede–Callisto dichotomy can be explained through differences in the energy received during a brief period of frequent planetary impacts about 700 million years after planet formation, termed the late heavy bombardment12,13,14,15. We propose that during the late heavy bombardment, impacts would have been sufficiently energetic on Ganymede to lead to a complete separation of rock and ice, but not on Callisto. In our model, a dichotomy between Ganymede and Callisto that is consistent with observations is created if the planetesimal disk that supplied the cometary impactors during the late heavy bombardment is about 5–30 times the mass of the Earth. Our findings are consistent with estimates of a disk about 20 times the mass of the Earth as used in dynamical models that recreate the present-day architecture of the outer solar system and the lunar late heavy bombardment15,16.