Asteroid and comet bombardment melted one of Jupiter's moons
John Timmer in Ars Technica describes a new model for formation in Jupiter’s moons:
The authors of the new paper focus on a period early in our Solar System’s history called the Late Heavy Bombardment. The record of impacts on the Moon suggest that gravitational disturbances sent about 1.6 x 10²²g of material into its surface, equally divided between comets and asteroids. That implies the disruption of a planetesimal disk that contained about 20 times the Earth’s mass, most likely via gravitational interactions with the gas giants. Obviously, a gravity well like Jupiter’s would attract a significant number of these bodies. More significantly, the planet would focus their trajectories such that the closer the moon to Jupiter, the more impacts it would receive. According to the authors, the Late Heavy Bombardment would strike Ganymede with about 80 times the mass than Callisto was hit with. They estimate that the total energy imparted to Ganymede could be up to 10³⁶erg [10³⁰ joules], enough to melt its ice five times over and allow all of the rocky material to sink to the core.
