Ganymede and Callisto are the two largest moons of Jupiter. They’re similar in size and chemical composition (half rock, half ice), and both may have subsurface oceans, but their differences — Ganymede has a differentiated core and is the only moon in the solar system with a magnetosphere — have been confounding planetologists. Researchers at the Southwest Research Institute have proposed an explanation that has to do with the Late Heavy Bombardment, approximately four billion years ago, when comets and asteroids pelted the Earth’s Moon and other terrestrial planets. When these impactors hit Ganymede and Callisto, the ice at the impact site melted. Ganymede is a lot closer to Jupiter than Callisto; according to the researchers’ model, thanks to Jupiter’s gravity, about twice as many impactors hit Ganymede than hit Callisto — and they hit a lot harder, too. The net effect is that a lot more of Ganymede’s ice turned to liquid water, and the remaining rocky materials sank and settled in the core. Neat. News coverage: Astronomy, CBC News, Universe Today.
Callisto escaped a lot of the Late Heavy Bombardment because it’s so far away from Jupiter — at about 1.9 million kilometres, it’s the most distant of Jupiter’s four major (Galilean) moons. That distance means it’s not in an orbital resonance with the other three moons (Io, Europa and Ganymede are in a 1:2:4 resonance, which means that Io completes four orbits for every two of Europa’s and one of Ganymede’s). It’s still tidally locked, though. Callisto is also far enough away that it does not receive nearly as much radiation as the other major moons. Its 0.01 rem per day is a lot more manageable than Ganymede’s 8 rem per day (nearly six times the yearly limit for radiation workers), Europa’s 540 rem per day (60 percent fatality after 30 days), or Io’s 3,600 rem per day (100 percent fatality after seven days). If human beings are going to set up a base in the Jupiter system, it’s going to be on Callisto.