Was Ganymede once closer to Jupiter than it is now? Io, Europa, and Ganymede are in resonance around Jupiter: Europa's orbital period is exactly twice Io's, and Ganymede's is exactly twice Europa's. This was not always the case, and some amount of moon-shuffling had to take place for this resonance to occur. Some scientists think it possible that Ganymede was once much closer to Jupiter than it is now, and was then kicked outward by the gravitational interactions with Io and Europa [R. Malhotra, Icarus 94, 399 (1991)]. It's impossible to be certain, though, because multiple-body problems in astrophysics tend to be chaotic over long time periods. But there certainly are indications of this, for example Ganymede's magnetic field.