Other issues also seem difficult to explain. For instance, one might expect the outermost Galilean satellite to have sig- nificantly less angular momentum than the preceding satellite. If we insist in forming all the Galilean satellites out of a more or less uniform accretion disk, the size of Callisto would seem to require a disk with a very sharp outer cut-off outside Callisto. It seems unlikely that the satellite disk would have enough surface density to make a satellite the size of Callisto at 26 Rj , but form no smaller objects further out. Furthermore, the sep- aration between Ganymede and Callisto (~ l(LRj) is so large that one is led to wonder why there are no satellites in between at ~ 20 Rj.
