  The Jovian regular satellite system mainly consists of four Galilean
satellites that have similar masses and are trapped in mutual mean motion
resonances except for the outer satellite, Callisto. On the other hand, the
Saturnian regular satellite system has only one big icy body, Titan, and a
population of much smaller icy moons. We have investigated the origin of these
major differences between the Jovian and Saturnian satellite systems by
semi-analytically simulating the growth and orbital migration of
proto-satellites in an accreting proto-satellite disk. We set up two different
disk evolution/structure models that correspond to Jovian and Saturnian
systems, by building upon previously developed models of an actively-supplied
proto-satellite disk, the formation of gas giants, and observations of young
stars. Our simulations extend previous models by including the (1) different
termination timescales of gas infall onto the proto-satellite disk and (2)
different evolution of a cavity in the disk, between the Jovian and Saturnian
systems. We have performed Monte Carlo simulations and show that in the case of
the Jovian systems, four to five similar-mass satellites are likely to remain
trapped in mean motion resonances. This orbital configuration is formed by type
I migration, temporal stopping of the migration near the disk inner edge, and
quick truncation of gas infall caused by Jupiter opening a gap in the Solar
nebula. The Saturnian systems tend to end up with one dominant body in the
outer regions caused by the slower decay of gas infall associated with global
depletion of the Solar nebula. The total mass and compositional zoning of the
predicted Jovian and Saturnian satellite systems are consistent with the
observed satellite systems.
