  We study the possibility that the mutual interactions between Jupiter and
Saturn prevented Type II migration from driving these planets much closer to
the Sun. Our work extends previous results by Masset and Snellgrove (2001), by
exploring a wider set of initial conditions and disk parameters, and by using a
new hydrodynamical code that properly describes for the global viscous
evolution of the disk. Initially both planets migrate towards the Sun, and
Saturn's migration tends to be faster. As a consequence, they eventually end up
locked in a mean motion resonance. If this happens in the 2:3 resonance, the
resonant motion is particularly stable, and the gaps opened by the planets in
the disk may overlap. This causes a drastic change in the torque balance for
the two planets, which substantially slows down the planets' inward migration.
If the gap overlap is substantial, planet migration may even be stopped or
reversed. As the widths of the gaps depend on disk viscosity and scale height,
this mechanism is particularly efficient in low viscosity, cool disks. We
discuss the compatibility of our results with the initial conditions adopted in
Tsiganis et al. (2005) and Gomes et al. (2005) to explain the current orbital
architecture of the giant planets and the origin of the Late Heavy Bombardment
of the Moon.
