Gap-opening and the Survival and Composition of the Galilean Satellites

We expect that large objects forming in the planetary subnebula truncated the gas disk in which they were formed (Mosqueira and Estrada 2002a,b). The inertial mass (Ward and Hourigan 1989) can be used to constrain the disk surface density as a function of position at the time of gap-opening. Given weak Type II migration, this may explain the survival of the Galilean satellites in the subnebula. It has often been noted that the compositional gradient of the Galilean satellites suggests a temperature gradient in the subnebula at the time of their formation. However, this is not a sufficient condition to expect such a trend. There are at least three issues that complicate the picture considerably. First, the satellites themselves may have migrated from cool regions to hot regions of the disk due to the gas tidal torque. Second, water may be delivered to satellites inside the water-ice condensation line by satellitesimals that form outside this location and then drift in by the effects of gas drag. Third, a satellite inside the water-ice condensation line at one time may still receive water from its own feeding zone following planetary cooling and water condensation, i.e., the water-ice condensation line itself will drift inwards, which might lead to inhomogeneous accretion, but would not deprive a satellite of its water component. Yet, a detailed model of accretion in the dense Jovian subnebula in the presence of a strong temperature gradient may lead to a compositional gradient as observed. First, because Ganymede probably underwent limited radial migration before it opened a gap. Second, because Ganymede probably intercepted most satellitesimals originating outside the water-ice condensation line. Third, because gap-opening will clear the gas between Io and Ganymede in a timescale considerably shorter than the planetary cooling time.