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[25.02] Satellite Torques in a Two-Component Giant Planet Subnebula

P. R. Estrada (Cornell University), I. Mosqueira (NASA Ames Research Center)

We model the subnebulae of Jupiter and Saturn wherein satellite accretion took place. We expect a giant planet subnebula to be composed of an optically thick (given gaseous opacity) inner region inside of the planet's centrifugal radius (located at r_c^J ~15 R_J for Jupiter and r_c^S ~22 R_S for Saturn), and an optically thin, extended outer disk out to a fraction of the planet's Roche lobe, which we choose to be ~ R_roche/5 (located at ~150 R_J near the inner irregular satellites for Jupiter, and ~200 R_S near Phoebe for Saturn). This places Titan and Ganymede in the inner disk, Callisto and Iapetus in the outer disk, and Hyperion in the transition region. The inner disk is the leftover of the gas accreted by the protoplanet at the end of planetary accretion. The outer disk results from the solar torque on nebula gas flowing into the protoplanet during the time of giant planet gap opening (Bryden et. al. 1999, ApJ 514, 344). For the sake of specificity, we use a cosmic mixture "minimal mass" model to constrain the gas densities of the inner disks of Jupiter and Saturn. For the total mass of the outer disk we use the simple scaling M_disk ~M_P t_gap/t_acc, where M_P is the mass of the giant planet, t_gap is the gap opening timescale, and t_acc is the giant planet accretion time. This gives a total outer disk mass of ~ 100 M_Callisto for Jupiter and possibly ~100 M_Iapetus for Saturn (which contain enough condensables to form Callisto and Iapetus respectively). Our model has Ganymede at a subnebula temperature of ~250 K, Titan at ~100 K and Rhea at ~250 K. The outer disks of Jupiter and Saturn have constant temperatures of 130 K and 90 K respectively.

We compute the torque as a function of position and show that although the torque exerted on the satellite is generally negative, which leads to inward migration as expected (Ward 1997, Icarus 126, 261), there are regions of the disk where the torque is positive. For our model these regions of positive torque correspond roughly to the locations of Callisto and Iapetus. Though the outer location of zero torque depends on the (unknown) size of the transition region, the result that Saturn's is found much farther out (at ~3 r_c^S) than Jupiter's (at ~2 r_c^J) is mostly due to Saturn's less massive outer disk, and larger Hill radius.

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