DDA 33rd Meeting, Mt. Hood, OR, April 2002
Session 14. Satellites in the Outer Solar System
Wednesday, April 24, 2002, 10:20am-12:00noon

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[14.05] Satellite formation around gas giant planets

R. M. Canup, Wm. R. Ward (Southwest Research Institute)

We consider a scenario in which the regular satellites of gas giants form within circumplanetary accretion disks produced during the end stages of gas accretion (e.g., Lubow et al. 1999; D'Angelo et al. 2002). Assuming some inflow rate of gas and solids, a steady-state circumplanetary gas disk is produced through a balance of the inflow supply and the disk's internal viscous evolution (Lynden-Bell and Pringle 1974). The disk's radial thermal profile is determined by a balance of radiative cooling from the disk with heating from the planet's luminosity, viscous dissipation, and ambient nebular insolation. Once in circumplanetary orbit, inflowing solids accumulate into objects large enough to decouple from the gas on time scales much shorter than their lifetime against inward decay due to gas drag. Both the total mass of solids and the solids-to-gas mass ratio in the disk thus build-up over time, with satellites accreting at a rate regulated by the inflow flux.

In the Jovian system, the ice-rich composition of Ganymede and Callisto, as well as the apparently incomplete differentiation of Callisto (e.g., Anderson et al. 1998, 2001), both provide constraints on the disk environment in which the regular satellites formed. In addition, the presence of the four large Galilean satellites implies that some satellites (at least the last generation) were able to survive against inward orbital decay due to Type I interaction with their precursor disk. We have found that these constraints can be best satisfied for a circumjovian disk supplied by a slow gas inflow rate, F, of F < MJ/(few x 106 yrs), where MJ is a Jovian mass (Canup and Ward 2001). Such a slow inflow rate yields a much lower steady-state gas surface density than is implied by augmenting the mass of the current satellites to solar elemental composition, as has been done previously (Lunine and Stevenson 1982; Coradini et al. 1989). Here we examine whether such a "gas starved" accretion disk could be applicable to the Saturnian system as well. A particular puzzle is how a Jovian protosatellite accretion disk could yield a system of multiple large satellites, while in the case of Saturn, the single satellite Titan overwhelmingly dominates the total satellite system mass.

Support of NASA's Planetary Geology and Geophysics program is gratefully acknowledged.

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Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.