Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p43b..04e&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P43B-04
Physics
6062 Satellites, 6218 Jovian Satellites, 6280 Saturnian Satellites
Scientific paper
Given our presently inadequate understanding of the turbulent state of the solar nebula and planetary nebulae, there are two sensible approaches to satellite formation that avoid over-reliance on specific choices for essentially free parameters. The first one postulates turbulence decay. If so, Keplerian disks must eventually pass through quiescent phases, so that the survival of satellites (and planets) ultimately hinges on gap-opening. In this scenario, the criterion for gap-opening itself sets the value for the gas surface density of the satellite disk (Mosqueira and Estrada 2003b). The second approach assumes that steady turbulence is sufficiently strong to cause the evolution of the gas disk on a shorter timescale than that for satellite formation. This approach uses the turbulence of the subnebula to remove gas from the disk but not to fine-tune the conditions of the subnebular environment. In this case, the gas surface density is left unspecified, though the presence of some gas may help to explain the observations. Satellite formation is then understood in terms of planetesimal dynamics that are largely uncoupled from the gas (somewhat analogous to the case of the terrestrial planets). We will discuss a gas-poor model with the following features: First, collisions between planetesimals in the vicinity of the giant planet leads to the formation of a protosatellite swarm of prograde and retrograde objects extending as far as ˜ RH/2 (Ruskol 1975, Safronov et al. 1986). Second, this circumplanetary swarm has a small net specific angular momentum which results in the formation of close-in, prograde satellites. Third, close to the planet, hypervelocity impacts can ultimately lead to a variety of outcomes (i.e., Jovian-like versus Saturnian-like satellite systems). Fourth, satellitesimal collisional removal from the outer disk is balanced by planetesimal collisional capture. Excluding satellite embryos, at any given time this disk mass is less than the mass of the regular satellites. Fifth, a satellite formation timescale of 105-10^6 years (consistent with a partially differentiated Callisto) controlled by the feeding of planetesimals onto the circumplanetary disk (Mosqueira et al. 2000). It might be possible to concoct a turbulent mechanism operating following a giant impact between Titan and a Triton-sized differentiated interloper (Mosqueira and Estrada, this conference) that leads to the spread of a volatile-rich disk. However, such a mechanism is very unlikely to work inasmuch as it would require an unrealistic angular momentum budget, particularly if one considers gas drag inward migration of Iapetus (gas drag would be needed to account for the lack of objects between Titan and Iapetus). Instead, the angular momentum of material fed from heliocentric orbit (gas or solids) strongly implies that Iapetus (like Callisto, ρ = 1.85 g cm-3) should be of roughly solar composition. This statement constitutes a prediction of this model and requires that the present value of the density of Iapetus (1.14± 0.1 g cm-3, Jacobson, pers. comm.) be in error. That is, within the context of a planetesimal feeding model, Phoebe's density suggests that one should expect ρ > 1.6 g cm-3 for Iapetus.
Estrada Paul R.
Mosqueira Ignacio
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