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I. Mosqueira (NASA Ames/SETI Institute), P. R. Estrada (NASA Ames)
Phoebe's retrograde, eccentric and inclined orbit marks it as an object captured from heliocentric orbit. Accordingly, its composition may be indicative of its origin in the solar nebula. Analogous arguments have been made extensively in connection with the origin of Pluto-Charon (see, e.g., McKinnon et al. 1997) as well as Triton (McKinnon and Mueller 1989). Indeed, the demarcation between nebula and subnebula objects has led a number of workers (see, e.g., Johnson et al. 1987; Lunine et al. 1993; Podolak et al. 1993) to argue that the regular satellites of the giant planets did not derive the bulk of their material directly from heliocentric orbit.
The recent Cassini flyby of Phoebe has yielded a mass for this object of GM = 0.5527 ±0.001 km3/s2 Jacobson et al. 2004 (this conference). Its density of 1.6 g/cm3 indicates a rock to ice ratio of at least 50 % (Porco et al. 2004; Science, to be submitted). Phoebe's high rock/ice ratio when compared to the icy Saturnian satellites reinforces the argument that Phoebe is an object that formed in heliocentric orbit and became captured. Yet, given that it may be misleading to lump together satellites with quite different formation histories, we refine the comparison on the basis of models for regular satellite formation. Because it derives condensables directly from heliocentric orbit and fails to consider planetesimals, the model of Canup and Ward (2002) does not provide a context for understanding such compositional differences. We will therefore discuss two models of satellite formation we are developing, which differ mainly in their treatment of turbulence (decaying vs steady). In both models the inner (located inside Titan's orbit), icy Saturnian satellites represent a second generation of objects. Mosqueira and Estrada (2003a,b) has these satellites forming 104-105 years after Titan as the disk became optically thin and water rich due to preferential gas drag loss of silicates as Saturn cooled. On the other hand, the gas-poor planetesimal-capture model of Estrada and Mosqueira (2003, 2004, this conference) has them forming from the impact ejecta (which presumably avoided re-accretion by gas drag inward migration) between Titan and a Triton-sized differentiated interloper, leading to Titan's eccentricity and likely causing it to differentiate (assuming it was not differentiated to begin with). In either case, these satellites may not be representative of the bulk composition of regular satellites. Furthermore, Titan's higher density and possible size-selective devolatilization (Stevenson et al. 1986) may also cloud the link between origin and composition. However, we argue that Iapetus would not have been affected by these processes, and so it may furnish a more direct test of whether the regular satellites of Saturn could have derived the bulk of their material directly from heliocentric orbit. At the time of submission, Iapetus' mass GM = 118 ±11 km3/s2 (though a systematic source of error hasn't been ruled out; Jacobson, pers. comm.) and mean radius of 718 ±8 km (Dermott and Thomas 1988) yields a density of 1.14 ±0.1 g/cm3, which implies processing of planetesimal composition prior to regular satellite formation and favors the satellite formation model of Mosqueira and Estrada (2003a,b), but future Cassini Iapetus flybys may be needed to settle this issue. It is possible that by the time of this conference an improved constraint on this number will be available (Jacobson, pers. comm.).
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.