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[20.14] Titan and Phoebe: Predicted Bulk Chemical Composition and Structure for a Capture Origin

A. J. R. Prentice (Monash University, Victoria 3800, Australia)

Here I report the results of a new set of calculations for the gravitational contraction of the proto-solar cloud (PSC) and the expected bulk chemical composition of Titan and Phoebe, based on the idea that these bodies are captured moons of Saturn. That is, it is proposed that both bodies initially condensed in solar orbits from gas rings that were shed by the PSC at the orbits of Saturn and Quaoar respectively. For Phoebe, a capture origin is clear owing to the retrograde nature of its orbit. For Titan, the case for capture rests on the large disparity (by a factor of ~ 58) between the masses of Titan and Rhea. Rhea's mass is consistent with the value expected for a native moon of Saturn, had Rhea condensed from a gas ring shed by the proto-Saturnian cloud (Prentice, JPL Pub. 80-80 1980; Proc. Astron. Soc. Aust. 4 164 1981; Earth, Moon &Planets 30 209 1984; http://www.aas.org/publications/baas/v36n2/aas204/887.htm).

A new model for PSC has been constructed to include the effect of very strong super-adiabaticity in the outer layers of the cloud, following the numerical simulation of supersonic thermal convection due to Prentice & Dyt (MNRAS 341 644 2003). I also adopt the solar elemental abundances of Lodders (ApJ 591 1220 2003). At Quaoar's orbit, where the gas ring temperature is T_n=26 K and the mean orbit pressure p_n = 1.1 \times 10^-9 bar, the bulk chemical constituents of the condensate are rock (mass fraction 0.532), water ice (0.205), dry ice (0.175), CH₄ ice (0.069) and graphite (0.019). The mean density is 1.64 g/cm³. At Saturn's orbit (T_n = 94 K, p_n= 4.7 \times 10^-7 bar), we have rock (0.494), water ice (0.474), graphite (0.032) and mean density 1.52 g/cm³. Structural models for Titan yield a mean density of 2.09 g/cm³ (homogeneous case) and 1.93 g/cm³ (differen-tiated). For the latter, C/MR² = 0.32. It is predicted that Titan has no internal ocean or induced magnetic field but it may possess a small native dipole field due to thermoremanent magnetization. Capture of Titan was achieved by gas drag within the proto-Saturnian envelope whose initial size was ~60 R_Sat. Titan should thus have the same appearance as Triton. I thank John D. Anderson [NASA/JPL] for much support.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.