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A. J. R. Prentice (Monash University, Victoria 3800, Australia)
The fast approaching arrival of the Cassini-Huygens spacecraft at Saturn has renewed interest in the origin of this planet and its complex system of moons. Here I report a predicted bulk chemical composition for Phoebe and Titan based on the idea that these bodies are captured satellites which originally condensed as planetesimals within the gas ring shed by the proto-Solar cloud at Saturn's initial orbit. For Phoebe a capture origin is clear owing to the retrograde sense of its orbital motion about Saturn. For Titan the case for capture rests on the large disparity (by a factor ~58) between the masses of Titan and Rhea.
According to one theory of Solar system origin (Prentice,Moon & Planets 19 341 1978, Earth, Moon & Planets 87 11 2001, ref* http://www.lpi.usra.edu/meetings/mercury01/pdf/8061.pdf), both the planetary system and the regular satellite systems of the giant planets condensed from concentric families of orbiting gas rings. These rings are shed by the parent cloud as a means for disposing of excess spin angular momentum during gravitational contraction. The orbital radii Rn and masses mn (n = 0,1,2,... ) of the rings satisfy the equations Rn/Rn+1 = (1+mn/Mnfn)2 . Here Mn, fn denote the residual mass and moment-of-inertia factor of the cloud after shedding the nth ring. If the cloud contracts uniformly, then fn, mn/Mn and Rn/Rn+1 remain constant. Further, taking fn = 0.01 for the proto-Saturnian cloud (Earth, Moon & Planets 30 209 1984), Mn = MSat and (Rn/Rn+1) = 1.298, we get mn = 8.0 x 1026 g. Such a mass of gas of Solar composition contains a condensable mass of rock, H2O, and NH3 ices that is mcond = 1.07 x 1025 g. The mass of Rhea (2.32 x 1024 g) is consistent with this estimate, allowing for inefficiency in the process of satellite accretion. But Titan's mass exceeds mcond by a factor of ~13, so speaking against a native origin.
The temperature and mean-orbit pressure of the gas ring shed by the proto-Solar cloud at Saturn's initial orbit Rn = 8.1 AU, where Mn= 1.173MSun, are Tn = 99 K, pn = 5.7 x 10-7 bar (see ref*). The bulk chemical constituents of the condensate are rock (mass fraction 0.378), water ice (0.595) and graphite (0.027). The rock consists mostly of spinel (0.032), akermanite (0.044), MgO-SiO2 (0.494), Fe3O4 (0.193), FeS (0.189) and Ni-NiS (0.019). The mean density is \rhocond = 1.33 g/cm3. This is the predicted upper bound for Phoebe's mean density, assuming zero porosity. The lower bound is 0.67 g/cm3, for 50 % porosity.
The author(s) of this abstract have provided an email address for comments about the abstract: andrew.prentice@sci.monash.edu.au
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Bulletin of the American Astronomical Society, 36 #2
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