Astronomy and Astrophysics – Astronomy
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p41b..04p&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P41B-04
Astronomy and Astrophysics
Astronomy
5410 Composition (1060, 3672), 5422 Ices, 5455 Origin And Evolution, 6280 Saturnian Satellites, 6281 Titan
Scientific paper
I report the results of calculations for the bulk chemical composition and physical structure of Titan to coincide with the T-11 first gravity flypast by the Cassini spacecraft on 2006 February 27. The results are based on the hypothesis that Titan is a captured moon of Saturn (Prentice 1984 Earth Moon Planets 30 209-228; 2004 36th DPS meeting , abstract no. 20.14; http://arxiv.org/abs/astro-ph/0602512). The motivation for capture is that Titan is ~ 14 times more massive than the value expected of a native moon of Saturn. Rhea's mass and those of the smaller moons are consistent with the values anticipated for condensation within the family of gas rings that were shed by the proto-Saturnian cloud, during the cloud's gravitational contraction from initial radius 30RSat to Saturn's present size RSat = 60268 km (Prentice 2005, 36th Lunar Plan. Sci. Conf., abstract no. 2378; 2006, Publ. Astron. Soc. Australia (PASA) 23, 1-11). It is proposed that Titan condensed as a secondary embryo in the gas ring that was shed by the proto-solar cloud at Saturn's orbit. Saturn's core of mass ~ 10 M♁ was the primary solid aggregate within that gas ring. The gas ring temperature is 94 K and the mean orbit pressure is 4.9 × 10-7 Bar. The bulk chemical composition of the condensate is rock (mass fraction 0.492), water ice (0.474) and graphite (0.034). The rock is almost anhydrous and has mean density 3.667 g/cc (cc = cubic centimetre) at 94 K and 1 Bar pressure. The condensate mean density is 1.522 g/cc. A suite of thermally-evolved structural models has been constructed for this compositional mix. A chemically homogeneous model has mean density ρ = 2.10 g/cc. A 2-zone model possessing a rock-graphite core of mean temperature 600 K has ρ = 1.94 g/cc. This is close to the observed value, namely 1.88 g/cc. A 3-zone model with a solid central FeS-NiS-Ni core, having fraction 0.212 of the total satellite mass, has ρ = 2.03 g/cc. Lastly, a new 2-zone model was constructed with the ice mantle mass fraction increased to 0.506 in order to match the Titan mean density. The axial moment-of-inertia coefficient of this model is C/MR2 = 0.317. I predict that the upper mantle of Titan may contain mass anomalies. These correspond to the residue of two former Rhea-sized moons of Saturn at orbital radius ~ 17RSat and ~ 24RSat that were destroyed by impact when Titan was captured from solar orbit. Hyperion is a remnant of one such moon. The buried moons contain ammonia ice (mass fraction 0.240) and methane (as clathrate hydrate ice, 0.057) but no Ar hydrate. These ices are the source of Titan's N2 - CH4 atmosphere. No internal ocean is expected. I thank George W. Null (NASA/JPL) for much encouragement and support.
ph/0602512
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