HR: 1340h
AN: P53A-1447    [Abstracts]
TI: Titan: Predicted Bulk Chemical Composition and Interior Structure for a Capture Origin
AU: * Prentice, A J
EM: andrew.prentice@sci.monash.edu.au
AF: Monash University, School of Mathematical Sciences, Monash University, Clayton, VIC 3800 Australia
AB: I report a predicted bulk chemical composition and internal structure for Titan based on the idea that this body is a captured satellite of Saturn which originally condensed within the gas ring shed by the proto-Solar cloud (PSC) at Saturn's initial helio-centric distance $\sim 8.1$ AU . The case for capture rests on the large disparity (by a factor of $\sim $58) between the masses of Titan and Rhea. Rhea's mass ($2.3 \times 10^{24}$ g) is consistent with the mass $m_{cond} = 9.3 \times 10^{24}$ of rock, H$_{2}$O, and NH$_{3}$ ices expected for a native moon of Saturn, had Rhea condensed from a gas ring shed by the proto-Saturnian cloud (Prentice, \textit{JPL Pub.} 80-80 1980; \textit{Proc. Astron. Soc. Australia} 4 164 1981; \textit{Earth, Moon Planets} 30 209 1984). Here I assume an efficiency of 25% in the process of satellite accretion and adopt the proto-solar elemental abundances of Lodders (\textit{Astrophys.J} 591 1220 2003). Titan's mass exceeds $m_{cond}$ by a factor of $\sim 14$, so speaking against a native origin (http://www.aas.org/publications/baas/v36n2/aas204/887.htm). Previously it has been supposed that the process of shedding discrete gas rings by the parent gas cloud comes about solely through the action of large turbulent stresses arising from powerful convective motions (Prentice, \textit{Moon & Planets} 19 341 1978, \textit{Earth, Moon & Planets} 87 11 2001, http://www.lpi.usra.edu/meetings/mercury01/pdf/8061.pdf). This has necessitated convective speeds $v_{t}$ up to $\sim 5$ times the local adiabatic sound speed $v_{s}$, which is unacceptable. An exact numerical simulation of supersonic turbulent convection in a model atmosphere which represents the outer layers of the PSC shows, however, that the upper layers are strongly super-adiabatic (Prentice & Dyt, \textit{MNRAS} 341 644 2003). This results in a natural density inversion at the top boundary . Gas ring shedding can now be achieved for speeds $v_{t} \leq 3v_{s}$, which is OK. A new model for PSC has thus been constructed to include the influence of very strong super-adiabaticity. The controlling paramters are chosen so that the mean density of the condensate at the orbit of Mercury matches the inferred uncompressed value $\rho_{unc} = 5.3$ g/cm$^{3}$ and that the fraction of water vapour in the gas ring at Jupiter's orbit which condenses is $\phi_{\mathrm{H_{2} O}}$ = 0.665. This later accounts for the densities of Ganymede and Callisto, following condensation from the gas rings shed by proto-Jovian cloud (Prentice 2001). At Saturn's initial orbit, where the gas ring temperature is $T_{n}= 94$ K and the mean orbit pressure $p_{n}= 4.7 \times 10^{-7}$ bar, the bulk chemical constituents of the condensate are anhydrous rock (mass fraction 0.494), water ice (0.474) and graphite (0.032). The mean density is 1.52 g/cm$^{3}$. Structural models for a present-day Titan based on this composition yield mean densities of 2.10 g/cm$^{3}$ (homogeneous case) and 1.93 g/cm$^{3}$ (differentiated 2-zone case). For the latter, $C/MR^{2}$ = 0.32. Titan is thus most likely fully differentiated between its rock, graphite and water ice constituents. It is predicted that Titan has no internal ocean or induced magnetic field but it may possess a small magnetic dipole moment of magnitude $\sim 2\times 10^{11}$ T m$^{3}$. This was acquired through thermoremanence at $\sim 1.5 \times 10^{9}$ yr after satellite formation. Capture of Titan was achieved by gas drag within the proto-Saturnian envelope whose initial size was $\sim 60 R_{Sat}$. Titan's surface should thus look much like that of Triton. I thank John D. Anderson [NASA/JPL] for much support, and Nicole Rappaport and Bob Jacobson for helpful discussions.
DE: 5410 Composition
DE: 5430 Interiors (8147)
DE: 5440 Magnetic fields and magnetism
DE: 5455 Origin and evolution
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting