Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13c1432p&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P13C-1432
Astronomy and Astrophysics
Astronomy
5410 Composition (1060, 3672), 5430 Interiors (8147), 5455 Origin And Evolution
Scientific paper
I report calculations for the predicted chemical composition and physical structure of Iapetus. The results are based on the hypothesis that Iapetus is a native moon of Saturn which initially formed closer to the planet at orbital distance of ~ 12RSat , where RSat = 60268 km (Prentice 1984 Earth Moon Planets 30 209- 228; 2006 Publ. Astron. Soc. Australia (PASA) 23, 1-11). Iapetus was scattered tidally to its current orbit by Titan, which is assumed to be a captured moon of Saturn (2005 LPSC XXXVIII, # 2402). That is, Titan condensed as a secondary body in the same solar orbit as Saturn. Initially Titan's orbit was very eccentric, having a peri-Saturn distance of ~ 10RSat. All of Saturn's native moons, including 2 former moons that existed at radii ~ 17RSat and ~ 24RSat, condensed from a concentric family of gas rings. The rings were shed by the proto-Saturnian cloud to rid spin angular momentum during gravitational contraction. If the contraction of the cloud is homologous, then the sequence of gas rings (n = 0,1,2,...) are geometrically spaced and their temperatures Tn vary with mean orbital distance Rn according as Tn ~ 1/Rn, setting aside the heat due to the early Sun. If the Tn are scaled so that Enceladus condenses just inside the stability field of liquid water, then for Iapetus Tn=95 K and the condensate consists of hydrous rock (mass fraction 0.336), water ice (0.344), ammonia ice (0.267) and clathrated methane (0.053). The density of the rock at 76 K and 0.1 Mpa is 3.154 g/cc. The condensate mean density is ρ = 1.185 g/cc. A chemically uniform Iapetus model for this mix has ρ = 1.210 g/cc. The Iapetus density is 1.083 g/cc. Next, a 4-zone differentiated model was constructed. This consists of a rocky core surrounded by separate layers of water ice, ammonia ice and methane ice - all in the same proportions as above. This model has ρ = 1.123 g/cc. Lastly, a 5-zone model was made by adding a crust composed of the initial homogeneous mix and having a porosity of 58%. A crustal thickness of ~ 16 km yields a model whose ρ=1.083 g/cc. The axial moment-of-inertia coefficient of this model is 0.311. It is proposed that tidal heating due to the satellite's orbital displacement by Titan caused all of the ices (except for those of the crust) to melt and for the moon to differentiate. Most likely, the equatorial ridge of Iapetus was formed by the subsequent solidification and contraction of the liquid interior.
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