Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33f..04m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P33F-04
Astronomy and Astrophysics
Astrophysics
[5422] Planetary Sciences: Solid Surface Planets / Ices, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
Cassini gravity data for Titan is consistent with a hydrostatic interior and implies an incomplete separation of rock from ice. Simple 2-layer models of the interior have been proposed, in which Titan possesses a "core" and an "ice" layer. In the following, this ice layer should be understood to itself likely consist of higher and lower pressure phases of ice (and/or clathrate) separated by an internal ocean. This is not the "layering" in question in the title; here we address the gross structure of Titan, and what this might tell us about the accretion, evolution, and bombardment history of large icy satellites. Two-layer models face fundamental difficulties. If the "core" is a rock-ice mixture, its average density is ≈2500-2600 kg/m3 (to match Titan's moment-of-inertia [MOI]), and the silicate volume fraction implied may be too high to permit ice-mediated convection and efficient heat transport from the interior. Alternately, if the core is assumed to be a low-density carbonaceous rock, it must be iron deficient (non-solar) and hydrated. We have reexamined models of solar-composition rock for the outer solar system, and guided by high-pressure (multi-GPa) experiments, constructed an appropriate mineralogy based on up-to-date solar abundances (NCFMASNiSu system) and a maximum degree of hydroxylation and carbonation. In order of decreasing abundance, this rock model consists of iron-bearing antigorite, pyrrhotite, calcite, natrite, diaspore, and millerite, minerals stable at Titan core pressures and moderate temperatures. The STP density is 3,000 kg/m3, and is predominantly antigorite (74%) with appreciable sulfide (18%), but no iron metal or magnetite. No simple 2-layer model of Titan can be constructed with this rock that matches Titan's density and MOI. (And such rock as part of a rock-ice "core" guarantees that the rock volume fraction would exceed the critical value (~60%) where the viscosity of the mixture is controlled by a close-packed rock framework. Ice melting and catastrophic differentiation should ensue.) More critically, a low-density rock core is unlikely to resist dehydration and densification as it heats radiogenically. Even assuming such favorable conditions as late core formation at 500 m.y. after accretion (e.g., carapace infall) and leaching of appreciable 40K (say, 30%) to the ice (ocean) layer, antigorite dehydration begins at 2 b.y. for realistic thermal properties, and is complete for the majority of the core by 4.5 b.y. (the outer portion of such a core always remains cool). Under a variety of thermal evolution scenarios, only extreme assumptions permit Titan's core to remain cool and undehydrated. Thus, thermochemically realistic, solar-composition-rock-bearing, interior models of Titan that satisfy its density and MOI must contain 3 (or more) layers, specifically, a mixed ice-rock layer of some sort must exist between the complex "ice" layer and a rock core. This structure (and the atmosphere) offer constraints on conditions during accretion (rate, timing, rock "particle" size distribution) and the role (if any) of a Late Heavy Bombardment in the outer solar system. It remains to be seen how such a structure could evolve to be Ganymede-like (plausibly driven by tidal heating) and form an metallic core without either iron metal or magnetite.
Bland Michael T.
McKinnon William B.
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