Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p22a..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P22A-01
Mathematics
Logic
[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
The 40Ar measured by the Huygens Gas Chromatograph Mass Spectrometer in Titan’s lower atmosphere represents approximately 7-9% of the radiogenic argon produced within Titan to date, assuming a rock mass fraction of ≈55% and a CI-chondrite-like potassium abundance. As such, the overall Ar-degassing efficiency of Titan is more akin to that of Mars, or possibly Venus, than that of the Earth. Titan’s normalized moment-of-inertia (0.34) implies a partially differentiated structure, which can be generally described as possessing a rock+metal core, a middle layer of mixed rock+ice, and a rock-poor upper sequence of, in order of decreasing depth, high-pressure ices, ocean, and ice I and/or clathrate. Assuming hydrostatic equilibrium, Titan is at least 40% differentiated, meaning rock separation from ice, with the principal uncertainty being the mass of the discrete rock+metal core. Such a core may have reaching magmatic temperatures due to long-lived radiogenic heating, and 40Ar would partition into such melts, but at Titan core pressures (>2 GPa), argon is highly soluble in silicate melts and would not degas as do magmas that reach the surfaces of the terrestrial planets. Whether radiogenic argon produced within cool rock fragments suspended in the slowly convecting, mixed ice-rock middle layer diffuses into the ice matrix depends on whether the rock is altered (hydrated and oxidized) or not. Argon forms a clathrate with water ice to high pressures, however (at least to 3 GPa), so 40Ar produced in the mixed ice+rock layer and released to the surrounding ice matrix has likely remained trapped there. In addition, material transfer across the boundary between the mixed rock+ice layer and the ice layer above should have been quite limited (that convection may not penetrate the ice II-ice V boundary either [McKinnon 1998, in Solar System Ices, Kluwer] does not help). The logical source of Titan’s atmospheric 40Ar is thus the upper sequence of ocean and ice layers, which while nominally rock free may nonetheless have acquired a non-negligible amount of suspended rock fines and dissolved potassium during early melting and differentiation. Over time, such fines and potassium should concentrate in the internal ocean as the bounding ice layers thicken with declining heat flow. At least two pathways appear viable for 40Ar release to the atmosphere from Titan’s ocean: cryovolcanism, and impact disruption of a thin, clathrate-dominated crust, but in both cases the Ar-degassing efficiency from the ocean itself, as opposed to Titan as a whole, would have to have been high. A third alternative posits that Titan accreted undifferentiated, and that its differentiation was a product of the Late Heavy Bombardment. Melting and differentiation of >40% of Titan at 3.9 GYA could have released more than enough extant 40Ar to account for Titan’s present atmospheric inventory.
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