Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmmr13a0994l&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #MR13A-0994
Physics
0545 Modeling (4255), 8115 Core Processes (1213, 1507), 8125 Evolution Of The Earth (0325), 8147 Planetary Interiors (5430, 5724, 6024)
Scientific paper
The early stages of terrestrial planetary accretion and differentiation related to core formation are largely enigmatic and require extensive realistic numerical modelling efforts especially in 2D(a cross-section of a spherical planet) and 3D geometries. One early stage of terrestrial planets was assumed to have a gravitationally unstable three-layer structure, the innermost undifferentiated solid core, the intermediate metal-melt layer, and the outermost silicate-melt layer, which leads to a Rayleigh-Taylor instability of various orders. We have developed a 2D thermomechanical numerical model for studying core formation in a self-gravitating planetary body surrounded by mass-less weak medium by using a combination of finite-differences with a Lagrangian marker-in-cell technique on a fully staggered Cartesian grid. We include a free planetary surface, spontaneously evolving gravity field, visco(elasto)plastic rheology of materials and feedback from shear heating. Benchmarking of this novel numerical method against available analytical solutions (Ida et al., 1987, Earth Moon Planets, 44, 149-174) has demonstrated high accuracy of the numerical results in the non inertial reshaping regime. Assuming the three-layered model (primordial protocore, metal and silicate layers) we investigated the influence of the viscosity contrast between the layers on the geometrical mode of planetary reshaping. In contrast to a previously conducted numerical study (Honda et al., 1993, JGR, 98, 2075-2089) we explored a broad range of viscosity ratios between the metallic layer and the protocore (0.001-1000) as well as between the silicate layer and the protocore (0.001-1000). A new important prediction from our study is that realistic modes of planetary reshaping characterized by a high viscosity contrast between the cold protocore and hot molten silicate layer always results in the transient exposure of the prorotocore to the planetary surface during the early stages of core formation. This causes large lateral variations in the thickness of a global magma ocean (which should have major geochemical consequences) and creates possibility of destruction and reworking of the exposed protocore by ongoing planetesimal impacts. The dominant L=1 nature of this instability may explain the crustal dichotomy on planets such as Mars.
Gerya Taras V.
Lin Juhn-Jong
Tackley Paul J.
Yuen Dave A.
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