HR: 1340h
AN: MR13A-0994 [Abstracts]
TI: Modes of Planetary Reshaping During Core Formation: Numerical Study
AU: * Lin, J
EM: linj@erdw.ethz.ch
AF: Institute of Oceanography, National Taiwan University, Taipei, 106, Taiwan
AU: Gerya, T V
EM: taras.gerya@erdw.ethz.ch
AF: Geophysical Fluid Dynamics Group, Institute of Geophysics, Department of
Geosciences,ETH-Zurich, HPP L.8, Hoenggerberg, Zurich, 8093, Switzerland
AU: Tackley, P J
EM: paul.tackley@erdw.ethz.ch
AF: Geophysical Fluid Dynamics Group, Institute of Geophysics, Department of
Geosciences,ETH-Zurich, HPP L.8, Hoenggerberg, Zurich, 8093, Switzerland
AU: Yuen, D A
EM: daveyuen@gmail.com
AF: University of Minnesota Supercomputing Institute and Department of Geology and
Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AB:
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.
DE: 0545 Modeling (4255)
DE: 8115 Core processes (1213, 1507)
DE: 8125 Evolution of the Earth (0325)
DE: 8147 Planetary interiors (5430, 5724, 6024)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting