Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v12b..04d&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V12B-04
Physics
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5475] Planetary Sciences: Solid Surface Planets / Tectonics
Scientific paper
The differentiation of terrestrial planets with segregation of metal from silicates took place contemporaneously with the last stage of planetary accretion and was likely completed in few tens of Myrs. The separation of metal occurs when at least the metallic phase is liquid and proceeds via a combination of transport by diapiric instabilities, a more diffuse percolation flow and liquid separation in a magma ocean. The complex nature of the problem (i.e., the presence of multiple phases, density anomalies comparable in magnitude to the density itself) has posed challenges to numerical modeling attempts. We present a formalism derived from a two-phase model by Bercovici et al. (2001). Our model can handle two components simultaneously, silicates and metal. Depending on local temperature, the metal is present either in solid or liquid state. Below the melting point of the metal, the two components are locked together and their mixture is treated as a single phase fluid where density is a function of composition (iron/silicate proportions). When the metal is liquid, it can separate from the silicates and the two phases interact through shear stress (e.g., Darcy flow) and normal stress. Energy conservation takes into account the different mechanisms by which the gravitational energy is dissipated as heat. We show 1-D spherical calculations of the thermal evolution of growing planetesimals. These accreting bodies are initially heated from within by the decay of short-lived radionuclides and in later stages by impacts in a near-surface layer. We further present 2-D Cartesian numerical simulations of core-mantle differentiation set off by an impact event on a planetary embryo. We show that the first impact that melts the iron phase near the surface is potentially able to trigger the whole core-mantle segregation in a runaway phenomenon, and possibly results in the formation of a magma ocean at the surface of the growing planet. The segregation of the metal in regions where the silicates remain solid occurs by a mechanism that has not been suggested before and which is a combination of the usual diapiric instability and a porosity wave.
Dubuffet Fabien
Ricard Y. R.
Šrámek Ondřej
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