CORE FORMATION: A NEW MODELLING APPROACH

摘要

Core formation in terrestrial planets is still not well understood although this process is of importance for our understanding of the thermal evolution of a planet and the history of its magnetic field. Because core formation is among the earliest processes in planet formation and evolution, the initial conditions for thermal evolution models are, to a significant extent, determined by this process. The initial temperature of the core and its state are determined by the amount of energy dissipated during core formation. One possible scenario for the formation of a planetary core is the settling of liquid iron from a solid matrix (Stevenson, 1990). Assuming that a planet in the late state of accretion has a magma ocean, there soon will form a layer of molten iron at the bottom of the magma ocean. Since the iron has a higher density than the underlying planetary mantle, it will probably sink due to Rayleigh―Taylor instability. According to Woidt (1978) the sinking iron will attain the shape of spheres because the viscosity of the liquid iron should be much smaller than that of silicates. We model the Stokes falling of an iron sphere through a silicate mantle with temperature dependent viscosity of the mantle material by using a finite element code (FEATFLOW) written by Turek (1998). We solve the incompressible Navier-Stokes equation coupled with the energy and mass equation. With these models the effect of the temperature dependence of the silicate rock viscosity on the differentiation rate and the temperature of the core after core formation can be estimated.