NUMERICAL-SIMULATION OF EARTHS CORE FORMATION

Formation process of Earth's core is studied by numerical simulations of flow field in a self-gravitating fluid sphere. The proto-Earth was assumed to have gravitationally unstable three-layered structure initially, which consists of the uppermost silicate melt layer, the middle iron layer, and the central undifferentiated silicate-rich protocore. This structure of the heavy iron layer overlying the light protocore leads to Rayleigh-Taylor instability. The numerical simulation supports that the fastest mode of the instability is the translation (l = 1 mode in spherical harmonics) of the protocore relative to the iron layer, as pointed out by previous studies. Furthermore, the present calculation shows that the translational mode is followed by growth of a gigantic iron drop and intrusion of the iron drop into the protocore. This late stage process is found to be the controlling process of Earth's core formation. In order to complete the core formation within 10(9) years required by various geophysical and geochemical constraints, the protocore viscosity eta1, should be less than 10(26) Pas, which is larger than the present mantle viscosity but much smaller than the viscosity estimated for the protocore under low temperature and high pressure. Heating of the protocore by gravitational energy released during the core formation, change of flow mechanisms from diffusional flow to either power law creep or plastic flow and effect of volatiles contained in the protocore on the effective viscosity may assist acceleration of the core formation process.