Fragmentation and Thermochemical Exchanges during Planetary Core Formation - an Experimental Approach

Abstract:

Telluric planet formation involved the settling of large amounts of liquid iron coming from impacting planetesimals into an ambient viscous magma ocean. The initial state of planets was mostly determined by exchanges of heat and elements during this iron rain. Up to now, most models of planet formation simply assume that the metal rapidly equilibrated with the whole mantle. Other models account for simplified dynamics of the iron rain, involving the settling of single size drops at the Stokes velocity. But the fluid dynamics of iron sedimentation is much more complex, and influenced by the large viscosity ratio between the metal and the ambient fluid, as shown in studies of rising gas bubbles (e.g. Bonometti and Magnaudet 2006).

We aim at developing a global understanding of the iron rain dynamics. Our study relies on a model experiment, consisting in popping a balloon of heated metal liquid at the top of a tank filled with viscous liquid. The experiments reach the relevant turbulent planetary regime, and tackle the whole range of expected viscosity ratios. High-speed videos allow determining the dynamics of drop clouds, as well as the statistics of drop sizes, shapes, and velocities. We also develop an analytical model of turbulent diffusion during settling, validated by measuring the temperature decrease of the metal blob. We finally present consequences for models of planet formation.