Experiments on Metal-Silicate Mixing by Planetary Impacts

Abstract:

The timing of accretion of terrestrial planets and physical conditions of core formation are constrained by geochemical data such as geochronometers and estimated partition coefficients between the core and the mantle. However, in order to interpret these observations, the degree of chemical equilibration between the metal provided by the planetesimals’ cores and the terrestrial silicates of the magma ocean is required. Here we use fluid dynamics laboratory experiments to investigate the metal-silicate equilibration following the impact of a differentiated planetesimal onto a magma ocean. We present results obtained using two experimental set-ups designed to investigate the impact dynamics, and the post-impact flow, in dynamical regimes similar to planetary conditions. We first focus on the equilibration processes involved during the opening of the impact crater, using an experimental set-up in which a liquid drop is released into the air and strikes a deep pool of a second, less dense, liquid. A second series of experiments is devoted to the mixing dynamics of the downward propapating turbulent thermal following the impact. This time, an initial volume of fluid is released instantaneously within the deep pool of the less dense liquid. Unlike numerical simulations, our experiments capture small-scale mixing processes crucial for planetary impacts. During the crater opening, a radial instability due to the initial density contrast produces a mixing layer around the crater. A scaling law is obtained for the mass of equilibrated silicates with the metal of the impactor’s core. During the post-impact flow, metal-silicate equilibration is governed by stirring processes and the deformation of the metal phase. Using simultaneous velocity field and tracer concentration measurements, small-scale equilibration dynamics and metal-silicate equilibration timescale in the magma ocean are obtained.