Giant impacts onto magma ocean: metal-silicate mixing constrained by coupling analogue laboratory experiments and numerical modelling

Planetary formation models suggest that Earth experienced multiple high-energy impacts. Among those, the Moon-forming event is thought to be responsible for melting a large fraction of proto-Earth’s silicate mantle. Mixing of the impactor’s metallic core into Earth's silicate mantle controlled the chemical equilibration between metal and silicates, and hence the respective compositions of Earth's core and mantle. The extent of this mixing is, however, still debated. Previous studies explore mixing upon large impacts either with numerical modelling or with analog laboratory experiments. Numerical simulations are efficient in that they reproduce the shock physics of hypervelocity impacts. However, their spatial resolution is too limited to produce the turbulent features responsible for metal-silicate mixing in a magma ocean. Liquid impact experiments on the other hand are subsonic and hence neglect compressibility effects. However, they produce small-scale mixing and turbulence, which is crucial in estimating metal-silicate equilibration. Recent simulations and experiments disagree on the degree of mixing between the impactor and target materials. The origin of these differences is still unclear and requires further investigation. We present a scaling-law developed to extend the laboratory experiments results to hypervelocity cases and its further application the the metal-silicate mixing upon impact. We find that the Mach number (impact velocity to sound speed ratio) affects the metal-silicate mixing upon impacts, but that its effect depends on the other impact parameters such as the impactor size.