Core Merging After Giant Impacts and Core Stratification in the Late Stages of Earth's Formation

English Abstract:

The fluid dynamics of core merging after giant impacts during planetary accretion provides constraints on core stratification, early magnetic field generation and metal-silicate equilibration. The energy released during giant impacts, such as those thought to have formed Earth's Moon and the crustal dichotomy on Mars, likely resulted in melting of the impactor and much or all of the protoplanet's mantle. Under these conditions, the liquid core of the impactor migrates through a fully-liquid magma ocean, and merges with the protoplanet's core.

We present experiments on liquid blobs released into another liquid consisting of two immiscible layers, representing the magma ocean and protocore, respectively. In contrast with the laminar flow in numerical simulations, liquid impact experiments can produce turbulence, as expected during core formation. The released liquid is immiscible in the upper layer, miscible in the lower layer, and denser than the upper layer. We characterize the impact at the immiscible interface varying the upper layer depth and the released fluid density. With a shallow upper layer, the relevant regime for giant impacts, a turbulent cloud of released and upper liquid penetrates in the lower layer, collapses and spreads at the immiscible interface. This behavior contrasts with direct core merging found in impact simulations, and suggests that, because of turbulence, metal-silicate chemical equilibration extends deep inside the protocore. Experimental scalings for low-density releases suggest that compositional stratification is likely in the aftermath of core formation, and the stratified layer detected by seismology at the top of Earth's core is compatible with a moon-forming impact. By implication, the early core dynamo had to overcome compositional stratification to initiate.