Laboratory Experiments on Core Merging and Stratification After Giant Impacts

Abstract:

The fluid dynamics of core merging after giant impacts in the late stages of accretion provides constraints on metal-silicate equilibration, core stratification, and early magnetic field generation. 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.

Unlike the laminar flow in numerical simulations, liquid impact experiments can produce turbulence, as expected during core formation. We present experiments on liquid blobs of variable density released into another liquid consisting of two immiscible layers, representing the magma ocean and protocore, respectively. The released liquid is denser than the upper layer, immiscible in the upper layer, and miscible in the lower layer. With a shallow upper layer, the relevant regime for giant impacts, a turbulent cloud of released and upper liquids penetrates into the lower layer, collapses and spreads along the interface between the upper and lower layers. This behavior contrasts with the laminar core merging observed in impact simulations or the classical iron rain scenario, and suggests that metal-silicate chemical equilibration extends inside the protocore. Experimental scalings for low-density releases predict that compositional stratification of the core is likely in the aftermath of planet 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.