FORMATION AND EVOLUTION OF THE PROTOLUNAR DISK AND THE MAGMA OCEAN

Giant and large-scale impacts are ubiquitous features of early solar systems that dominate the end of the accretion stage. The last major accretion event in Earth’s history was the Moon-forming giant impact. Depending on the impact parameters, the outcome of this impact was the formation of such a protolunar disk. During cooling, liquids and gases separate according to the liquid-vapor stability dome. The liquid droplets rain toward the center, forming the planet as a magma ocean (MO). The leftover gas forms the hot dense disk atmosphere.

We study the formation and condensation of the disk and the evolution of the MO using molecular dynamics simulations based on ab initio calculations. We work on a multi-component silicate fluid with bulk silicate Earth composition. From the pressure-density variation and the Maxwell construction, we determine the limits of stability of the molten silicate and the position of the critical point. We find that the Earth’s protolunar disk reached the supercritical state of the silicate mantle. Then we follow the chemical evolution of the disk during its cooling.

We characterize the structure and behavior of the MO. The onset of a mushy layer rich in bridgmanite crystals profoundly marks its evolution. This layer exhibits neutral buoyancy and effectively separates the MO at middle depths into a shallow MO and a basal MO that evolve separately.

We also characterize the composition of the disk atmosphere and find that it is extremely rich in molecules. Oxidized phases like SiO, O, O2, MgO, and cations like Na and Mg dominate the gas phase. But a plethora of other phases are present in the system, with lifetimes that allow them to play a role in the chemical and isotopic exchanges. Many of the gas molecules that we find in our simulations are not present in databases like JANAF. This suggests that a huge field of investigation lies bare ahead of us.