C2 - Modelling giant impacts and the subsequent thermochemical evolution of liquid metal in a convecting magma ocean

Research areas: Geophysics


Giant impacts on Earth heavily influenced core formation and may have contributed to late accretion of mate- rial, but the extent to which the present-day geochemical signature of Earth’s mantle reflects the processes of core formation and late accretion and how much of the delivered material was incorporated into the core remains unclear. To better understand these processes, it is key to comprehend:

  1. how much of the material delivered by impacts was retained,
  2. how impactor material is dispersed upon impact, and
  3. how the metal (or sulfide) settles in a global or partial magma ocean.

First, we investigated the ‘retention’ of material as a function of impact parameters (impactor size, impact velocity and angle), and our findings showed that two to three times less material is retained on the Moon than on the Earth. Second, to investigate how the delivered material is dispersed upon impact, we implemented a particle-based approach in our Eulerian (grid-based) shock-physics code. We found that the way in which the impact-delivered metal interacts with the ambient magma strongly depends on the size-frequency distribution of particles or clumps the impactor is dispersed into. The subsequent settling of impactor material is strongly affected by the thermodynamic convection state of the magma ocean. Another key parameter is the rotation of the planet, which influences the sinking dynamics of the material. This implies that the fate of the sinking material depends on the latitudinal position of the impact. Understanding how the magma ocean interacts with the sinking metal particles is crucial for understanding core formation. For example, we must understand how both of these factors – particle size and convection state of the ambient magma – affect the material’s sinking speed: While particles that sink quickly leave little time for chemical equilibration, those that sink sufficiently slowly, or are even kept in suspension by the convection flow, could allow time for equilibration between metal and silicate materials, thus potentially shaping the chemical signatures we see in Earth’s mantle today. We have made substantial progress in modelling how the flow style of the magma ocean affects the interaction of metal and silicates. Our first attempts at combining impact modelling with sophisticated models of the magma ocean that account for solidification processes are promising for gaining a better understanding of how the changing magma ocean reacts to impacts and the how the sedimentation of impactor material changes during magma ocean evolution.


First results on the settling of impact-derived metal in a convecting and rotating magma ocean.


Figure: First results on the settling of impact-delivered metal in a convecting and rotating magma ocean. The colours in the snapshots display the temperature field of the convecting fluid: red denotes hot fluid and blue denotes cold fluid. The metal droplets are shown in black. The three snapshots show the metal distribution at some time after the impact of an impactor with a radius of 750 km, impacting at (a) the pole (b) the mid-latitudes and (c) the equator. The stark differences in metal distribution are due only to the different impact locations.