Enhanced mixing in giant impact simulations with a new Lagrangian method.

Giant impacts (GIs) are common in the late stage of planet formation. The Smoothed Particle Hydrodynamics (SPH) method is widely used for simulating the outcome of such violent collisions, one prominent example being the formation of the Moon. However, a decade of numerical studies in various areas of computational astrophysics has shown that the standard formulation of SPH suffers from several shortcomings, such as its inability to capture subsonic turbulent, which can suppress mixing when two different fluids come into contact. In order to quantify how severe are these limitations when modeling GIs we carry out a comparison of simulations with identical initial conditions run with standard SPH and the novel Lagrangian Meshless Finite Mass (MFM) method. We confirm the lack of mixing between impactor and target, the proto-Earth, when SPH is employed, while MFM is capable of driving turbulence and leads to significant mixing between the two bodies. Modern SPH variants with artificial conductivity, different formulation of the hydro force or reduced artificial viscosity, do not improve as significanyly, and their initial conditions suffer from numerical density discontinuity at the core-mantle boundary. These early results of MFM already go quite far in explaining the compositional similarity between the Earth's mantle and the Moon. The effect of different impact configurations remains to be explored.