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 have shown that the standard formulation of SPH suffers from several shortcomings, for example, its inability to capture subsonic turbulence, which can suppress mixing when two different fluids come into contact. In order to quantify how severe are these limitations when modeling GIs we did a comparison of simulations with identical initial conditions run with the standard SPH and a novel Lagrangian Meshless Finite Mass (MFM) method. We confirm the lack of mixing between the impactor and target 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 resolving density discontinuity at the core-mantle boundary.