Title: Enhanced mixing in giant impact simulations with a new Lagrangian method,
Abstract: 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 artificial surface tension and its
tendency to promptly damp turbulent motions on scales much larger than the
physical dissipation scale, both resulting in the suppression of mixing. In
order to quantify how severe these limitations are when modeling GIs we carried
out a comparison of simulations with identical initial conditions performed
with the standard SPH as well as with the novel Lagrangian Meshless Finite Mass
(MFM) method in the GIZMO code. We confirm the lack of mixing between the
impactor and target when SPH is employed, while MFM is capable of driving
vigorous sub-sonic turbulence and leads to significant mixing between the two
bodies. Modern SPH variants with artificial conductivity, a different
formulation of the hydro force or reduced artificial viscosity, do not improve
mixing as significantly. Angular momentum is conserved similarly well in both
methods, but MFM does not suffer from spurious transport induced by artificial
viscosity, resulting in a slightly higher angular momentum of the proto-lunar
disk. Furthermore, SPH initial conditions exhibit an unphysical density
discontinuity at the core-mantle boundary which is easily removed in MFM.