Magnetized Models of Moon-Forming Giant Impacts
3-D simulations of the Moon-forming giant impact have been studied extensively, however, all have neglected the potential role of magnetic fields. Our preliminary calculations treat the impactor and proto-Earth as ideal polytropes, seeded with dipole magnetic fields. Evolving the impact under the assumptions of ideal MHD, we find that the resulting disk of debris sent into orbit about the proto-Earth (the “protolunar disk”) hosts a toroidal magnetic field. Shear in the disk amplifies field strengths linearly in time, resulting in a field unstable to the magnetorotational instability (MRI). Resolving the MRI is a difficult feat; a large simulation domain is required to contain the impact, while a high-resolution is necessary to capture the onset of magnetic turbulence. If resolved, we predict that our simulations may show rapid accretion of material onto the proto-Earth due to the MRI, and may even source magnetized bipolar outflows, thus removing angular momentum from the system. Mixing from magnetic turbulence may help explain the observed isotopic similarities between the Earth and Moon, while the formation of bipolar outflows may provide an avenue to relax the angular momen- tum constraint on Moon-formation theories. Our idealized models will be extended to include (1) better material data (i.e., realistic equations of state); (2) multiple material evolution (i.e., separately track iron cores and silicate mantles); and (3) resistive magnetohydrodynamics (i.e., not all of the protolunar disk is perfectly coupled to the magnetic field). The resulting tool will be applicable to a large set of planetary science problems, including (exo)moon/(exo)planet formation.
