The effect of pre-impact spin on the Moon-forming collision

Abstract

We simulate the hypothesized collision between the proto-Earth and a Mars-sized impactor that created the Moon. Among the resulting debris disc in some impacts, we find a self-gravitating clump of material. It is roughly the mass of the Moon, contains $\sim 1{{\ \rm per\ cent}}$ iron like the Moon, and has its internal composition resolved for the first time. The clump contains mainly impactor material near its core but becomes increasingly enriched in proto-Earth material near its surface. The formation of this Moon-sized clump depends sensitively on the spin of the impactor. To explore this, we develop a fast method to construct models of multilayered, rotating bodies and their conversion into initial conditions for smoothed particle hydrodynamical (SPH) simulations. We use our publicly available code to calculate density and pressure profiles in hydrostatic equilibrium and then generate configurations of over a billion particles with SPH densities within 1 per cent of the desired values. This algorithm runs in a few minutes on a desktop computer, for 107 particles, and allows direct control over the properties of the spinning body. In comparison, alternative relaxation or spin-up techniques take hours on a supercomputer and the structure of the rotating body cannot be known beforehand. Collisions that differ only in the impactor’s initial spin reveal a wide variety of outcomes: a merger, a grazing hit-and-run, or the creation of an orbiting proto-Moon.