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

Ruiz-Bonilla, S and Eke, V R and Kegerreis, J A and Massey, R J and Teodoro, L F A (2021) 'The effect of pre-impact spin on the Moon-forming collision.', Monthly notices of the Royal Astronomical Society, 500 (3). pp. 2861-2870.


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 ∼1 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.

Item Type:Article
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal astronomical Society ©: 2020 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:20 October 2020
Date deposited:29 June 2021
Date of first online publication:04 December 2020
Date first made open access:29 June 2021

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