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

Ruiz-Bonilla, S; Eke, VR; Kegerreis, JA; Massey, RJ; Teodoro, LFA

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S Ruiz-Bonilla

LFA Teodoro


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.


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

Journal Article Type Article
Acceptance Date Oct 20, 2020
Online Publication Date Dec 4, 2020
Publication Date 2021-01
Deposit Date Jun 29, 2021
Publicly Available Date Jun 29, 2021
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 500
Issue 3
Pages 2861-2870


Published Journal Article (2.8 Mb)

Copyright 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.

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