DiRAC resources used: COSMA8

A new understanding of the “canonical” model for Moon formation was provided by Kegerreis et al (2022). Our paper showed that a Moon-sized body can be immediately placed into orbit following the collision of a Mars-sized body, called Theia, with the proto-Earth.

Previous low resolution hydrodynamical simulations of this planetary collision had produced a disk of debris that would subsequently accrete to form the Moon over a timescale of years. Using the SWIFT simulation code, we were able to simulate this impact at a wide range of resolutions. Only when we included at least three million particles, which is more than previous studies had typically employed, did the initial clump of debris split into two after which the outer, Moon-sized, satellite was torqued onto a stable orbit. This moment of splitting is shown in the figure. At lower resolutions, the clump as a whole smashes back into Earth, leaving just a disk of orbiting debris.

The numerical resolution in our simulations allowed us to determine the radial variation of the provenance of material in the Moon-sized clump, with the outer layers containing at least as much proto-Earth as Theia material. Depending upon the extent of subsequent mixing within the satellite this radial variation could have a significant bearing upon the interpretation of isotope ratio measurements of lunar samples and Earth rocks. Immediately forming the Moon during a giant impact could lead to a thinner lunar magma ocean than when the Moon is entirely accreted from a diffuse disk. This would produce a thinner crust, which is what recent measurements show. Furthermore, there is more scope for imparting an orbital inclination to the Moon, relative to Earth’s equator, if it forms immediately.