Immediate origin of the Moon as a post-impact satellite

TL;DR

This paper proposes that the Moon could have formed immediately after a giant impact, creating a satellite with Earth-like composition directly in orbit, simplifying lunar origin models.

Contribution

It demonstrates that giant impacts can produce a Moon-sized satellite directly in orbit, even from within the Roche limit, challenging traditional multi-stage formation theories.

Findings

Simulations show immediate satellite formation at high impact energies.

Surviving satellites can be formed from within the Roche limit by partial stripping.

The formed satellites have molten outer layers with 60% proto-Earth material.

Abstract

The Moon is traditionally thought to have coalesced from the debris ejected by a giant impact onto the early Earth. However, such models struggle to explain the similar isotopic compositions of Earth and lunar rocks at the same time as the system's angular momentum, and the details of potential impact scenarios are hotly debated. Above a high resolution threshold for simulations, we find that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside the Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon's Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon's early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.