Two-stage formation of the Moon from accreting fragmentation and resonance captures

¹ Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Versoix, Switzerland
² LTE, Obs. de Paris, Univ. PSL, Sorbonne Univ., Univ. de Lille, LNE, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
³ Dept. of Physics and Astronomy, University of Rochester, Bausch & Lomb Hall, Rochester, NY 14627, USA
⁴ Dept. of Earth and Environmental Siences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 December 2025
Accepted: 27 February 2026

Abstract

Context. In the canonical Moon-forming model, a Mars-sized object collided with Earth to produce a disk of debris from which the Moon is believed to have accreted.

Aims. In order to build on past works, we simulated disks containing up to 10⁵ debris (called moonlets) and we took fragmentation of the debris into account in the case of violent collisions.

Methods. We used the new software NcorpiON, and in particular its module FalcON, for fast gravity computation using multipole expansions. The built-in fragmentation model of NcorpiON was used to resolve collisions. The initial conditions of our N-body simulations were output of smoothed particle hydrodynamic (SPH) simulations.

Results. Unlike previous works, we find that the Moon probably did not form in one step but rather in two stages. The first stage lasts a few months to a few tens of years and is dominated by collisions and gravitational scattering. It often leads to several large submoons. In the second stage, of length 10³ to 10⁵ years, tidal forces and subsequent migration allow these submoons to be captured in mean motion resonances (MMR), cleaning the system and completing the formation of the Moon through ejections and collisions.

Conclusions. A significant mass is lost during the accretion process, and we favor protolunar disks with initially at least 2M_☾.