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

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 10 5 debris (called moonlets) and we took fragmentation of the debris into account in the case of violent collisions. Methods . We used the new software Ncorpi O N, and in particular its module FalcON, for fast gravity computation using multipole expansions. The built-in fragmentation model of Ncorpi O N 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 10 3 to 10 5 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 2 M ☾ .