The Role of Multiple Giant Impacts in the Formation of the Earth–Moon System

Abstract

The Earth-Moon system is suggested to have formed through a single giant collision, in which the Moon accreted from the impact-generated debris disk. However, such giant impacts are rare, and during its evolution the Earth experienced many more smaller impacts, producing smaller satellites that potentially coevolved. In the multiple-impact hypothesis of lunar formation, the current Moon was produced from the mergers of several smaller satellites (moonlets), each formed from debris disks produced by successive large impacts. In the Myrs between impacts, a pre-existing moonlet tidally evolves outward until a subsequent impact forms a new moonlet, at which point both moonlets will tidally evolve until a merger or system disruption. In this work, we examine the likelihood that pre-existing moonlets survive subsequent impact events, and explore the dynamics of Earth-moonlet systems that contain two moonlets generated Myrs apart. We demonstrate that pre-existing moonlets can tidally migrate outward, remain stable during subsequent impacts, and later merge with newly created moonlets (or re-collide with the Earth). Formation of the Moon from the mergers of several moonlets could therefore be a natural byproduct of the Earth's growth through multiple impacts. More generally, we examine the likelihood and consequences of Earth having prior moons, and find that the stability of moonlets against disruption by subsequent impacts implies that several large impacts could post-date Moon formation.

Impact Dynamics of Moons Within a Planetary Potential

“…Citron et al () found that most impacts are at a velocity <1.75 V esc , and these will have a high merger efficiency if the impact angle is low enough for accretion, but these also include hit‐and‐run impacts for higher angles. Because grazing angles are the most probable (∼45°), most similar size impacts will usually not accrete at velocities > V esc .…”

Collisional formation of massive exomoons of superterrestrial exoplanets

“…For the latter case, we wish to investigate what would be the likely result. Based on the statistical analysis of Citron et al (2018), most moonfalls have extremely grazing geometries, with θ ∼90°, as shown in Figure 2. It remains to be checked if extremely grazing moonfalls do not always result in the complete-loss of moon material, but instead give rise to a new generation of intact moons that retain most of their original mass (as was done by Malamud et al (2018) for Earth-sized planets).…”

“…Multiple-impact formation of massive moons is still possible, but it requires that the gravitational interaction between moonlets would exclusively result in mergers, while avoiding ejections and moonfalls, since both would terminate growth. Further studies are required in order to establish the relative probabilities of mergers, ejections and moonfalls, since the latter depend on the relative masses of the interacting moons, and since studies so far (Citron et al 2018) have targeted the Earth rather than super-Earth planets. With more studies and statistics, we may be able to reach more decisive conclusions.…”

“…• Dynamical and tidal evolution of multiple exomoons -as mentioned in Section 5.2 (in the context of exomoon growth through multiple impacts) we require a detailed statistical understanding of gravitational interactions among exomoons, including their tidal evolution about their Superterrestrial host planets. Such a study can easily follow the design of Citron et al (2018), with the required modifications.…”

“…The distrbution of moonfall impact angles triggered by the gravitational perturbations between an inner and an outer moon (θ=0°: head-on impact; θ=90°: exteremely grazing impact) based on analysis of data from theCitron et al (2018) study.…”