A "New Hope" for Moon Formation: Presenting a Multiple Impact Pathway

TL;DR

The paper addresses the Moon’s origin by testing the multiple impact hypothesis, which relaxes strict compositional constraints of a single giant impact. Using SPH simulations with the SWIFT code, it models chains of four impacts on rapidly rotating Earth-like planets, tracking disk properties, moonlet formation via the semi-analytic relation $M_{moonlet}=1.14\frac{L_d}{\sqrt{GM_\oplus a_R}}-0.67M_d-2.3M_\infty$, and a compositional distance metric $d_c$ across multiple impactors. The study finds that several chains can yield Moon-sized moons with Earth-like angular momentum and modest compositional differences, with chains 4 and 8 satisfying all prescribed criteria and chain 4 emerging as the best compromise between lunar mass, angular momentum, and compositional similarity. These results suggest that a series of moderate impacts could coherently build the Earth–Moon system in a more probable formation pathway than a single extreme impact, while highlighting uncertainties in long-term angular-momentum evolution and the role of moonlet interactions.

Abstract

The leading hypothesis for the origin of the Moon, that of a single giant impact, faces significant challenges. These include either the need for an impactor with a near-identical composition to Earth or an extremely high-mass or high-energy impact to achieve near-complete material mixing. In this paper we explore an alternative, the "multiple impact hypothesis", which relaxes the compositional constraints on both the target and projectile, and allows for the consideration of more probable, less extreme impacts that steadily grow the Earth and Moon to their current size over several impact events. Using the hydrodynamical code SWIFT, we simulate "chains" of impacts and follow the growth of a moon around a planet analogous to our own. Our results demonstrate that chains of three or more impacts can produce systems comparable to the Earth-Moon system whilst achieving higher compositional similarities than the canonical giant impact scenario. This presents the multiple impact hypothesis as a promising alternative to the single large impact scenario for the origin of the Moon.

Figures (4)

• Figure 1: Cartoon of the multiple impact Moon-forming scenario: A projectile impacts the proto-Earth forming a debris disk (a), from which an initial seed moon coalesces (b), a subsequent impact occurs (c), forming an additional debris disk (d), from which another moonlet forms and eventually merges with the previously formed moon, and in our case contributes to its mass (e). This process could have repeated until the modern day Moon was formed.
• Figure 2: Progression of the first two links of Chain 1. Illustrating an initial disk forming impact (a) from which we create a larger planet and an initial moonlet (b), to a subsequent impact (c) which forms another debris disk (d), from which we again create a larger planet and add to the mass of the moonlet, before repeating the process for the next chain link. The white particle with the trailing dashed line shows the point mass moon and a segment of its instantaneous orbit, the green particles indicate unbound material, and the orange particles represent the disk material. The remaining particles are coloured by their density with yellow indicating the most dense material. The images present a slice taken along the centre of mass.
• Figure 3: The accumulated mass of the moonlet (dashed lines) and of the planet (solid lines) over the course of the four impacts in Chains 1, 4, and 8. The grey lines at a value of 1.0 on their respective axes indicate the masses of the modern Earth and Moon.
• Figure 4: The compositional distance between the planet and moon, as defined in Equation \ref{['distance']}, over the accrued moon mass for each of the 4 links within the 12 chains. The two grey lines indicate the "successful" thresholds for the Distance and Mass at 0.3 and 1 respectively, with those chains ending in the lower-right-hand quadrant considered successful in this regard. Individual chains are coloured from orange to purple, ordered by increasing final moonlet mass.