If at first you don't succeed: Moon formation by sequential giant impacts
The origin of the Moon is an archetypal problem in planetary science, and is intimately connected to the origin of Earth. Approaches to the problem include geochemistry (the Moon's lack of volatiles, small iron core, and isotopic similarity to Earth), dynamics (the late stage of planet formation, and modern theories such as pebble accretion), thermodynamics (the behavior of materials under intense pressures and shocks and their unloading), and simulation physics (3D modeling of planet-scale collisions). The canonical model (Cameron and Ward 1976; Canup and Asphaug 2001) is that of a Mars-sized planet, Theia, that collided with proto-Earth at low velocity and was accreted, along with significant angular momentum that spun off a Moon made mostly out of Theia's mantle. This can explain the composition and dynamics of the Moon, but not the isotopic similarity, unless Theia was already nearly identical to Earth. This poses other problems: Where did Theia come from, to hit so slowly? And why did it wait so long, about 100 Myr, to collide?
Here I will review some of the later hypotheses diverging from the canonical model, and novel ideas about solar system formation in general, and conclude with our idea for "hit-and-run-return" collisions. We propose that Theia did not accrete the first time it collided with proto-Earth. Instead it had a hit-and-run (a non-accretionary collision, which is common), and then collided more slowly around 1 Myr later in an accretion that formed the Moon. This scenario starts with more realistic initial velocities and impact angles, and has a greater propensity for mixing. It also provides context for another puzzle, why Venus does not have a moon.
Relevant papers: Canup and Asphaug (Nature 2001), Emsenhuber et al. (PSJ 2021), Asphaug et al. (PSJ 2021).
