Signatures of hit-and-run collisions

Introduction Terrestrial planets grew in a series of similar-sized collisions (SSCs) that swept up most of the next-largest bodies (NLBs; see Box 2.1). Theia was accreted by the Earth to form the Moon according to the theory. Planetesimals likewise might have finished their accretion in a sequence of “junior giant impacts,” scaled down in size and velocity. Here we consider the complicated physics of pairwise accretion, as planetesimals collide and grow to planetary scales, and show how the inefficiency of that process is a foundation for the origin of planetesimals and the diversity of meteorites and primary asteroids. Detailed simulations show that planetary collisions are inefficient mergers. Accretion inefficiency gets concentrated, as it were, in the unaccreted bits and pieces, giving asteroids and meteorites their distinctive record, according to the arguments below. The impact axis is well off-center in a typical collision, relative to the combined center of mass, with dire consequences to the less massive object M². The target M¹ serves both as an “anvil” into which M² collides, and decelerates, and as a gravitational pivot around which it gets swung, causing it to be shredded. The post-collision system can be gravitationally bound or not, depending on the projectile mass γ = M²/(M ¹ + M²) normalized to the total mass, the relative velocity v^rel normalized to the escape velocity v ^esc, which scales with size, the impact angle θ with median value 45°, and composition, differentiation, rotation, and thermal tate. Outcomes of SSCs are diverse throughout this large parameter space, segregating what is gravitationally bound from what is not, thereby causing the disruption and compositional segregation of planetesimals and growing planets. For example, the standard model of Moon formation involves a graze-and-merge collision (GMC) where a Mars-size planet Theia is mostly accreted (ξ ~ 1) by a ~45° impact into proto-Earth at close to the mutual escape velocity v^esc (Canup and Asphaug, 2001), leaving a silicate protolunar disk. The Mars hemispheric dichotomy might have been caused by a faster (up to 2v^esc) collision (Marinova et al., 2008), in which case the impactor was not accreted by M¹.