A Systematic Survey of Moon-forming Giant Impacts. II. Rotating Bodies

Abstract In the leading theory of lunar formation, known as the giant impact hypothesis, a collision between two planet-sized objects resulted in a young Earth surrounded by a circumplanetary debris disk from which the Moon later accreted. The range of giant impacts that could conceivably explain the Earth–Moon system is limited by the set of known physical and geochemical constraints. However, while several distinct Moon-forming impact scenarios have been proposed—from small, high-velocity impactors to low-velocity mergers between equal-mass objects—none of these scenarios have been successful at explaining the full set of known constraints, especially without invoking one or more controversial post-impact processes. Allowing for pre-impact rotation of the colliding bodies has been suggested as an avenue that may produce more promising collision outcomes. However, to date, only limited studies of pre-impact rotation have been conducted. Therefore, in this second paper of the series, we focus on pairwise impacts between rotating bodies. Using nonrotating collisions as a baseline, we systematically study the effects of rotation on collision outcomes. We consider nine distinct rotation configurations and a range of rotation rates up to the rotational stability limit. Notably, we identify a population of collisions that can produce low post-impact angular momentum (AM) budgets and massive, iron-poor protolunar disks. Furthermore, even when pre-impact rotation is included, we demonstrate that the canonical Moon-forming impact can only generate sufficiently massive protolunar disks in the presence of excessive post-impact AM budgets; this casts doubt on the canonical impact scenario.