Similar-sized collisions and the diversity of planets

Abstract

It is assumed in models of terrestrial planet formation that colliding bodies simply merge. From this the dynamical and chemical properties (and habitability) of finished planets have been computed, and our own and other planetary systems compared to the results of these calculations. But efficient mergers may be exceptions to the rule, for the similar-sized collisions (SSCs) that dominate terrestrial planet formation, simply because moderately off-axis SSCs are grazing; their centers of mass overshoot. In a “hit and run” collision the smaller body narrowly avoids accretion and is profoundly deformed and altered by gravitational and mechanical torques, shears, tides, and impact shocks. Consequences to the larger body are minor in inverse proportion to its relative mass. Over the possible impact angles, hit-and-run is the most common outcome for impact velocities v imp between ∼ 1.2 and 2.7 times the mutual escape velocity v esc between similar-sized planets. Slower collisions are usually accretionary, and faster SSCs are erosive or disruptive, and thus the prevalence of hit-and-run is sensitive to the velocity regime during epochs of accretion. Consequences of hit-and-run are diverse. If barely grazing, the target strips much of the exterior from the impactor—any atmosphere and ocean, much of the crust—and unloads its deep interior from hydrostatic pressure for about an hour. If closer to head-on ( ∼ 30 – 45 ° ) a hit-and-run can cause the impactor core to plow through the target mantle, graze the target core, and emerge as a chain of diverse new planetoids on escaping trajectories. A hypothesis is developed for the diversity of next-largest bodies (NLBs) in an accreting planetary system—the bodies from which asteroids and meteorites derive. Because nearly all the NLBs eventually get accreted by the largest (Venus and Earth in our terrestrial system) or by the Sun, or otherwise lost, those we see today have survived the attrition of merger, evolving with each close call towards denser and volatile-poor bulk composition. This hypothesis would explain the observed density diversity of differentiated asteroids, and of dwarf planets beyond Neptune, in terms of episodic global-scale losses of rock or ice mantles, respectively. In an event similar to the Moon-forming giant impact, Mercury might have lost its original crust and upper mantle when it emerged from a modest velocity hit and run collision with a larger embryo or planet. In systems with super-Earths, profound diversity and diminished habitability is predicted among the unaccreted Earth-mass planets, as many of these will have be stripped of their atmospheres, oceans and crusts.