Abstract:
The main asteroid belt between Mars and Jupiter is phenomenally diverse, and is sparse by factors of thousands compared to the primordial population thought to have existed there. These aspects are probably related. The largest asteroids are too massive to have been catastrophically destroyed by later impacts, which has led to theories for a fossilized system of bodies that grew rapidly by some form of primary growth, and then was scattered by planetary migration. But this framework is insufficient to explain the diameters of the largest asteroids, their distribution of rotation rates, their striking diversity, and the petrological complexities of meteorites. These are better explained if the main belt, or the inner solar system at large, underwent a concluding epoch of hierarchical, pairwise mergers that lasted a few Myr until planet formation stirred up the relative velocities beyond the accretion threshold. Even in the slowest encounters, only about half of binary collisions are effective mergers; the other half are "hit and run" collisions of various kinds. As planetesimals merge and attempt to merge, this creates a powerful survivorship bias among the unaccreted. To be accreted is the common outcome, but removes those bodies from the sample, whereas avoiding accretion happens in multitudinous ways (the "Anna Karenina principle"). I apply stochastic representations of this process to make predictions for the diversity of final bodies as a decreasing function of their mass, and show that the main belt asteroids are consistent with this mode of formation, followed by dynamical depletion. These models also provide a framework for understanding the exhumation of iron and stony-iron meteorites from deep inside of planetesimals, and potentially the origin of chondrules. I will discuss asteroid (16) Psyche in this context, the 220-km metallic target of the NASA Psyche mission that is enroute. To stimulate conversation I will argue that the diversity of terrestrial planets around the Sun is similarly related to its sparsity compared to the tightly-packed exoplanetary systems that are so common, and that a "late stage" hierarchy of formation, as ended here with the origin of the Moon, is essential to complex life.