The Mix is In! Exploring the Origin of Exogenous Material in Asteroids and Meteorites

Remote and in situ observations indicate that many asteroids have notable amounts of non-indigenous material (e.g., Vesta, Bennu, Ryugu, possibly Psyche). While most of this material likely comes from impacts, the delivery process is poorly understood. Here we examine the essentials of impact mixing.

First, most main belt asteroids have the potential to strike one another. Using WISE data, we find that up to 85% of 5 < D < 50 km main belt bodies are carbonaceous chondrites. This trend should continue for sub-km bodies, and it suggests most exogenous material should have this composition.

Second, remote (or even in situ) detection of exogenous material is challenging unless a strong albedo/spectral contrast exists (e.g., C-type material on an S-type target).

Third, only superficial contamination comes from cratering events, with projectile material mixed into the near surface of a target. The highest contamination ratios, defined as the fraction of non-indigenous material mixed into a target, come from catastrophic disruption events. For 100 < D < 200 km asteroids, which can only be destroyed by relatively large projectiles, the median disruption creates fragments with a contamination ratio of ~1-10%, though a modest fraction of disruptions achieve ratios > 30-50%. This kind of rare event may explain the singular albedo/spectral properties of the Baptistina family (i.e., S-type family members mixed with 10-70% low albedo contaminant; Reddy et al. 2013).

Fourth, small asteroids are relatively easy to break up from an energy per mass perspective. This has two implications: (i) the median disruption event will produce lower contamination ratios (e.g., 0.02-0.1% for 1 < D < 10 km bodies), and (ii) many sub-km bodies experience multiple disruption events prior to escaping the main belt, with each event adding exogeneous material. Accordingly, the diversity of exogeneous materials on Bennu and Ryugu provide strong evidence they were once part of a “collisional cascade”. Moreover, the largest fraction of exogenous material on each body may tell us the composition of the projectile that disrupted its parent body.

Fifth, a collisional cascade is the most natural way of explaining Almahata Sitta, a loosely-aggregated ureilite several meters in diameter that disintegrated in our atmosphere. The blast, which eliminated 99.9% of the bolide, created a strewn field with clasts from numerous chondrite groups, none which had solar wind-implanted noble gases. We predict Almahata Sitta was the byproduct of numerous catastrophic collisions, with each projectile mixed into the surviving debris.