Characterization of Exogenic Boulders on Near-Earth Asteroid (101955) Bennu from OSIRIS-REx Color Images

TL;DR

This study identifies and characterizes exogenic boulders on asteroid Bennu, revealing their likely composition and estimating the amount of material delivered from other bodies, which informs our understanding of asteroid surface processes.

Contribution

The paper provides the first global analysis of exogenic boulders on Bennu, including spectral characterization and estimates of external material contribution.

Findings

Exogenic boulders similar to eucrites are present on Bennu.

Mixtures of eucrites with carbonaceous material are also found.

Estimated external ordinary chondrite material in Bennu is around 91,000-150,000 m3.

Abstract

A small number of anomalously bright boulders on the near-Earth, rubble-pile asteroid (101955) Bennu were recently identified as eucritic material originating from asteroid (4) Vesta. Building on this discovery, we explored the global presence of exogenic boulders on Bennu. Our analysis focused on boulders larger than 1 m that show the characteristic 1-micron pyroxene absorption band in the four-color MapCam data from the OSIRIS-REx mission. We confirm the presence of exogenic boulders similar to eucrites and find that mixtures of eucrites with carbonaceous material is also a possible composition for some boulders. Some of the exogenic boulders have spectral properties similar to those of ordinary chondrite (OC) meteorites, although the laboratory spectra of these meteorites have a higher albedo than those measured on Bennu, which could be explained by either a grain size effect, the presence of impact melt, or optical mixing with carbonaceous material owing to dust coating. Our Monte Carlo simulations predict that the median amount of OC mass added to the parent body of Bennu is 0.055% and 0.037% of the volume of a 100- and 200-km-diameter parent body, respectively. If Bennu was a uniformly mixed byproduct of parent body and S-type projectiles, the equivalent mass of OC material would be a sphere with a diameter of 36 to 40 m (or a volume of 24,200 to 33,600 m3). The total amount of OC material in the interior of Bennu estimated from the MapCam data is slightly higher (91,000-150,000 m3).