Boulder Diversity in the Nightingale Region of Asteroid (101955) Bennu and Predictions for Physical Properties of the OSIRIS‐REx Sample

Abstract

The sample of asteroid (101955) Bennu was collected from the Nightingale sample site by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft and arrived on Earth on 24 September 2023. To better understand Bennu's parent body, we identified boulders over 2 m in diameter around the Nightingale region and analyzed normal albedo, morphology, and surface roughness. We found that boulders can be separated into two groups based on albedo, and four groups using morphology including angularity, texture, and the presence or absence of clasts, layers, and bright spots: Type A is rounded, rugged, and clastic, with the highest root‐mean square deviation roughness; Type B is sub‐angular with intermediate roughness and polygonal surface fractures; Type C is angular, has distinct fractures, and the lowest roughness; and Type D is sub‐angular with intermediate roughness and bright spots. Unsupervised clustering algorithms showed that our Type A‐D classification represents the diversity in the morphology and albedo data. Using documented contacts between boulder groups, we conclude that boulders on Bennu originated on a single, heterogeneous parent body that experienced vertical mixing via impacts prior to or during its disruption. The boulder morphologies on Bennu bear striking resemblance to those on asteroid Ryugu, potentially suggesting a shared origin. Finally, from analyses of sample collection images, we predict that the sample will be heterogeneous in morphology, brightness, and degree of aqueous alteration and dominated by darker Type A and B material. These predictions are supported by initial analyses of the Ryugu sample.

Plain Language Summary

Asteroid Bennu is composed of fragments of an ancient, disrupted parent body. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft investigated Bennu and collected a sample, which arrived on Earth on 24 September 2023. We investigated the brightness, appearance, and roughness of boulders near where the sample was collected to better understand the parent body and the diversity of Bennu's surface materials. We found that boulders can be broken into four groups: Type A are dark, rough, and have clasts; Type B are smoother and are similarly bright and rough to Type A; Type C boulders are brighter and very smooth; and Type D boulders have distinct bright spots and similar brightness as Type C. Certain boulders contain more than one morphology, suggesting they formed near each other on the parent body, and impacts onto the parent body mixed rocks from different depths and cemented them into the boulders we observe on Bennu. Bennu boulders resemble those on Ryugu, which may mean they are related. We predict that dark Type A and B particles will be most abundant in the returned sample.

Key Points

• Boulders in the Nightingale sample region can be divided into four groups that vary in albedo, morphology, and surface roughness

• Contacts between groups indicate a single heterogeneous parent body modified by impacts and aqueous alteration prior to disruption

• We predict the returned sample will be diverse in reflectance and morphology that reflects the heterogeneity of meter‐scale boulders