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.

Rocks with Extremely Low Thermal Inertia at the OSIRIS-REx Sample Site on Asteroid Bennu

“…This value is consistent with a median regolith particle size of ∼0.5-2.0 cm, which is within the range of particle sizes observed in high-resolution images of Nightingale (Burke et al 2021) and in photos of particles drifting out of the TAGSAM following sample collection (Lauretta et al 2022). The rocks and fine particles in Hokioi Crater and at the TAG contact location are predominantly of the darker lithology, excavated from <1.5 m depth during the formation of the crater, and secondarily of the brighter lithology, likely derived from regolith surrounding the crater (Barnouin et al 2022;Lauretta et al 2022;Jawin et al 2023). Further interpretation of the physical properties of individual boulders and fine regolith has been somewhat limited by the spot size of observations obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES; Christensen et al 2018).…”

“…similar to those of Bennu (Hamilton et al 2019). A more recent study of Bennu's boulders indicated that the two dominant types may consist of multiple distinct subtypes on the basis of morphology and roughness (Jawin et al 2023); the dark boulders are subdivided into boulder Types A and B, while the bright boulders are subdivided into Types C and D, largely on the basis of morphology. It is not yet known if these subtypes are also distinct in thermal and physical properties.…”

“…The thermal inertia values are generally higher outside of Hokioi Crater than inside, consistent with previous studies (Rozitis et al 2020b(Rozitis et al , 2022; see Figure B5) though with lower values than previously noted in some locations. There are several instances (e.g., Figure 3(a)) of locally elevated thermal inertia values that spatially coincide with rock or rock clusters that appear to be the higher-reflectance lithology described by DellaGiustina et al (2020) and Rozitis et al (2020bRozitis et al ( , 2022 and classified as Type C or D in the scheme of Jawin et al (2023). Conversely, instances of locally depressed thermal inertia typically coincide with patches of finer regolith and in rarer cases identifiable instances of dark-toned (Type A and/or B) boulders (e.g., Figure 3(b)).…”