Asteroid (101955) Bennu in the Laboratory: Properties of the Sample Collected by OSIRIS-REx

Abstract: On 24 September 2023, the NASA OSIRIS-REx mission dropped a capsule to Earth containing approximately 120 g of pristine carbonaceous regolith from Bennu. We describe the delivery and initial allocation of this asteroid sample and introduce its bulk physical, chemical, and mineralogical properties from early analyses. The regolith is very dark overall, with higher-reflectance inclusions and particles interspersed. Particle sizes range from sub-micron dust to a stone about 3.5 cm long. Millimeter-scale and larger stones typically have hummocky or angular morphologies. A subset of the stones appears mottled by brighter material that occurs as veins and crusts. Hummocky stones have the lowest densities and mottled stones have the highest. Remote sensing of the surface of Bennu detected hydrated phyllosilicates, magnetite, organic compounds, carbonates, and scarce anhydrous silicates, all of which the sample confirms. We also find sulfides, presolar grains, and, less expectedly, Na-rich phosphates, as well as other trace phases. The sample composition and mineralogy indicate substantial aqueous alteration and resemble those of Ryugu and the most chemically primitive, low-petrologic-type carbonaceous chondrites. Nevertheless, we find distinct hydrogen, nitrogen, and oxygen isotopic compositions, and some of the material we analyzed is enriched in fluid-mobile elements. Our findings underscore the value of sample return, especially for low-density material that may not readily survive atmospheric entry, and lay the groundwork for more comprehensive analyses.

• The paper reveals that Bennu’s sample exhibits three distinct particle morphologies with a low bulk density of 1.55 g/cm³.
• It employs particle imaging, spectral analyses, and isotopic measurements to link elemental abundances with CI chondrites.
• The study indicates complex aqueous alteration through unique phosphate detections and enrichments in fluid-mobile elements.

Properties of Asteroid 101955 Bennu Samples Analyzed from OSIRIS-REx

The paper "Asteroid (101955) Bennu in the Laboratory: Properties of the Sample Collected by OSIRIS-REx" presents a detailed study of the 120 g carbonaceous regolith sample returned to Earth by NASA's OSIRIS-REx mission from asteroid Bennu. The mission, notable for being the first U.S. asteroid sample return mission, offers unprecedented insights into the asteroid's composition and evolution through analyses of both physical and chemical properties of the returned samples.

Bennu, a B-type near-Earth asteroid, is characterized by its carbonaceous composition, evident in its low albedo. The mission aimed to understand the primitive materials and processes from the early solar system by targeting Bennu due to its rich organic compounds and water-bearing minerals identified via telescopic observations. The OSIRIS-REx mission employed an array of analytical techniques on Earth to characterize the returned samples, including particle imaging, spectral analyses, and isotopic measurements.

The returned regolith exhibits fascinating properties, with the majority of the material being very dark. Detailed particle morphology studies identify three broad categories based on physical characteristics: hummocky, angular, and mottled particles. Hummocky stones, with their cauliflower-like surface features, are identified as having the lowest bulk density, averaging 1.55 g/cm³, aligning with expectations for materials rarely surviving atmospheric entry due to low density. The higher-density angular and mottled stones exhibit geometries suggestive of different lithological processes. These characteristics underscore the fractal nature of the asteroid's regolith, reflecting surface and sub-surface diversity.

Elemental and isotopic assessments further refine the understanding of Bennu’s composition. Comparisons of major and trace elemental abundances reveal strong alignments with CI chondrites, supporting Bennu’s classification as composed of highly aqueously altered materials. Markedly, isotopic compositions of hydrogen, carbon, and nitrogen denote distinct signatures compared to other known carbonaceous astromaterials. Additionally, oxygen isotopic data position Bennu in proximity to other chemically primitive chondrites, yet with unique enrichments in fluid-mobile elements, suggesting hydrothermal activity that possibly altered its primordial composition.

Notably, phosphate minerals discovered within the samples pose intriguing questions about Bennu's aqueous history, as this phase diverges from the carbonate veins previously detected via spacecraft observations. Such findings could indicate complex chemical environments, with the potential for altering regolith compositions through extraterrestrial metasomatic processes.

This preliminary analysis orchestrates a baseline for further explorative endeavors, beckoning advanced assessments into the intricate mineralogy and isotopic tales told by Bennu's regolith. As more rigorous studies unfold, the samples offer profound insights into the early solar system and potentially the primordial preconditions contributing to terrestrial habitability.

The OSIRIS-REx mission paves the way for future sample-return efforts, emphasizing the pronounced importance of such missions in decoding the early solar system's geological narratives. As analytical methods continue to advance, the sample's hidden complexities promise to unravel further layers of Bennu's enigmatic history. The data from these samples not only enrich planetary science but augment our broader understanding of the materials that constitute planetary building blocks. The research presented establishes a comprehensive foundation for ongoing and upcoming examinations by the scientific community, fostering expansive readings into the evolutionary histories of small solar system bodies.