Keywords:OSIRIS-REx, Bennu, Asteroid, Geology
NASA’s OSIRIS-REx sample return mission has been observing the near-Earth Asteroid (101955) Bennu since December 2018 and will be collecting a sample of the surface in the summer of 2020, to be returned to Earth in 2023. Preliminary and ongoing observations have yielded much information about Bennu’s formation, evolution, and ongoing geologic activity. Here we report on the global geology of Bennu based on analyses of images combined with shape, mass, and slope measurements. Initial results suggest that Bennu is a rubble-pile asteroid, formed from a parent body that was collisionally disrupted and reaccumulated. The asteroid has a “top” shape which is spherical with an equatorial bulge, similar to other near-Earth asteroids including Ryugu as observed by the Hayabusa2 sample return space mission. Candidate impact craters have been observed at a range of diameters between ~10-150 m across the surface of Bennu, leading to a crater retention age of 100 million to 1 billion years. An apparent concentration of large crater candidates at low latitudes suggest that the equatorial ridge is stratigraphically old, and may have formed early in Bennu’s history or been inherited from the reaccumulation event which created the asteroid. Several linear features have been identified on Bennu, the largest of which are topographic highs that extend longitudinally from the northern polar regions to the equator. Boulders are concentrated between several of these linear ridges, as well as in the interior of large candidate craters. Boulders on Bennu appear to be geologically diverse. Boulder sizes range from < 3 m to ~95 m in diameter with large variations in geometric albedo, morphology, degree of burial, and state of degradation. Fractured boulders have been identified with various numbers, orientations, and widths of fractures. Distinct clasts within several boulders indicate they may be breccias, while piles of boulders may represent disaggregated polymict or brecciated boulders. Current imaging resolution cannot resolve centimeter-sized particles, so direct detections of regolith or other sub-cm sized particles have not been made. However, several small candidate craters (< 20 m diameter) have a significant lack of boulders relative to the surrounding terrain, and additional datasets suggest the presence of a component of fine-grained particles. Improved image resolution will aid in the identification of regolith deposits on Bennu.