GLOBAL GEOLOGY OF BENNU FROM NASA'S OSIRIS-REX SPACE MISSION (Invited Presentation)

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. Bennu’s geology and physical properties are consistent with 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 mission. Candidate impact craters have been observed across the surface of Bennu at a range of diameters from ~10 to 150 m, from which we have estimated a crater retention age of 100 million to 1 billion years. An apparent concentration of large crater candidates at low latitudes suggests that the equatorial ridge is stratigraphically old; it may have formed early in Bennu’s history or been inherited from the reaccumulation event that 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, with large variations in size (< 3 m to ~95 m in diameter), normal 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 boulders indicate that they may be polymict breccias. Current imaging resolution cannot resolve sub-centimeter-sized particles, so direct detections of regolith or other fine-grained particles have not been made. However, several small candidate craters (< 20 m diameter) lack boulders relative to the surrounding terrain. Improved image resolution in forthcoming mission phases will aid in the identification of regolith deposits on Bennu.