GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 144-1
Presentation Time: 1:35 PM


JAWIN, Erica R.1, WALSH, Kevin J.2, MCCOY, Timothy J.3, CONNOLLY, Harold C.4, BALLOUZ, Ronald L.4, BARNOUIN, Olivier S.5, BEDDINGFIELD, Chloe B.6, BENNETT, Carina4, BIERHAUS, E. Beau7, BURKE, Keara N.4, CLARK, Beth E.8, DALY, Michael9, DELBO, Marco10, DELLAGIUSTINA, Daniella4, DWORKIN, Jason P.11, HARTZELL, Christine12, MARSHALL, John R.13, MICHEL, Patrick14, MOLARO, Jamie L.15, NOLAN, Michael4, PAJOLA, Maurizio16, PERRY, Mark E.5, RIZK, Bashar4, SANDFORD, Scott17, SCHEERES, Dan J.18, SCHWARTZ, Stephen4, TRANG, David19 and LAURETTA, Dante S.4, (1)Mineral Sciences, Smithsonian Institution National Museum of Natural History, Washington, DC 20013-7012, (2)Southwest Research Institute, Boulder, CO 80302, (3)Mineral Sciences, Smithsonian Institution, PO Box 37012, MRC 119, Washington, DC 20013, (4)Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85705, (5)Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, (6)SETI Institute, Mountain View, CA, (7)Lockheed Martin Space Systems Company, Littleton, CO 80120, (8)Ithaca College, Ithaca, NY 14850, (9)Earth and Space Science and Engineering, York University, 4700 Keele St, Toronto, ON M3J 1P3, Canada, (10)Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, 06100, France, (11)NASA Goddard Space Flight Center, Greenbelt, MD 20771, (12)University of Maryland, College Park, MD 20742, (13)SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94943, (14)Laboratoire Lagrange, Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS,, Nice, France, (15)7Planetary Science Institute, Tucson, AZ 85719, (16)INAF–Osservatorio Astronomico di Padova, Padova, Italy, (17)NASA Ames Research Center, Mountain View, CA 94035, (18)University of Colorado, Boulder, CO 80309, (19)Hawaii Institute of Geophysics and Planetology, University of Hawaii, 1680 East-West Road, POST 503, Honolulu, HI 96822

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.