P037-06
Ongoing shape modification of Bennu by terracing.
Thursday, 10 December 2020: 07:20
Virtual
Olivier S Barnouin1, Michael G Daly2, Jeff Seabrook3, Yun Zhang4, Patrick Michel5, Florian Thuillet6, James H Roberts7, Mark E Perry8, R. Terik Daly9, Hannah C M Susorney10, Erica R Jawin11, Manar Al Asad12, Lydia Philpott13, Catherine Johnson14, Ronald -L Ballouz15, Kevin John Walsh16, Robert W Gaskell17, Eric E Palmer18, John R Weirich18, Bashar Rizk19, Michael C Nolan19, Daniella N DellaGiustina19, Daniel Jay Scheeres20, Jay McMahon21, Harold Connolly22 and Dante S Lauretta19, (1)Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (2)York University, Centre for Research in Earth and Space Science, Toronto, ON, Canada, (3)York University, Toronto, ON, Canada, (4)Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France, (5)UNS-CNRS-Observatoire de la Cote d'Azur, Laboratoire Lagrange, NIce, France, (6)UNS-CNRS-Observatoire de la Cote d'Azur, Laboratoire Lagrange, Nice, France, (7)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (8)JHU/APL--Space Dept, Laurel, MD, United States, (9)Johns Hopkins University Applied Physics Laboratory, Laurel, United States, (10)University of Bristol, Bristol, United Kingdom, (11)National Museum of Natural History, Mineral Sciences, Washington, DC, United States, (12)University of British Columbia, Department of Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada, (13)University of British Columbia, Vancouver, Canada, (14)University of British Columbia, Vancouver, BC, Canada, (15)University of Arizona, Tucson, United States, (16)Southwest Research Institute Boulder, Boulder, United States, (17)Planetary Science Institute Tucson, Tucson, AZ, United States, (18)Planetary Science Institute, Tucson, AZ, United States, (19)University of Arizona, Lunar and Planetary Laboratory, Tucson, AZ, United States, (20)University of Colorado Boulder, Ann and H.J. Smead Department of Aerospace Engineering Sciences, Boulder, CO, United States, (21)University of Colorado at Boulder, Ann and H.J. Smead Department of Aerospace Engineering Sciences, Boulder, CO, United States, (22)Rowan University, Glassboro, United States
Abstract:
We explore in detail the geological characteristic of subtle latitudinal scarps or terraces that lie predominantly between 30 and 80° from the equator of Bennu. These terraces are a few meters in height and appear as short-wavelength variations in slope that overlay the broader hemispherical slope distribution of Bennu. In high-resolution global and regional digital terrain models produced by the scanning OSIRIS-REx Laser Altimeter (OLA), these terraces are present both in the northern and southern hemispheres of Bennu, but are more evident in the south.
We propose that the terraces are the result of surface creep due to YORP-driven spin-up of Bennu, which leads to localized surface failure across the upper latitudes of both hemispheres. There are several reasons to expect this mode of failure. First, geotechnical analyses indicate that at current spin rates, the latitude bands where the terraces are predominantly located are the most prone to localized failure, especially if the upper portions of the regolith are unconsolidated and cohesionless. Observations of surface failures near terraces would indicate that cohesion of few Pa in the upper few meters of regolith is unlikely, as it would suppress any evidence for surface slumping as Bennu spins up. Second, simplistic laboratory experiments that attempt to simulate quasi-static failure of a blocky surface reproduce many of the features seen regionally near Bennu’s terraces. Consistent with observations, the experimental results show creeping surface failure until catastrophic changes occur. The creep is a mixture of individual rock falls and localized slumping, especially where variation in the regolith’s substrata strength exists. The laboratory shows, as seen on Bennu, that large boulders prevent regolith surface displacement and contribute to the surface processes leading to terrace formation. Third, discrete finite-element simulations undertaken at Bennu’s gravity confirm the laboratory findings of creep before full catastrophic failure occurs, but with more subtle topographic expressions relative to the 1-g laboratory results. Such subtle expressions are consistent with the subtle meter-level changes observed at Bennu’s terraces.
