Asteroid Science 2019 (LPI Contrib. No. 2189) 2021.pdf

EXPLORING THE ORIGINS OF TERRACE FROMATION ON BENNU O. S. Barnouin1, M. E. Perry1, H. C. M. Susorney2, J. H. Roberts1, E. R. Jawin3, M. G. Daly4, J. A. Seabrook4, F. Thuillet5, P. Michel5, M. M. Al Asad2, C. L. Johnson2,6, R.-L. Ballouz7), S. R. Schwartz7, K. J. Walsh8, E. B. Bierhaus9, R. W. Gaskell6, E. E. Palmer6, J. Weirich6, B. Rizk7, C. Y. Drouet D’Aubigny7, M. C. Nolan7, D. N. DellaGiustina7, D. J. Scheeres10, J. W. Mahon10, H. C. Connolly Jr7,11 D. C. Richardson12, D. S. Lauretta7. (1) Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, (2) University of British Columbia, Vancouver, Canada, (3) Smithsonian National Museum of Natural History, Washington, DC, USA, (4) York University, Toronto, ON, Canada, (5) Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France, (6) Plane- tary Science Institute, Tucson, AZ, USA, (7) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA, (8) Southwest Research Institute, Boulder, CO, USA, (9) Lockheed Martin Space, Littleton, CO, USA, (10) Smead Aerospace Engi- neering Sciences Department, University of Colorado, Boulder, CO, USA, (11) Department of Geology, Rowan University, Glassboro, NJ, USA, (12) Dept. of Astronomy, Univ. of Maryland, College Park, MD, USA ([email protected])

Summary: We present evidence for latitudinal returns), OCAMS images and global imaging mosaics scarps or terraces across the northern and southern hem- all provide context for the slope and elevation results. ispheres of the asteroid Bennu at mid-to-high (40–70°) The slope stability analysis uses the “factor of latitudes, and explore their geological characteristics. safety” model for infinite slope [6], FS, that compares These features may be the result of surface creep due to the ratio of resisting frictional and cohesive stresses to YORP spin-up that leads to localized regional surface gravity ��� � � failure. Geotechnical analyses indicate that the latitude �� = + bands where the terraces are located are the most prone ��� � ��� ��� � where is the slope angle, is the frictional angle of the to failure. Simple laboratory experiments that attempt to q f surface material, c is the cohesion, is the depth-aver- simulate quasi-static slope failure of a blocky surface � aged total unit weight, and T is the thickness of the reg- indicate that regional terracing as observed on Bennu is olith/boulders that could fail. Values of FS < 1 imply likely. Similarly, numerical simulations of this slope the slope is prone to failure. We used the lowest-resolu- failure at gravitational acceleration commensurate with tion shape model available for our analysis (average Bennu’s show a similar failure pattern. Evidence for facet size of 12.5 m) to reduce local slope biases in our some infilling of larger mid-to-low latitude craters by surface flow could be evidence that the creeping surface assessment. We set f=40° for highly angular granular failure displacement creating terraces may be recent. materials, and used c=1 Pa based on the minimum co- Introduction: The Origins, Spectral Interpretation, hesion necessary to form the equatorial ridge of Bennu Resource Identification, and Security–Regolith Ex- [3]. plorer (OSIRIS-REx) spacecraft arrived at (101955) As a complement to the FS calculations, we per- Bennu in December 2018 [1]. A global digital terrain formed both a series of simple laboratory experiments model (GDTM) of Bennu was derived from imaging and numerical investigations to form terraces in granu- and laser altimetry observations [2]. Initial assessment lar material. In the laboratory investigation, we make of the surface slope and elevation relative to gravity [3, use of a gravel mixture composed of particles with three 4] indicates that Bennu possesses some interior stiff- distinct colors with diameters near ~0.2 cm, ~0.5 cm, ness, with significant evidence for surface mass-wast- and ~1 cm. The coarse-grained material employed was ing. intended as a proxy for the boulder-rich surface of Here, we explore manifestations of surface mass- Bennu. We placed the gravel in a clear-sided box, and wasting that result in terrace formation on Bennu. As in slowly lifted one of its sides to mimic slope increases on our earlier assessment [2], we use slope and elevation Bennu due to YORP spin-up. We used a high speed (100 relative to gravity along with a slope stability analysis frames/s) camera to observe the displacement of the sur- to gain insights into the possible origin of these features. face material. We supplement these geotechnical assessments with In the case of the numerical study, we used pkdgrav simple laboratory and numerical investigations. [7], with a soft-sphere model that includes four compo- Methods: The slope and elevation of Bennu are nents in the normal, tangential, rolling, and twisting di- computed using a recent GDTM developed from all rections to compute inter-particle contact forces [8,9]. OSIRIS-REx Camera Suite (OCAMS) images [5] col- These simulations evaluated the consequences of slowly lected between late November 2018 and April 2019, that steepening a slope composed of near equal-sized spher- is conditioned with OSIRIS-REx Laser Altimeter ical particles whose friction mimic that of coarse gran- (OLA) [6] data collected through April 2019. We also ular sand with friction angles near 40 deg in the Bennu use the asteroid mass reported in [3]. The RMS error of the global model is <0.7 m. Individual Orbital Phase B OLA scans (data collected <7cm intervals between Asteroid Science 2019 (LPI Contrib. No. 2189) 2021.pdf

gravitational en- to topple over. As the slope increases further, regional vironment. Cohe- failures occur to produce many of the characteristic of sion effects were the terraces seen on Bennu. The low g-numerical simu- ignored. The re- lation show a similar set of events; these do not show a sults could be need for an instability to cause surface failure. In the la- compared both to boratory, additional presence of smaller grain sizes in the observations certain areas enhances the likelihood of surface mass- on Bennu, and the wasting, which may explain why some scarps on Bennu simple laboratory often lack coarse grains. experiments. Discussion and Conclusion: We show that the for- Results: Ter- mation of terraces on Bennu could be the result of slope races are evident instabilities generated by YORP spin-up. Their mor- in polar and lati- phology and slope characteristic are consistent with la- tudinal assess- boratory-based assessments of regional slope failure, ments of the and the location of the observed terracing is consistent slope and eleva- with expectations from slope stability analyses using the tion of Bennu current spin-rates. A possible concern, however, for (Figure 1). They such an origin of these terraces is that YORP spin-up on are most preva- Bennu occurs very slowly [10], causing minute in- lent near latitudes creases in slope with time. For such conditions, several of ±60° but span studies [e.g., 11,12] indicate that slope failure should be Figure 1. Regional high slopes encir- 30–80°. The global and catastrophic. Our numerical studies at low-g cling the N (+Z) pole of Bennu terraces OCAMS data in- show creeping flows as seen in the laboratory can occur (top) indicate the presence of terraces. dicate that many on Bennu even when slopes increase very slowly. The The median slope distribution in 1° lat- of the terraces are slope distribution in Figure 1 shows that on Bennu, large itudinal bins (bottom) shows evidence steep scarps, with craters are long-lived, especially at the equator, but are for these features on both hemispheres; an accompanied slowly being obscured in the mid-latitudes where the their presence is also apparent in eleva- downslope ledge. slopes are steepest and the terraces are forming/present tion. Some variability in terracing ex- ists in longitude, but terracing remains The longest scarp [4]. The wide-spread evidence for small craters in many a dominant feature in the mid-to-high spans over 120° of the regions where the terraces are visible indicate latitudes. of longitude. Alt- these features are probably fairly recent [13]; these im- hough a statistical assessment is needed, the steep scarps pacts can help instigate surface failure to further build appear to lack discernible boulders (except when a scarp terraces and cause surface flows. itself is a boulder face). Some of the terraces are com- Acknowledgements: This material is based upon posed of rows of large step-faced boulders, with smaller work supported by NASA under Contract rocks accumulating both above and below these rock NNM10AA11C issued through the New Frontiers Pro- faces as the surface material slowly creeps along and gram and by the Canadian Space Agency. P.M. sometime precipitously fails to form the terraces. acknowledges support from CNES and UCA IDEX This formation scenario is supported by our stability JEDI (Acad. 2 & 3). We thank the OSIRIS-REx Team. analysis. For the case where c=0, the slopes are the least References: [1] Lauretta, D. S. et al. (2019) Nature, stable at the current spin rates in the latitude bands 568, 55-60. [2] Barnouin, O. S. et al. (2019) Nat. Ge- where terraces are observed. Reducing f to 32°, a more osci. 12, 247-252. [3] Scheeres, D. J. et al., Nat. Astron. plausible friction angle for Bennu surface material, or (2019) 3, 352-361. [4] Walsh, K. J. et al. (2019) Nat. continuing to increase the spin rate, we find regions that Geosci. 12, 242-246. [5] Rizk, B. et al. (2018) Space Sci. are at risk of slope failure today near where terraces are Rev. 214, 26. [6] Daly, M.G. et al. (2017) Space Sci. located. A cohesion of c=1 Pa completely hinders any Rev. 212 (1-2), 899–924,. [6] Iverson, R.M. et al. (1997) surface failure at current spin rates, which is incon- Annu. Rev. Earth Planet. Sci. 25, 85-138.[7] Richardson sistent with observations; the failure would be deep- et al. (2000) Icarus, 143, 45. [8] Schwartz, S. R., et al. seated for such cohesion conditions. (2012) Granular Matter, 14, 363. [9] Zhang, Y., et al. At Earth gravity, the laboratory experiments provide (2017) Icarus, 294, 98. [10] Hergenrother, C.W. et al. some useful insights in understanding what to expect at (2019) Nat. Commun., 10, 1291. [11] Walsh, K.J., Rich- Bennu. The experiments reveal that initially, as the ardson, D.C. and Michel, P. (2008) Nature, 454, 188- slope steepens, individual boulders are the most likely 191. [12] Delbo, M. et al. (2014) Nature, 508, 233-236. [13] Bierhaus et al., this meeting.