51st Lunar and Planetary Science Conference (2020) 1372.pdf

THE SURFACE ROUGHNESS OF (101955) BENNU. H. C. M. Susorney1, C. L. Johnson1,2, O. S. Barnouin3, M. G. Daly4, J. A. Seabrook4, M. M. Al Asad1, E. B. Bierhaus5, K. J. Walsh6, E.R. Jawin7 B. Rozitis8, R. W. Gaskell2, E. Palmer2, J. Weirich2, D. N. DellaGiustina8, B. Rizk8, M. C. Nolan8, D. S. Lauretta8. 1Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada ([email protected]), 2Planetary Sci- ence Institute, Tucson, AZ, USA, 3Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 4York University, Toronto, ON, Canada, 56Lockheed Martin Space, Littleton, CO, 6Southwest Research Institute, Boulder, CO, 7Smithsonian National Museum of Natural History, Washington, DC, and 8Lunar and Planetary Laboratory, Uni- versity of Arizona, Tucson, AZ.

Introduction: The Origins, Spectral Interpretation, surface roughness, except for elevated surface rough- Resource Identification, and Security–Regolith Ex- ness on the rims of the largest craters associated with plorer, OSIRIS-REx, mission is currently in orbit exposure of large boulders. around the ~490-m-diameter asteroid (101955) Bennu Small-scale roughness (L = 80 cm). The global map [1]. The mission goal is to return a sample of the primi- of surface roughness at a baseline of 80 cm has similar tive asteroid material to Earth. To plan and execute a spatial patterns as the 6.4-m baseline surface roughness. safe sampling of the surface, the asteroid’s shape and However, the relative difference between boulder fields topography was mapped in unprecedented detail [2]. In- and the surrounding regions are more subdued and the formation about Bennu’s topography can be used to un- large crater rims are not clearly different from the sur- derstand and explore Bennu’s geologic and geophysical rounding terrain. As in the large-scale maps, there are history. Geologic processes affect Bennu at multiple no clearly different surface roughness units. scales; to understand one way in which topography re- Nightingale primary sampling site (L = 10 cm). flects these processes, we generate global maps of sur- Fine-scale surface roughness at baselines of 10 cm is face roughness—the change in topography over a spec- fairly uniform across Nightingale, with the exception of ified horizontal scale [3]. the edges of large boulders. Clear spatial variations in Studies of global surface roughness of asteroids surface roughness due to the presence of boulders is not have highlighted the relative roles of regolith, boulders seen until baselines of 40 cm and above. and impact craters in shaping an asteroid’s surface [4, Discussion: The surface roughness of Bennu is 5], and in driving Bennu’s YORP spin-up [6]. The sur- dominated by the contrast between exposed boulder face roughness of small asteroids is particularly im- fields and regions with lower boulder density. The faces portant as centimeter-scale surface roughness affects as- of the largest boulders (tens of meters) are relatively teroids’ thermophysical properties [7]. smooth, but the edges of these large boulders have Method: We use RMS deviation as a metric of sur- higher surface roughness. The spatial distribution of face roughness as it is simple to understand [3] and is surface roughness may reflect the migration of fines ex- directly related to the surface roughness measure used posing the largest boulders. While Bennu possesses in thermophysical modeling [7]. RMS deviation is the many impact craters [8, 9, 10], their main contribution root-mean-square of the detrended difference in topog- to surface roughness is from exposed large boulders on raphy at a specified baseline, L, in each mapped bin. To- some crater rims. Further work will investigate spatial pography was derived from stereophotoclinometry variations in surface roughness across craters of differ- (SPC) and data from the OSIRIS-Rex Laser Altimeter ent sizes at various baselines. Further work will investi- (OLA) (see 8 and references therein). gate spatial variations in surface roughness across cra- Surface Roughness Maps: We generated global ters of different sizes at various baselines and variations surface roughness maps from scales of 80 cm to 24 m at centimeter-scale baselines are expected to show and show results at 80 cm and 6.4 m here (Figure 1). For larger regional variations. the final four candidate landing sites, surface roughness In comparison to (25143) Itokawa, a similarly sized maps at several baselines were also generated, and we ruble-pile asteroid, the surface roughness of Bennu is discuss the results for Nightingale, the selected primary more uniform and does not show the large scale spatial sample collection site. variations associated with Itokawa’s highlands and low- Large-scale roughness (L = 6.4 m). At these base- lands [5]. The uniform surface roughness of Bennu may lines surface roughness is dominated by boulder den- reflect less efficient size sorting of regolith on this as- sity, with larger values at boulder fields and smaller val- teroid versus on Itokawa. ues in regions with fewer boulders. The equator shows References: [1] Lauretta, D. S., et al. (2019) Nature, slightly higher surface roughness compared to that at 568, 55-60. [2] Barnouin, O. S., et al. (2019) Nature Ge- mid-latitudes and there are no obvious hemispherical oscience, 12, 247-252. [3] Shepard M. K., et al. (2001) differences. Impact craters do not appear to affect J. Geophys. Res. 106, 32777-32796. [4] Susorney H. C. 51st Lunar and Planetary Science Conference (2020) 1372.pdf

M., Barnouin, O. S., et al. (2018) Icarus, 314, 299-310. in press. [9] Walsh, K. J., et al., (2019) Nat. Geosci., [5] Susorney H. C. M. et al. (2019) Icarus, 325, 141- 12, 242-246. [10] DellaGiustina, D. N., Emery, J. P., et 152. [6] Roberts, J.P. et al., (2019) LPSC L, Abstract al. (2019) Nat. Astron., 3, 341-351. [9] Bierhaus, E. B., #2132. [7] Rozitis B., and Green, S. F. (2012) R. Astron. et al. (2019) EPSC-DPS, Abstract #1134. Soc. 423, 367-388. [8] Barnouin O.S., et al., (2019) PSS,

Figure 1. Maps of the surface roughness from SPC at baselines of 80 cm (a,b) and 6.4 m (c-f). The 6.4-m maps are also overlain with all potential craters identified as yellow circles from [9].