Characterization of a mass movement site in Bennu’s Bralgah Crater and implications for other asteroids
Yuhui Tang1, D.S. Lauretta1, R.-L. Ballouz2, D.N. DellaGiustina1, C.A. Bennett1, D.R. Golish1
1
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
2
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
Near-Earth asteroids such as Bennu and Ryugu have boulder-covered surfaces and latitude-dependent slope
distributions, signifying that mass movements of boulders perform significant roles in the surface evolution of these smallbodies. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission[1] documented numerous locations on Bennu exhibiting evidence of mass movements of surface material, mostly towards thelower elevation equatorial region [2]. One such location, contained within Bralgah Crater, was considered (but not used) by themission as a candidate site for sample collection; thus, high-resolution images collected during reconnaissance of this area
allow a more detailed survey than would be possible elsewhere [3] [Fig. 1]. An apparent flow “wake” downhill from a 4.8-mdiameter central boulder in the area, where medium-sized boulders are much less prevalent than in the surrounding terrain,provides an opportunity to investigate the characteristics of mass flow on the asteroid. Through boulder mapping andtopographic analysis, we found evidence of a pileup behind the central boulder, as well as a strong orientational preference ofthe boulders in this area [Fig. 2]. This preferential long-axis orientation exhibits the same westward deflection from theexpected downslope direction that we observed at another mass movement site in Bennu’s northern hemisphere [4], implyingthat a global mechanism with effects mirrored between the northern and southern hemispheres is affecting the mass flows.
We performed dynamical simulations of seismic shaking using the discrete-element N-body code PKDGRAV [5–8]to better constrain the conditions that may have formed the landscape. In these simulations, we used boulder sizes similar to
those surveyed at the Bralgah Crater site and an environment with a similar 4.8-m-diameter central boulder. We were able toreplicate the pileup of material uphill from the central boulder and the wake downhill from it, producing similar elevation
profiles created by a lack of downflowing material in the wake area [Fig. 3].
Simulations also showed a preferential orientation emerging, with a strong possibility for preferential orientation
parallel to direction of motion. Combined with the orientational preference observed in this and other studies of Bennu’s
landscape, this indicates that the Coriolis effect could be a major factor in the mass movement of boulders and would help to
explain the westward deflection of the boulders’ long-axis orientation. Numerical simulations show that the Coriolis effect isstrong enough to induce an east-west displacement of the boulders, which can amount to 10% of the downslope displacement,
which could lead to the observed orientational preferences. This effect would be expected to occur on other fast-spinning
asteroids, such as Ryugu, where this hypothesis predicts a similar orientational preference, with eastward deviation from thepoleward migration of the boulders.

Figure 1. A map of the survey area (magenta outline) within Bralgah Crater, projected in ArcGIS, with the central boulder (long axis, 4.8 m)
highlighted by the light blue best-fit ellipse. The downhill direction is north in this area.

Figure 2. Orientations of the surveyed boulders as a rose diagram (A) and histogram (B), with the data best fitted by a bimodal von Mises
distribution (green) with peaks at 94.1° and 164.2° (p-value = 0.839). (C) The empirical distribution function (black line) and cumulative
distribution function (green line) of the orientations, used for the K-S tests to derive the p-values.

Figure 3. (A) Elevation map of the study site, overlayed with arrows indicating the direction of greatest slope, with the locations of theelevation profiles along longitudes 321°E and 323°E marked in cyan and orange, respectively. (B) The elevation profiles indicated in (A),with the 321°E line profile (cyan) crossing the middle of the central boulder. The blue dashed line represents the best-fit slope of the pileup
area uphill from the central boulder, extrapolated across the entire mass movement length. The extrapolated profile ranges from 0.8 to 1.6 m
above the actual elevation profile downhill from the central boulder, with greater differences further north (downhill). (C) Elevation profileof simulations after one shaking. The cyan solid line shows elevation along the center of the wake, and the orange dashed-dot line shows theelevation along a parallel transect away from the wake.

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