Properties of the asteroid Bennu’s surface based on contact measurements and their implications
D.J. Scheeres1, G. Brown1, R.-L. Ballouz2, E.B. Bierhaus3, E.R. Jawin4, P. Sánchez1, and K.J. Walsh5
1University of Colorado, Boulder, CO, USA
2Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
3Lockheed Martin Space, Littleton, CO, USA
4National Air and Space Museum, Smithsonian Institution, Washington, DC, USA
5Southwest Research Institute, Boulder, CO, USA
When the OSIRIS-REx S/C touched the surface of Bennu to carry out its sampling operations, it opened a new view into themechanical properties of rubble pile asteroids. The OREX S/C was able to take measurements of its deceleration as it
penetrated the surface, in advance of initiating its sampling process. Based on these measurements, optical observations ofsurface deformations during its penetration, and detailed granular mechanics modeling of the interaction, it was determinedthat the surface regolith covering was highly porous and had a very low level of cohesion [1]. Figure 1 (from [1]) shows themain data used in determining the Bennu surface mechanical properties. Figure 2 shows granular mechanics simulations in a
micro-gravity environment. The simulations capture the total spacecraft mass properties, the constant force spring in thesampling arm, and the geometry of the sampling head. The simulations show that for sufficiently low cohesion, the spacecraft
head penetrates the regolith and displaces it, without activating the constant force spring. This is the scenario that was seen in
the actual sampling event. Simulating regolith with stronger cohesive strength forces the spacecraft to stop and rebound,engaging the constant force spring.
Based on these measurements and modeling the surface packing fraction was determined to be as low as 0.25, half the inferredbulk packing fraction of the asteroid, and the cohesive strength on the order of 1 Pa or less. These properties imply that thesurface regolith may be significantly weaker than the sub-surface, potentially due to the lack of fines, which have the net effectof strengthening a rubble pile structure [2]. This is also consistent with other observations of the Bennu surface geology [3]and crater profiles [4].
The implications of these regolith properties can be explored relative to the larger-scale mass distributions detected on Bennu,
which found that the equatorial bulge region had a lower overall density than other regions of the asteroid [5]. Migration ofregolith into Bennu’s equatorial region brings this material within the rotational Roche lobe of the body where it isenergetically trapped, allowing for accumulation [5]. This process should have been accelerating over time, as the YORP effectcauses Bennu to spin more rapidly [6], moving the Roche lobe down to lower latitudes. Current predictions are that the surfaceof Bennu should start to lose regolith directly in about 400 Ky, assuming the YORP effect stays constant. In this talk, we willconsider the mechanics of this overall cycle of behavior, and note what likely outcomes should be found. These can then beinterpreted in light of the asteroid Ryugu, which has also been hypothesized to have undergone a period of past rapid spin [7].
References[1] K.J. Walsh, et al. Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by
spacecraft contact, Sci. Adv., 8 (27), eabm6229. • DOI: 10.1126/sciadv.abm6229[2] P. Sánchez and D.J. Scheeres. 2014. “The Strength of Regolith and Rubble Pile Asteroids,” Meteoritics and Planetary
Science 49(5): 788-811[3] E. R. Jawin, et al. 2020. Global Patterns of Recent Mass Movement on Asteroid (101955) Bennu, JGR Planets
10.1029/2020JE006475[4] E.B. Bierhaus, et al. Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface, Nature
Geoscience 15, pages 440–446 (2022)[5] D.J. Scheeres, et al. Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu, Science Advances 6,eabc3350 (2020)[6] C. Hergenrother, et al. (2019). The Operational Environment and Rotational Acceleration of Asteroid (101955) Bennu fromOSIRIS-REx Observations, Nature Communications 10: 1291
[7] M. Hirabayashi, et al. (2019). The western bulge of 162173 Ryugu formed as a result of a rotationally driven deformation
process, ApJ Letters 874(1): L10

Figure 1: From Reference [1], these figures show the acceleration and surface imaging data used to constrain the mechanicalproperties of the regolith surface.

WeakCohesion

t=0s

t = 2.7 s

t = 7.0 s

StrongerCohesion

Figure 2: Granular mechanics simulations of the OSIRIS-REx TAGSAM sampling in a micro-gravity environment. Weakly
cohesive regolith (top) allow the sampler head to penetrate and push the regolith. Stronger cohesion will stop the spacecraft
and cause a small rebound. The actual sampling event mimics the weakly cohesive case.