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Low surface strength of the asteroid Bennu inferred from impact ejecta deposit

Abstract

The surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. Here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) Bennu to constrain the asteroid’s surface properties. We infer that the ejecta deposited near the crater must have been mobilized with velocities less than Bennu’s escape velocity (20 cm s–1); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally ≤2 Pa. This value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, Ryugu. We find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since Bennu’s initial assembly. Compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates—although the heterogeneous structure of rubble piles complicates interpretation.

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Fig. 1: Bralgah Crater and the surrounding uniform terrain.
Fig. 2: Relationships between the mass and velocity of ejecta from scaling relationships.
Fig. 3: Simulation results of ejecta leaving the rim of Bralgah Crater according to gravity scaling.

Data availability

OCAMS data are available via the Planetary Data System (PDS) at https://sbn.psi.edu/pds/resource/orex/ocams.html41. The global image mosaic of Bennu is available in ref. 13. OLA data underlying the DTMs used for slope calculations are available via the PDS at https://sbn.psi.edu/pds/resource/orex/ola.html42. The v.42 global DTM is available from the Small Body Mapping Tool (SBMT) at https://sbmt.jhuapl.edu. The output of the ejecta simulations is archived at https://lib.jhuapl.edu/.

Code availability

The ejecta-simulation programmes are available at https://lib.jhuapl.edu/.

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Acknowledgements

This material is based on work supported by NASA under contracts NNM10AA11C and NNG12FD66C, issued through the New Frontiers Program. The OSIRIS-REx Laser Altimeter and the Canadian authors were supported by the Canadian Space Agency. P.M. acknowledges funding support from the French space agency CNES, from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 870377 (project NEO-MAPP) and from Academies of Excellence: Complex systems and Space, environment, risk, and resilience, part of the IDEX JEDI of the Université Côte d’Azur. This work used the Small Body Mapping Tool (http://sbmt.jhuapl.edu). We are grateful to C. Wolner for her indispensable editing support and to the entire OSIRIS-REx Team of engineers, operators, scientists and administrators for making the encounter with Bennu possible.

Author information

Authors and Affiliations

Authors

Contributions

M.E.P. led the data analysis and writing. O.S.B. led the Altimetry Working Group that produced the DTMs. O.S.B., R.T.D. and C.M.E. contributed analyses and expertise on crater processes. M.G.D. and J.A.S. provided the altimetry data for the high-resolution DTMs. E.B.B. and R.-L.B. provided analyses on crater-retention age. K.J.W., M.C.N. and P.M. contributed to writing. D.N.D. and D.R.G. provided image and spectral analyses. J.P.E., M.M.A., E.R.J., W.F.B. and C.L.J. provided analytical insight. D.S.L. is principal investigator of the OSIRIS-REx mission.

Corresponding author

Correspondence to M. E. Perry.

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Nature Geoscience thanks Akbar Whizin, Jennifer Anderson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin and James Super, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Table 1 Fitted parameters used for analyses and simulations24

Extended Data Fig. 1 Color-phase slope of Bennu’s surface.

The colors are the phase slope (Golish et al. 2021) from the linear (in magnitude space) phase function, averaged over 1 degree and normalized to the Bennu average. The underlying data are the PolyCam albedo basemap (Golish et al. 2021). Notionally, low phase slope values (blue) indicate a smoother surface. The blue area north of Bralgah Crater is centered at −45°, 325° E. The scale is –10%/+5%, so the flow region is approximately a 10% effect in the phase slope.

Extended Data Fig. 2 Laser altimetry topography of the flow field around boulder 2.

Laser altimetry topography of the flow field around boulder 2 showing elevations 4 to 5 m lower behind (north) of the bolder. The blue and red lines shown in a correspond to the profiles in b.

Extended Data Fig. 3 Topography of Bralgah Crater from laser altimetry data.

Topography of Bralgah Crater from laser altimetry data (Figs. 1, 2)16. a, DTM overlaid onto an OCAMS image (ocams20190419t204556s223_map_iofl2pan_92585). North (downslope) is to the left. b, Eight profiles of the crater. The value d/Delevation is crater depth (calculated from elevation) divided by crater diameter. The apparent asymmetry is due to the prevailing ~23° slope of the local region. Because of compaction and uplift near the crater rim, the total volume of material excavated from an impact crater is typically about 2/3 of the crater volume24,37.

Extended Data Fig. 4 Ejecta mass and velocity as a function of target strength.

Calculations of ejecta velocities and the resulting ejected mass using the equations in Table 1 and published parameters24 for three different low-strength material analogs for Bennu’s regolith. WCB is weakly cemented basalt, and ‘Base’ has the constant C3 = 1 in the strength equation for ejection velocity. a, Fraction of ejecta landing within one crater radius of Bralgah as a function of target strength for the three different types of low-strength materials. The red line represents the fraction for gravity-regime scaling. The highest strengths that produce as much low-velocity ejecta as in the gravity regime are 0.1, 1.2, and 1.9 Pa (marked with circles). We consider this the range of possible strengths for Bennu’s regolith. b, The minimum ejection velocity for the different strength parameterizations. The red line represents the lowest observed speed based on ejecta as close as 1 crater radius from the rim. The solid lines use the Table-1 equations, and the dashed lines include an additional factor that assumes ejecta velocities are not truncated and must smoothly approach zero. Although many of the potential surface properties do not have sufficiently slow velocities at 20 Pa, all of the strength parameterizations have high velocities at 100 Pa.

Extended Data Fig. 5 Higher-resolution view of boulder 2 showing more material on the side of the boulder facing the flow.

Higher-resolution view (global mosaic13) of boulder 2 showing more material on the side of the boulder facing the flow. This indicates that material flowed against the south-east side of the boulder. Extended Data Fig. 2 shows the drop in elevation to the northwest.

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Perry, M.E., Barnouin, O.S., Daly, R.T. et al. Low surface strength of the asteroid Bennu inferred from impact ejecta deposit. Nat. Geosci. 15, 447–452 (2022). https://doi.org/10.1038/s41561-022-00937-y

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