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Bennu’s near-Earth lifetime of 1.75 million years inferred from craters on its boulders

Abstract

An asteroid’s history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu’s metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years.

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Fig. 1: Craters are observed on Bennu’s boulders in images and laser altimetry data.
Fig. 2: The maximum crater size on a boulder depends on boulder strength.
Fig. 3: Bennu’s boulders are relatively weak and have short lifetimes in the main asteroid belt.
Fig. 4: The surface exposure age of Bennu’s metre-size boulders is ~1.75 Myr.

Data availability

OCAMS images and OLA data from the Orbital A, Detailed Survey and Orbital B phases of the OSIRIS-REx mission are available in the Planetary Data System at https://sbn.psi.edu/pds/resource/orex/. Measured dimensions and locations of craters and host boulders are available in Extended Data Tables 1, 2 and Supplementary Table 1.

Code availability

The Small Body mapping tool is available at http://sbmt.jhuapl.edu/.

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Acknowledgements

We thank D. P. Hamilton, K. A. Holsapple and P. Pravec for insights and feedback. This material is based upon work supported by NASA under contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. M.D., C.A. and P.M. acknowledge the French space agency CNES. C.A. and M.D. acknowledge support from ANR “ORIGINS” (ANR-18-CE31-0014). C.A. was supported by the French National Research Agency under the project “Investissements d’Avenir” UCAJEDI ANR-15-IDEX-01. P.M. acknowledges funding support from the European Union’s Horizon 2020 research and innovation program under grant agreement number 870377 (project NEO-MAPP) and from Academies of Excellence: Complex Systems and Space, Environment, Risk, and Resilience, part of the IDEX JEDI of Université Côte d’Azur. M.P. was supported by the Italian Space Agency (ASI) under ASI-INAF agreement number 2017-37-H.0. S.R.S. is supported by contract number 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program.

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Authors and Affiliations

Authors

Contributions

R.-L.B. led the conceptualization of the study, the images and OLA data analysis, construction of the analytical formalism to measure boulder impact strength from crater sizes, the interpretation of results, and manuscript preparation efforts. K.J.W. contributed to the conceptualization of the study, the interpretation of results, and manuscript preparation efforts. O.S.B. created OLA DTMs of boulders, measured crater dimensions with these products, contributed to the interpretation of results and the preparation of the manuscript. D.N.D., E.R.J. and C.A.B. provided GIS guidance and expertise, contributed to the image analysis, interpretation of the results and the preparation of the manuscript. M.A.A., M.G.D. and R.T.D. contributed to the OLA data analysis, interpretation of the results and the preparation of the manuscript. W.F.B., P.M., C.A., M.D., J.L.M., E.A., E.B.B., M.C.N., M.P., H.C.C. Jr, S.R.S., D.T. and C.W.V.W. contributed to the interpretation of the results and the preparation of the manuscript. B.R. and D.R.G. processed the PolyCam images presented in the manuscript. D.S.L. leads the mission and contributed to the analysis and writing.

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Correspondence to R.-L. Ballouz.

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Peer review information Nature thanks Dan Britt and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Examples of boulders with craters and OLA profiles of the craters.

Boulders are outlined with dashed orange polygons. Craters are outlined with dashed white circles. The crater profile shown below each image corresponds to the dashed yellow line, with the letters A (start) and B (end) in the image indicating the direction of the corresponding profile (from left to right). a, Boulder 1 (image 20190328T191143S208_pol) has a circle-equivalent diameter of 2.9 m and an OLA-measured crater diameter of 1.21 ± 0.09 m. b, Boulder 2 (image 20190328T182010S618_pol) has a circle-equivalent diameter of 3.06 m and an OLA-measured crater diameter of 1.24 ± 0.07 m. c, Boulder 3 (image 20190329T205259S821_pol) has a circle-equivalent diameter of 4.24 m and an OLA-measured crater diameter of 1.60 ± 0.13 m. d, Boulder 4 (image 20190321T185825S567_pol) has a circle-equivalent diameter of 11.3 m and an OLA-measured crater diameter of 4.18 ± 0.47 m.

Extended Data Table 1 Summary of OLA crater profile measurements for a subset of boulders with crater size close to the maximum allowable before disruption
Extended Data Table 2 Locations of boulders with flat faces that exhibit multiple impact craters on their surface

Supplementary information

41586_2020_2846_MOESM1_ESM.pdf

Supplementary Table 1 Dimensions of craters and their host boulders. We tabulate the locations of boulders with craters and their dimensions based on shape model–projected images with pixel scales of 5 cm. We measure their largest crater radius \({R}_{{\rm{c}}}\), the boulder areal extent \({A}_{{\rm{b}}}\), and the ratio of the crater radius to the host boulder circle-equivalent radius \({R}_{{\rm{c}}}/{R}_{{\rm{t}}}\). We assume 3-pixel uncertainties (15 cm) for these measurements.

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Ballouz, RL., Walsh, K.J., Barnouin, O.S. et al. Bennu’s near-Earth lifetime of 1.75 million years inferred from craters on its boulders. Nature 587, 205–209 (2020). https://doi.org/10.1038/s41586-020-2846-z

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