Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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

Nature Geoscience volume 15, pages 447–452 (2022)Cite this article

Subjects

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.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Get just this article for as long as you need it

$39.95

Learn more

Prices may be subject to local taxes which are calculated during checkout

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/.

References

  1. Walsh, K. J. Rubble pile asteroids. Annu. Rev. Astron. Astrophys. 56, 593–624 (2018).

    Article  Google Scholar 

  2. Holsapple, K. A. The scaling of impact processes in planetary sciences. Annu. Rev. Earth Planet. Sci. 21, 333–373 (1993).

    Article  Google Scholar 

  3. Melosh, H. J. Impact Cratering: A Geologic Process (Oxford Monographs on Geology and Geophysics Series no. 11, Clarendon Press, 1989).

  4. Scheeres, D. J. Disaggregation of small, cohesive rubble pile asteroids due to YORP. Icarus 304, 183–191 (2018).

    Article  Google Scholar 

  5. Sugita, S. et al. The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Science https://doi.org/10.1126/science.aaw0422 (2019).

  6. Lauretta, D. S. et al. The unexpected surface of asteroid (101955) Bennu. Nature 568, 55–60 (2019).

    Article  Google Scholar 

  7. Lauretta, D. S. in Sample Return Missions (ed. Longobardo, A.) 163–194 (Elsevier, 2021).

  8. Rozitis, B. et al. Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator. Sci. Adv. 6, eabc3699 (2020).

    Article  Google Scholar 

  9. Ballouz, R. L. et al. Bennu’s near-Earth lifetime of 1.75 million years inferred from craters on its boulders. Nature https://doi.org/10.1038/s41586-020-2846-z (2020).

  10. Arakawa, M. et al. An artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime. Science https://doi.org/10.1126/science.aaz1701 (2020).

  11. Rizk, B. et al. OCAMS: the OSIRIS-REx camera suite. Space Sci. Rev. 214, 26 (2018).

    Article  Google Scholar 

  12. Golish, D. R. et al. Ground and in-flight calibration of the OSIRIS-REx camera suite. Space Sci. Rev. 216, 12 (2020).

    Article  Google Scholar 

  13. Bennett, C. A. et al. A high-resolution global basemap of (101955) Bennu. Icarus https://doi.org/10.1016/j.icarus.2020.113690 (2021).

  14. DellaGiustina, D. N. et al. Variations in color and reflectance on the surface of asteroid (101955) Bennu. Science https://doi.org/10.1126/science.abc3660 (2020).

  15. Barnouin, O. S. et al. Digital terrain mapping by the OSIRIS-REx mission. Planet. Space Sci. 180, 104764 (2020).

    Article  Google Scholar 

  16. Daly, R. T. et al. The morphometry of impact craters on Bennu. Geophys. Res. Lett. 47, e2020GL089672 (2020).

  17. Daly, M. G. et al. Hemispherical differences in the shape and topography of asteroid (101955) Bennu. Sci. Adv. 6, eabd3649 (2020).

    Article  Google Scholar 

  18. Jawin, E. R. et al. Global patterns of recent mass movement on asteroid (101955) Bennu. J. Geophys. Res. Planets 125, e2020JE006475 (2020).

    Article  Google Scholar 

  19. Takizawa, S. & Katsuragi, H. Scaling laws for the oblique impact cratering on an inclined granular surface. Icarus 335, 113409 (2020).

    Article  Google Scholar 

  20. Walsh, K. J. et al. Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface. Nat. Geosci. 12, 242–246 (2019).

    Article  Google Scholar 

  21. Scheeres, D. J. et al. The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements. Nat. Astron. 3, 352–361 (2019).

    Article  Google Scholar 

  22. Chesley, S. R. et al. Trajectory estimation for particles observed in the vicinity of (101955) Bennu. J. Geophys. Res. Planets 125, e2019JE006363 (2020).

    Article  Google Scholar 

  23. Housen, K. R., Schmidt, R. M. & Holsapple, K. A. Crater ejecta scaling laws: fundamental forms based on dimensional analysis. J. Geophys. Res. 88, 2485–2499 (1983).

    Article  Google Scholar 

  24. Housen, K. R. & Holsapple, K. A. Ejecta from impact craters. Icarus 211, 856–875 (2011).

    Article  Google Scholar 

  25. Bierhaus, E. B. et al. Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface. Nat. Geosci. https://doi.org/10.1038/s41561-022-00914-5 (2022).

  26. Slyuta, E. N. Physical and mechanical properties of the lunar soil (a review). Sol. Syst. Res. 48, 330–353 (2014).

    Article  Google Scholar 

  27. Barnouin, O. S. et al. Shape of (101955) Bennu indicative of a rubble pile with internal stiffness. Nat. Geosci. 12, 247–252 (2019).

    Article  Google Scholar 

  28. Honda, R. et al. Resurfacing processes on asteroid (162173) Ryugu caused by an artificial impact of Hayabusa2’s Small Carry-on Impactor. Icarus 366, 114530 (2021).

    Article  Google Scholar 

  29. McCaffrey, W. D. (ed.) Particulate Gravity Currents Vol. 31 (Blackwell Science, 2001).

  30. Iverson, R. M. The physics of debris flows. Rev. Geophys. 35, 245–296 (1997).

    Article  Google Scholar 

  31. Lauretta, D. S. & the OSIRIS-REx TAG team. The OSIRIS-REx touch-and-go sample acquisition event and implications for the nature of the returned sample. In Proc. 52nd Lunar and Planetary Science Conference LPI contribution no. 2548, abstract no. 2097 (2021).

  32. Vokrouhlický, D., Bottke, W. F., Chesley, S. R., Scheeres, D. J. & Statler, T. S. in Asteroids IV (eds Michel, P. et al.) 509–531 (Univ. Arizona Press, 2015).

  33. Bottke, W. F. et al. Interpreting the cratering histories of Bennu, Ryugu, and other spacecraft-explored asteroids. Astron. J 160, 14 (2020).

    Article  Google Scholar 

  34. Roberts, J. H. et al. Rotational states and shapes of Ryugu and Bennu: implications for interior structure and strength. Planet. Space Sci. 204, 105268 (2021).

  35. Bottke, W. F. et al. In search of the source of asteroid (101955) Bennu: applications of the stochastic YORP model. Icarus 247, 191–217 (2015).

    Article  Google Scholar 

  36. Walsh, K. J., Delbo, M., Bottke, W. F., Vokrouhlicky, D. & Lauretta, D. S. Introducing the Eulalia and new Polana asteroid families: re-assessing primitive asteroid families in the inner Main Belt. Icarus 225, 283–297 (2013).

    Article  Google Scholar 

  37. Daly, M. G. et al. The OSIRIS-REx Laser Altimeter (OLA) investigation and instrument. Space Sci. Rev. 212, 899–924 (2017).

    Article  Google Scholar 

  38. Schultz, P. H., Ernst, C. M. & Anderson, J. L. B. Expectations for crater size and photometric evolution from the deep impact collision. Space Sci. Rev. 117, 207–239 (2005).

    Article  Google Scholar 

  39. Housen, K. R., Sweet, W. J. & Holsapple, K. A. Impacts into porous asteroids. Icarus 300, 72–96 (2018).

    Article  Google Scholar 

  40. Golish, D. R. et al. A high-resolution normal albedo map of asteroid (101955) Bennu. Icarus 355, 114133 (2020).

  41. Rizk, B., Drouet d’Aubigny, C., Golish, D., DellaGiustina, D. N. & Laurentta, D. S. Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS‐REx): OSIRIS‐REx Camera Suite (OCAMS) Bundle (NASA Planetary Data System, 2019).

  42. Daly, M., Barnouin, O., Espiritu, R. & Lauretta, D. Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx): OSIRIS-REx Laser Altimeter Bundle (NASA Planetary Data System, 2019).

Download references

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

  1. Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA

    M. E. Perry, O. S. Barnouin, R. T. Daly & C. M. Ernst

  2. Lockheed Martin Space, Littleton, CO, USA

    E. B. Bierhaus

  3. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    R.-L. Ballouz, D. N. DellaGiustina, M. C. Nolan, D. R. Golish & D. S. Lauretta

  4. Southwest Research Institute, Boulder, CO, USA

    K. J. Walsh & W. F. Bottke

  5. The Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada

    M. G. Daly & J. A. Seabrook

  6. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA

    J. P. Emery

  7. Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada

    M. M. Al Asad & C. L. Johnson

  8. National Museum of Natural History, Smithsonian Institution, Washington, DC, USA

    E. R. Jawin

  9. Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France

    P. Michel

Authors
  1. M. E. Perry
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. O. S. Barnouin
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. R. T. Daly
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. E. B. Bierhaus
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. R.-L. Ballouz
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. K. J. Walsh
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. M. G. Daly
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. D. N. DellaGiustina
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. M. C. Nolan
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. J. P. Emery
    View author publications

    You can also search for this author in PubMed Google Scholar

  11. M. M. Al Asad
    View author publications

    You can also search for this author in PubMed Google Scholar

  12. C. L. Johnson
    View author publications

    You can also search for this author in PubMed Google Scholar

  13. C. M. Ernst
    View author publications

    You can also search for this author in PubMed Google Scholar

  14. E. R. Jawin
    View author publications

    You can also search for this author in PubMed Google Scholar

  15. P. Michel
    View author publications

    You can also search for this author in PubMed Google Scholar

  16. D. R. Golish
    View author publications

    You can also search for this author in PubMed Google Scholar

  17. W. F. Bottke
    View author publications

    You can also search for this author in PubMed Google Scholar

  18. J. A. Seabrook
    View author publications

    You can also search for this author in PubMed Google Scholar

  19. D. S. Lauretta
    View author publications

    You can also search for this author in PubMed Google Scholar

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.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

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.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Table 1 Fitted parameters used for analyses and simulations24
Full size table

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.

Supplementary information

Supplementary Information

Additional discussion material.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41561-022-00937-y

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing