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Reconstructing the formation history of top-shaped asteroids from the surface boulder distribution

Nature Astronomy volume 5, pages 134–138 (2021)Cite this article

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Abstract

Finding the basic mechanism governing the surface history of asteroids of various shapes is essential for understanding their origin and evolution. In particular, the asteroids (162173) Ryugu1 and (101955) Bennu2 currently being visited by Hayabusa2 and OSIRIS-REx appear to be top shaped. This distinctive shape, characterized by a raised equatorial bulge, is shared by other similarly sized asteroids, including Didymos A3, 2008 EV54 and 1999 KW4 Alpha5. However, the possibly common formation mechanism that causes the top-like shape is still under debate. One clue may lie in the boulders on their surfaces. The distribution of these boulders, which was precisely measured in unprecedented detail by the two spacecraft1,2, constitutes a record of the geological evolution of the surface regolith since the origin of these asteroids. Here, we show that during the regolith migration driven by Yarkovsky–O’Keefe–Radzievskii–Paddack spin-up6,7,8,9 the surface boulders coevolve with the underlying regolith and exhibit diverse dynamical behaviours: they can remain undisturbed, sink into the regolith layer and become tilted, or be totally buried by the downslope deposition, depending on their latitudes. The predominant geological features commonly observed on top-shaped asteroids, including the boulder-rich region near the pole1,10, the deficiency of large boulders in the equatorial area10,11 and partially buried, oblique boulders exposed on the regolith surface12,13, are commensurate with this coevolution scenario. The surface regolith migration thus is the prevalent mechanism for the formation history of the top-shaped asteroids with stiffer cores.

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Fig. 1: Asteroid morphological change during the YORP spin-up process.
Fig. 2: The simulated geomorphology.
Fig. 3: Visualization of the force networks excited by the creeping boulder.
Fig. 4: Typical landforms of boulders sculpted by YORP evolution.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Data used in Fig. 4d, e and f can be found at http://darts.isas.jaxa.jp/pub/hayabusa2/onc_bundle/ under image indexes hyb2_onc_20180831_101059_tvf_12a, hyb2_onc_20180801_160642_tnf_l2c and hyb2_onc_20180720_075208_tvf_l2b, respectively.

Code availability

The code used to generate the datasets is available from the corresponding authors on reasonable request.

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Acknowledgements

This work is funded by the National Science Fund for Distinguished Young Scholars of China (no. 11525208). Y.Y. acknowledges support from the Natural Science Foundation of China (no. 11702009). P.M. acknowledges funding 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. We acknowledge support from the JSPS Core-to-Core programme ‘International Network of Planetary Sciences’. We are grateful to the Hayabusa2 ONC team for providing images, which are available at the JAXA Data Archives and Transmission System (DARTS) at http://darts.isas.jaxa.jp/pub/hayabusa2/onc_bundle.

Author information

Authors and Affiliations

  1. School of Aerospace Engineering, Tsinghua University, Beijing, China

    Bin Cheng & Hexi Baoyin

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

    Bin Cheng & Erik Asphaug

  3. School of Aeronautic Science and Engineering, Beihang University, Beijing, China

    Yang Yu

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

    Patrick Michel

  5. Department of Astronomy, University of Maryland, College Park, MD, USA

    Derek C. Richardson

  6. Department of Aerospace Engineering, Auburn University, Auburn, AL, USA

    Masatoshi Hirabayashi

  7. Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan

    Makoto Yoshikawa

  8. The Graduate University for Advanced Studies (SOKENDAI), Kanagawa, Japan

    Makoto Yoshikawa

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Contributions

B.C. performed the soft-sphere numerical simulations and analysed the numerical results. H.B. and Y.Y. initiated the project, designed the simulations and led the research. E.A. and D.C.R. contributed to the discussion of the two-layer model and the creeping process. M.H. provided essential comments on the interior structure and deformation. P.M. initiated the collaboration between the institutions with Y.Y. and provided outstanding questions on the scope of the research with M.Y. All authors contributed to interpretation of the results and preparation of the manuscript.

Corresponding authors

Correspondence to Yang Yu or Hexi Baoyin.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks David Polishook and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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

Supplementary information

Supplementary Information

Supplementary methods, Tables 1 and 2 and Figs. 1–7.

Supplementary Video 1

Moderate friction. 75° LBx boulder. Coloured by slip heat.

Supplementary Video 2

Moderate friction. 75° LBx boulder. Coloured by latitude.

Supplementary Video 3

Moderate friction. 87° LBx boulder. Coloured by latitude.

Supplementary Video 4

Moderate friction. 45° LBx boulder. Coloured by latitude.

Supplementary Video 5

Moderate friction. 15° LBx boulder. Coloured by latitude.

Supplementary Video 6

Gravel-like friction. 75° LBx boulder. Coloured by latitude.

Supplementary Video 7

Moderate friction. Regolith bed with width of 160 m.

Supplementary Video 8

Moderate friction. Regolith bed with wedge-shaped boundaries.

Supplementary Video 9

Moderate friction. Regolith bed of both hemispheres.

Supplementary Video 10

Moderate friction. 45° LBx boulder initially buried in regolith.

Supplementary Video 11

Moderate friction. 15° LBx boulder initially buried in regolith.

Supplementary Video 12

Moderate friction. Multiple boulders.

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Cheng, B., Yu, Y., Asphaug, E. et al. Reconstructing the formation history of top-shaped asteroids from the surface boulder distribution. Nat Astron 5, 134–138 (2021). https://doi.org/10.1038/s41550-020-01226-7

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