Collisional history of Ryugu’s parent body from bright surface boulders

Abstract

The asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1,2,3. Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids.

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Fig. 1: Bright boulders observed by the ONC-T.
Fig. 2: Visible spectral variety of S-type bright boulders.
Fig. 3: Visible spectral variety of C-/X-type bright boulders.
Fig. 4: S-type bright boulders absorptions at p band (0.95 µm), band I depth, observed by ONC-T and 2 µm band depth, band II depth, observed by NIRS3.

Data availability

All images and input data used in this study are available at the JAXA Data Archives and Transmission System (DARTS) at http://www.darts.isas.jaxa.jp/pub/hayabusa2/paper/Tatsumi_2020, and higher-level data products will be available in the Small Bodies Node of the NASA Planetary Data System (https://pds-smallbodies.astro.umd.edu/) one year after mission departure from the asteroid.

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Acknowledgements

The Hayabusa2 spacecraft was developed and built under the leadership of JAXA, with contributions from DLR and CNES, and in collaboration with NASA, Nagoya University, University of Tokyo, NAOJ, University of Aizu, Kobe University, and other universities, institutes and companies in Japan. We thank many engineers, including N. Inaba of JAXA and T. Masuda, S. Yasuda, K. Matsushima and T. Ohshima of NEC Corp. for their dedicated work on the Hayabusa2 mission, K. Sato at NEC Corp. for ONC-T development, and S. Kashima at NAOJ for optical calculations. Funding: E.T. acknowledges financial support from the project ProID2017010112 under the Operational Programmes of the European Regional Development Fund and the European Social Fund of the Canary Islands (OP-ERDF-ESF), as well as the Canarian Agency for Research, Innovation and Information Society (ACIISI). S.W., T. Morota, M. Arakawa, T.O., T.N., N.N., M. Abe, S. Sasaki and S. Sugita were supported by KAKENHI from the JSPS (grant numbers 17H06459, 19H01951, 16H04044, 19K03958 and 18H01267) and the JSPS Core-to-Core program ‘International Network of Planetary Sciences’. T.H. acknowledges funding support from NASA Emerging World/Planetary Data Archiving and Restoration. D.D. acknowledges funding through the NASA Hayabusa2 Participating Scientist Program (grant number NNX16AL34G) and NASA’s Solar System Exploration Research Virtual Institute 2016 (SSERVI16) Cooperative Agreement (NNH16ZDA001N) for TREX (Toolbox for Research and Exploration). P.M. and M.A.B. acknowledge funding support from the French space agency CNES. P.M. also acknowledges funding 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.

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Authors

Contributions

E.T. coordinated co-author contributions, led the ONC-T data analyses and interpretations; ONC-T data acquisitions and reductions: R.H., N.S., Y. Yokota, M.Y., S. Sugita, T. Morota, S. Kameda, H. Sawada, M.M., T.K., E.T., C. Honda, K.O., H. Suzuki, M.I., K. Yoshioka, M.H., Y.C., C.S. and M.S.; NIRS3 data acquisitions and reductions: L.R., K.K., C.P., T.I., M. Abe and M. Ohtake; shape modeling and spacecraft trajectory: N.H. (Kobe), N.H. (Aizu). and Y. Yamamoto; science operations of spacecraft: S. Tanaka, F.T., S.N., S.K., T.Y., N.O., G.O., Y.M., K. Yoshikawa, S. Sugita, T.T., Y. Takei, A.F., H.T., Y. Yamamoto, T.O., M.Y., Y.S., K.S., N.H. (Kobe), Y.I., K.O., C.H., S.H., O.M., H.S., T. Shimada, S. Soldini, R.T., T.I., M.H., H.Y., M. Ozaki, M. Abe, M. Yoshikawa, T. Saiki, S.W. and Y. Tsuda; project administration: Y. Tsuda, S.W., M. Yoshikawa, T. Saiki, S. Tanaka, F.T., S.N., Y. Yamamoto, K.K., S. Sugita, N.N., M. Abe, S. Tachibana, M. Arakawa, H.I., M.I. and K.W.; interpretation and writing contribution: E.T., C.S., L.R., T.N., S. Sugita, T. Morota, M.P., M.M., S.W., T.H., M.T., N.S., D.D., M.A.B., P.M., H.Y. and J.d.L. All authors discussed the results and commented on the manuscript.

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Correspondence to E. Tatsumi.

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Extended data

Extended Data Fig. 1 Bright boulder morphologies.

The resolutions of these images are 0.28–0.30 m/pixel, as listed in Extended Data Fig. 4. Scale bars in the images are 5 m. M6 is shown in Fig. 1 of the main text. Source data

Extended Data Fig. 2 Power index for boulder size distribution from close-up images.

The power index is calculated based on the maximum-likelihood fitting method by ref. 35. The cumulative boulder size distribution is expressed as \(N\left( { > D} \right) \propto D^{ - \alpha }\), where D is the diameter of the bright boulder, and α is the power-law index.

Extended Data Fig. 3 Normalized reflectance spectra of bright boulders (black lines) compared with the average spectrum of Ryugu (gray lines).

Lines are offset by 0.7 for clarity. The bright boulders which display two spectra were observed twice in different multiband image sets. Source data

Extended Data Fig. 4

The locations and sizes of bright boulders.

Extended Data Fig. 5 Bright boulders (enclosed by red lines) in close-up images during the MINERVA-II deployment operation on 21 September 2018.

a, hyb2_onc_20180921_043010_tvf, altitude of 636 m (68 mm/pixel). b, hyb2_onc_20180921_041826_tvf, altitude of 335 m (36 mm/pixel). c, hyb2_onc_20180921_034938_tvf, altitude of 148 m (16 mm/pixel). d, hyb2_onc_20180921_040154_tvf, altitude of 75 m (8 mm/pixel). e, hyb2_onc_20180921_040634_tvf, altitude of 68 m (7 mm/pixel). Source data

Extended Data Fig. 6 S-type bright boulders observed by NIRS3.

(left) The NIRS3 spectra of the area including bright boulders (black) and adjacent areas (gray). The footprint size of NIRS3 is in Extended Table 2. (right) The normalized spectra of area including bright boulders by the adjacent areas. Only the M13 shows small absorption around 2 µm. Source data

Extended Data Fig. 7 Peak pressure during a collision between an ordinary chondrite and a carbonaceous chondrite.

The red solid and dashed lines indicate the average and standard deviation of compressive strength among ordinary chondrites, respectively39. The black solid and dashed lines indicate then peak pressure for the target densities of 2.2 g/cc and 1.2 g/cc (Ryugu bulk density), respectively.

Supplementary information

Supplementary Information

Supplementary methods, Figs. 1–6, and Tables 1 and 2.

Supplementary Data

The list of meteorite spectra used in Figs. 2 and 4.

Source data

Source Data Fig. 1

Original images used for Fig. 1 and size distribution of bright boulders.

Source Data Fig. 2

Bright boulder spectra, spectral slope-PC2’ values and the result of principal component analysis.

Source Data Fig. 3

Spectral slope and UV-index values of C-type bright boulders.

Source Data Fig. 4

NIRS3 spectra and calculated band I and band II values for all S-type bright boulders.

Source Data Extended Data Fig. 1

Original images used to compose Extended Data Fig. 1.

Source Data Extended Data Fig. 3

Bright boulder spectra.

Source Data Extended Data Fig. 5

Original images used to compose Extended Data Fig. 5.

Source Data Extended Data Fig. 6

NIRS3 spectra for the S-type bright boulders.

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Tatsumi, E., Sugimoto, C., Riu, L. et al. Collisional history of Ryugu’s parent body from bright surface boulders. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1179-z

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Further reading

  • Exogenic basalt on asteroid (101955) Bennu

    • D. N. DellaGiustina
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    • , A. A. Simon
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    • , J. de Leon
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    • , M. M. Al Asad
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    Nature Astronomy (2020)

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