Abstract
Analyses of meteorites and theoretical models indicate that some carbonaceous near-Earth asteroids may have been thermally altered due to radiative heating during close approaches to the Sun1,2,3. However, the lack of direct measurements on the subsurface doesn’t allow us to distinguish thermal alteration due to radiative heating from parent-body processes. In April 2019, the Hayabusa2 mission successfully completed an artificial impact experiment on the carbonaceous near-Earth asteroid (162173) Ryugu4,5, which provided an opportunity to investigate exposed subsurface material and test potential effects of radiative heating. Here we report observations of Ryugu’s subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. The strength and shape of the OH feature suggests that the subsurface material experienced heating above 300 °C, similar to the surface. In contrast, thermophysical modelling indicates that radiative heating cannot increase the temperature above 200 °C at the estimated excavation depth of 1 m, even at the smallest heliocentric distance possible for Ryugu. This supports the hypothesis that primary thermal alteration occurred on Ryugu’s parent body.
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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 author upon reasonable request. The raw and calibrated NIRS3 data will be made available through the JAXA Data Archives and Transmission System (DARTS) website (https://darts.isas.jaxa.jp/planet/project/hayabusa2/).
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Acknowledgements
The Hayabusa2 NIRS3 was funded by JAXA and built by Meisei Electric, Genesia and Hamamatsu Photonics. We thank H. Murao, Y. Sakata, A. Ikeda and K. Taguchi for their efforts in the development of NIRS3. We acknowledge the support from JAXA, CNES and ASI. D.L.D. and D.T. were supported by NASA’s Hayabusa2 Participating Scientist Program (NNX17AL02G, NNX16AL34G). D.L.D. was supported by the SSERVI16 Cooperative Agreement (NNH16ZDA001N). A part of this study was supported by the JSPS Grant-in-Aid for Scientific Research (16H04044, 17H06459, 17K05639, 17H01175) and the JSPS Core-to-Core Program ‘International Network of Planetary Sciences’.
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K.K. led the study, performed the data analysis and thermophysical modelling and wrote the manuscript. R.E.M. contributed to the interpretation of the results and assisted in the writing. T.I. led the development of NIRS3. M. Abe, M. Ohtake, S.M., M.M., L.R., Y.N., K.T. and T.A. (Ashikaga) contributed to the development and operation of NIRS3. L.R., C.P., D.L.D., E.P. and A.G. contributed to the data analysis. Y. Takagi, T.N., T.H., M.M., L.R., M.A.B., R.B., C.P., F.P., D.L.D., F.V., D.T., E.P. and A.G. participated in the interpretation of the results. All authors participated in science data acquisition, mission planning, mission operations, or project management, and/or contributed to discussion of the results. The entire Hayabusa2 project team made this mission possible.
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Peer review information Nature Astronomy thanks Pierre Beck, Benoit Carry and Ellen Howell for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 Individual spectra from each site used to derive the average spectra of the SCI crater region.
The spectra are divided by the surface standard spectrum of the day before and vertically shifted for clarity.
Extended Data Fig. 2 NIRS3 spectra of the surface standard.
a, Spectra averaged over regions having the similar surface temperature to the SCI crater region. The details of these spectra are listed in Extended Data Table 2. b, Ratios between the normalized spectra shown in a. The non-flat shape of the ratio-spectra indicates the residual of thermal correction. The spectra are normalized and vertically shifted for clarity. Note that the vertical scale of b is much larger than that of a to show the curvature and uncertainties of the ratio-spectra.
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Kitazato, K., Milliken, R.E., Iwata, T. et al. Thermally altered subsurface material of asteroid (162173) Ryugu. Nat Astron 5, 246–250 (2021). https://doi.org/10.1038/s41550-020-01271-2
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DOI: https://doi.org/10.1038/s41550-020-01271-2
