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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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/).
Nakamura, T. Post-hydration thermal metamorphism of carbonaceous chondrites. J. Miner. Petrol. Sci. 100, 260–272 (2005).
Marchi, S., Delbo, M., Morbidelli, A., Paolicchi, P. & Lazzarin, M. Heating of near-Earth objects and meteoroids due to close approaches to the Sun. Mon. Not. R. Astron. Soc. 400, 147–153 (2009).
Chaumard, N., Devouard, B., Delbo, M., Provost, A. & Zanda, B. Radiative heating of carbonaceous near-Earth objects as a cause of thermal metamorphism for CK chondrites. Icarus 220, 65–73 (2012).
Arakawa, M. et al. An artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime. Science 368, 67–71 (2020).
Saiki, T. et al. Hayabusa2’s kinetic impact experiment: operational planning and results. Acta Astronaut. 175, 362–374 (2020).
Brunetto, R., Loeffler, M. J., Nesvorny, D., Sasaki, S. & Strazzulla, G. in Asteroids IV (eds Michel, P. et al.) 597–616 (Univ. Arizona Press, 2015).
Iwata, T. et al. NIRS3: the near-infrared spectrometer on Hayabusa2. Space Sci. Rev. 208, 317–337 (2017).
Kitazato, K. et al. The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Science 364, 272–275 (2019).
Tsuda, Y. et al. Hayabusa2 mission status: landing, roving and cratering on asteroid Ryugu. Acta Astronaut. 171, 42–54 (2020).
Beck, P. et al. Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids. Geochim. Cosmo. Acta 75, 4881–4892 (2010).
Takir, D. et al. Nature and degree of aqueous alteration in CM and CI carbonaceous chondrites. Meteorit. Planet. Sci. 48, 1618–1637 (2013).
Potin, S., Beck, P., Schmitt, B. & Moynier, F. Some things special about NEAs: geometric and environmental effects on the optical signatures of hydration. Icarus 333, 415–428 (2019).
Sakatani, N. et al. Thermophysical property of the artificial impact crater on asteroid Ryugu. In 51st Lunar Planet. Sci. Conf. abstr. 2326 (Lunar and Planetary Institute, 2020).
Lantz, C. et al. Ion irradiation of carbonaceous chondrites: a new view of space weathering on primitive asteroids. Icarus 285, 43–57 (2017).
Brunetto, R. et al. Hyperspectral FTIR imaging of irradiated carbonaceous meteorites. Planet. Space Sci. 158, 38–45 (2018).
Rubino, S. et al. Space weathering affects the remote near-IR identification of phyllosilicates. Planet. Sci. J. 1, 61 (2020).
Sugita, S. et al. The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Science 364, eaaw0422 (2019).
Michel, P. et al. Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu. Nat. Commun. 11, 2655 (2020).
Amsellem, E., Moynier, F., Mahan, B. & Beck, P. Timing of thermal metamorphism in CM chondrites: implications of Ryugu and Bennu future sample return. Icarus 339, 113593 (2020).
Michel, P. & Delbo, M. Orbital and thermal evolutions of four potential targets for a sample return space mission to a primitive near-Earth asteroid. Icarus 209, 520–534 (2010).
Morota, T. et al. Sample collection from asteroid (162173) Ryugu by Hayabusa2: implications for surface evolution. Science 368, 654–659 (2020).
Morbidelli, A., Bottke, W. F., Froeschlé, C. & Michel, P. in Asteroids III (eds Bottke, W. F. et al.) 409–422 (Univ. Arizona Press, 2002).
Bottke, W. F. et al. Debiased orbital and absolute magnitude distribution of the near-Earth objects. Icarus 156, 399–433 (2002).
Greenstreet, S., Ngo, H. & Gladman, B. The orbital distribution of near-Earth objects inside Earth’s orbit. Icarus 217, 355–366 (2012).
Granvik, M. et al. Debiased orbit and absolute-magnitude distributions for near-Earth objects. Icarus 312, 181–207 (2018).
King, A. J., Solomon, J. R., Schofield, P. F. & Russell, S. S. Characterising the CI and CI-like carbonaceous chondrites using thermogravimetric analysis and infrared spectroscopy. Earth Planet. Space 67, 198 (2015).
Bates, H. C. et al. Linking mineralogy and spectroscopy of highly aqueously altered CM and CI carbonaceous chondrites in preparation for primitive asteroid sample return. Meteorit. Planet. Sci. 55, 77–101 (2020).
Tatsumi, E. et al. Updated inflight calibration of Hayabusa2’s optical navigation camera (ONC) for scientific observations during the cruise phase. Icarus 325, 153–195 (2019).
Spencer, J. R. et al. Systematic biases in radiometric diameter determinations. Icarus 78, 337–354 (1989).
Watanabe, S. et al. Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu—a spinning top-shaped rubble pile. Science 364, 268–272 (2019).
Jaumann, R. et al. Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites. Science 365, 817–820 (2019).
Grott, M. et al. Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu. Nat. Astron. 3, 971–976 (2019).
Okada, T. et al. Highly porous nature of a primitive asteroid revealed by thermal imaging. Nature 579, 518–522 (2020).
Shimaki, Y. et al. Thermophysical properties of the surface of asteroid 162173 Ryugu: infrared observations and thermal inertia mapping. Icarus 348, 113835 (2020).
Murray, C. D. & Dermott, S. F. Solar System Dynamics (Cambridge Univ. Press, 1999).
Ishiguro, M. et al. Optical properties of (162173) 1999 JU3: in preparation for the JAXA Hayabusa 2 sample return mission. Astrophys. J. 792, 74 (2014).
Wada, K. et al. Asteroid Ryugu before the Hayabusa2 encounter. Prog. Earth Planet. Sci 5, 82 (2018).
Hiroi, T. et al. Reflectance spectra (UV-3µm) of heated Ivuna (CI) meteorite and newly identified thermally metamorphosed CM chondrites. In 27th Lunar Planet. Sci. Conf. abstr. 551 (Lunar and Planetary Institute, 1996).
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’.
The authors declare no competing interests.
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 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.
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
Nature Communications (2021)
Nature Astronomy (2021)