Abstract
Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.
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Code availability
The display tool for the TIR thermal images is AiGIS, developed by author Naru Hirata of the University of Aizu, which is available at https://arcspace.jp/aigis/. The basic code of the asteroid thermal model that supports the calculation of thermal images using several thermal inertia values was constructed by ref. 17, and is applicable to any asteroid shape model even of more than one million nodes and capable of calculating the self-heating effect between nodes facing each other and the shadowing effect of insolation by geological features. This code is accessible from the corresponding author upon reasonable request.
Data availability
The source data of the TIR global thermal infrared images used for Fig. 1a–d are prepared from hyb2_tir_20180801_142608_l2.fit for Fig. 1a, from hyb2_tir_20180801_162120_l2.fit for Fig. 1b, from hyb2_tir_20180801_181632_l2.fit for Fig. 1c, and from hyb2_tir_20180801_201144_l2.fit for Fig. 1d. The source data of the TIR close-up thermal images used for Fig. 4a, c are prepared from hyb2_tir_20181015_133444_l2.fit, and hyb2_tir_20181015_134420_l2.fit, respectively. A TIR radiance image plotted on the Ryugu shape model (SHAPE_SFM_200k_v20180804) in Fig. 2 is from hyb2_tir_20180801_181632_l3b.txt. The maximum temperature plot in Fig. 3a and the plots of Fig. 3b–g are obtained from the datasets of hyb2_tir_20180801_150432~215056_l3b.txt. All the data related to this manuscript, including the raw thermal images (L1), the temperature-converted thermal images (L2), and the temperature plots on the shape model (L3) of TIR and the corresponding ancillary data for the SPICE tool are archived in the DARTS database (darts.isas.jaxa.jp/planet/project/hayabusa2/), and will be archived in the PDS4 within one year after the end of its nominal mission, according to the Hayabusa2 science data policy.
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Acknowledgements
We thank all the members of the Hayabusa2 Project and supporting staff for their technical assistance and scientific discussions, and especially S. Matsuura of Kwansei Gakuin University the use of the cavity blackbody and collimator to calibrate TIR. This research is supported in part by the JSPS KAKENHI (numbers JP26287108, JP17H06459, JP17K05639, JP19H01951 and JP19K03958), and the JSPS Core-to-Core programme “International Network of Planetary Sciences”. M.D. acknowledges support from the French space agency CNES. T.G.M. received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under grant agreement number 687378, as part of the project “Small Bodies Near and Far” (SBNAF). A.H. was supported by STFC grant ST/S001271/1.
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Contributions
T.O. led TIR development and experiments, including interpretations of TIR data. TIR development and calibrations: T.O., T.F., S. Tanaka, M.T., T.A., H. Senshu, N.S., H.D., Y.O., T. Sekiguchi, T.K., J.T., T.M., T. Imamura, T.W., S. Hasegawa, J.H., T.G.M. and A.H. TIR data acquisitions and reductions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., Y.O., K. Suko and T.K. Thermophysical modelling and discussions: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., M.D., J.B., M.G. and M.H. Shape modelling contributions: Naru Hirata, Naoyuki Hirata, Y. Yamamoto, K.M. and A.M. Landing site selection discussions: K.W., C. Honda, R.H., Y.I., K.M., M.M., T.M., A.M., T.M., H.N., R.N., K.O., K. Shirai, E.T., H. Yabuta, Y. Yokota, H. Yano and M. Yamada. Science operations of spacecraft: M. Abe, M. Hayakawa, T. Iwata, M.O., H. Yano, S. Hosoda, O.M., H. Sawada, T. Shimada, H.T., R.T., A.F., C. Hirose, S.K., Y.M., N.O., G.O., T.T., Y. Takei, T.Y., K.Y., F.T., T. Saiki, S.N., M. Yoshikawa, S.W. and Y. Tsuda; Project administration: S.S., N.N., K.K., T.O., M. Arakawa, S. Tachibana, H.I., M.I., S. Tanaka, F.T., T. Saiki, S.N., M. Yoshikawa, S.W., and Y. Tsuda, Interpretation and writing contribution: T.O., S. Tanaka, T.A., H. Senshu, N.S., Y.S., H.D., J.H., J.B., M.G., M.D., T.G.M., A.H., E.T., T.M. and S.S. All authors discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 A modelled thermal image of asteroid Ryugu before the arrival of Hayabusa2.
An example thermal image of the asteroid reference model ‘Ryugoid’ produced by the Hayabusa2 Landing Site Selection Data Preparation team shows the surface covered with granular regolith (with thermal inertia 300 tiu) and dense boulders (with thermal inertia 1,600 tiu), with most of the boulders identifiable as ‘cold spots’.
Extended Data Fig. 2 Temperature calibration of TIR using multiple apparatuses.
DN, digital number. a, An example of the TIR pixel response of a centre pixel at (164, 124) obtained by the ground-based calibration tests. The purple dots and line show the data for the blackbody plate and their linear regression. The blue and green dots show the data for the cavity blackbody and the collimator, respectively. b–d, Thermal images from TIR using the different apparatuses for the 100 °C target: b, the blackbody plate (total area coverage), c, the cavity blackbody (simulation to Mid-Altitude, 5 km), and d, the collimator (simulation to Home Position).
Extended Data Fig. 3 Effective diameter dependence of the LUT for TIR calibration.
This figures show the values of the slope (a) and the intercept (b) at the centre pixel at (164, 124) in terms of the effective diameter D of the target. For D ≤ 300 pixels, the red lines are derived from the data of the collimator and the cavity blackbody sources. For 300 pixels <D ≤ 322 pixels, the green lines are derived from the blackbody (BB) plate source.
Extended Data Fig. 4 A formation scenario of Ryugu from a porous parent body.
(1) Formation began with fluffy dust in the solar nebula. (2) Porous planetesimals were formed by accretion of dust or pebbles. (3) The parent body of Ryugu might have remained porous owing to a low degree of consolidation. A clear boundary of the inner core is illustrated but a gradual increase of consolidation by depth might be expected. (4) Impact fragmentation of the parent body occurred. Some large fragments are the boulders on Ryugu. (5) Part of fragments re-accreted to form Ryugu, with porous boulders and sediments on the surface, and some dense boulders originating from the inner core. (6) Re-shaping caused by a change in rotation rate to form a double-top-shape.
Supplementary information
Supplementary Information
This file contains additional information about Supplementary Videos 1 and 2.
Supplementary Data
This zipped file contains source data for Figures 1-4 and the Shape Model.
Video 1: Global one rotation thermal image set at HP
Video showing one-rotation global thermal images by Hayabusa2 TIR from Home Position.
Video 2: Global one rotation thermal image set at Mid-Altitude
Video showing one-rotation global thermal images by Hayabusa2 TIR from Mid-Altitude.
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Okada, T., Fukuhara, T., Tanaka, S. et al. Highly porous nature of a primitive asteroid revealed by thermal imaging. Nature 579, 518–522 (2020). https://doi.org/10.1038/s41586-020-2102-6
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DOI: https://doi.org/10.1038/s41586-020-2102-6
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