Abstract
Planetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts1,2, and subsequent thermal and compaction processes reduce their porosity3. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. 4,5,6), higher than meteorites but lower than cometary nuclei7, which are considered to be remnants of the original planetesimals8. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average6 and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment9 is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction3. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations10,11.
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Data availability
All images and essential 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/Sakatani_2021. The other data that support the plots within this paper are available from the corresponding author upon reasonable request.
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Acknowledgements
We thank all of the members of the Hayabusa2 project. This research is supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (grant nos. JP20K14547 and JP17H06459) and Core-to-Core programme ‘International Network of Planetary Sciences’. W.N. acknowledges support by the Klaus Tschira foundation. M. Hamm was financially supported by Geo.X, grant number: SO_087_GeoX. M.D. acknowledges the French space agency CNES and support from the ANR ‘ORIGINS’ (ANR-18-CE31-0014). We thank E. Posner, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/ac) for editing a draft of this manuscript.
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N.S. led this study and analysis TIR and ONC-T data. TIR development, data acquisitions, reductions: N.S., S. Tanaka, T.O., T.F. T. Kouyama, H. Senshu., Y.S., T.A., J.T., H.D., T. Sekiguchi, M.T., Y.O., T. Matsunaga, T.W., S. Hasegawa, J.H. and A.M. Thermal modelling of Ryugu’s surfaces and TIR data analysis: N.S., S. Tanaka, H. Senshu, J.T., T.G.M., A. Hagermann, J.B., M.G., M. Hamm and M.D. ONC-T development, data acquisitions, reductions and analysis: N.S., S. Sugita, R.H., T. Morota, S. Kameda, Y. Yokota, E.T., K. Yumoto, M. Yamada, H. Suzuki, C. Honda, K.O., M. Hayakawa, K. Yoshioka, M.M., Y.C. and H. Sawada. NIRS3 development, data acquisitions, reductions and analysis: L.R., K.K., T.I., M. Abe, M. Ohtake and S.M. Shape model construction and evaluation: N. Hirata (Univ. Aizu), N. Hirata (Kobe Univ.), S. Sugita, R.N., Y.S., S.W. and A.M. Thermal evolution modelling of the parent body: W.N., N.S. and M.G. LIDAR-derived trajectory: K.M., H.N., Y. Ishihara, K. Yamamoto, A. Higuchi, N.N. and G.O. SCI development and operations: T. Saiki, H. Imamura, Y. Takagi, H. Yano, K.S., C.O., S.N., Y. Iijima, M. Arawaka, K.W., T. Kadono, K.I. Operations of spacecraft: S. Tanaka, T.O., H. Sawada, T.I., M. Abe, G.O., T. Saiki, H. Yano, S.N., F.T., S. Kikuchi, T.Y., N.O., Y.M., K. Yoshikawa, T.T., Y. Takei, A.F., H.T., Y. Yamamoto, C. Hirose, S. Hosoda, O.M., T. Shimada, S. Soldini, R.T., M. Ozaki, M. Yoshikawa and Y. Tsuda. Project administration: S. Tanaka, T.O., S. Sugita, K.K., N.N., M. Arakawa, S. Tachibana, H. Ikeda, M.I., H. Yabuta, F.T., T. Saiki, S.N., M. Yoshikawa, S.W. and Y. Tsuda. All authors discussed the results and commented on the manuscript.
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Extended data
Extended Data Fig. 1 Field of view by high-resolution TIR data and mosaic images by TIR during the descent operations.
a, Global map of Ryugu by ONC-T6 and coverage of the TIR images during the descent sequences used in this paper (see also Supplementary Table 1). Abbreviations of the sequences are, MNRV: MINERVA-II rovers separation, MSCT: MASCOT lander separation, TD1-R1A and TD1-R3: Touch-down rehearsals, TD1-L08E1: First touch-down operation, and DO-S01: Descent operation for S01 region. The TIR coverage is about 5% of the total Ryugu surface area. Dashed circles denote the craters (IDs 3, 8, 9, 46, 59, 69, and 126) in the TIR footprints. First touch-down (TD1) site and SCI crater site are also shown in this map. b-f, Mosaics of TIR images acquired during the descent operations (images in hovering and ascending phases are not included). The right and left panels show images at higher and lower altitudes, whose footprints on the global map are roughly indicated as yellow and green regions in a, respectively. The values of D adjacent to the crater IDs represent the diameters of the crater17. The hotspots HS1 and HS2 are indicated as red arrows. The cold spot boulders detected by Okada et al.5 are shown as CS1 and CS2. Blue squares in d indicate the regions A to F shown in Fig. 3.
Extended Data Fig. 2 All TIR images of hotspots HS1 and HS2.
a-b, HS1 in TD1-R1A sequence. c-d, HS1 in TD1-R3 sequence. e-g, HS1 in TD1-L08E1 sequence. h-l, HS2 in MNRV sequence. File names are indicated as labels. Coloured pixels indicate regions that are hotter or colder by more than 2σ of the mean temperature relevant to each operation (Supplementary Table 1). Gray-scaled pixels are within 2σ of the averages. The temperatures of HS1 and HS2 are consistently higher than the surroundings although the observation geometries (for example, solar incident angle, emission angle) differ, which suggests lower TI regardless of microscopic roughness.
Extended Data Fig. 3 Estimation of the thermal inertia of hotspots.
a, Comparison of simulated temperature averaged in the ID126 crater with variable thermal inertia (black line) and observed temperature for the hotspot (red horizontal line). The error bars show the standard deviation within the relevant pixels. By comparing the observed HS1 temperature with the simulated average temperatures in the ID126 crater, the best-fit TI value is 73 tiu for the hotspot. b, Simulated temperature image around ID126 crater (dashed circle) with TI = 200 tiu. The position of HS1 is indicated as red point. c, Similar to a but for hotspot HS2. The simulated data are the average of the hotspot (red-filled circle) in the simulated image of d.
Extended Data Fig. 4 TIR and NIRS3 data for ID46 crater without the hotspot.
TIR (a) and ONC-T (b) images for the ID46 crater (7.8°N, 173.0°E) without hotspots (hyb2_tir_20180921_031212_l2 and hyb2_onc_20190307_222828_tvf_l2b, respectively). The crater rim is denoted as dashed circles. Only the hot or cold regions are coloured in the TIR image (see also Extended Data Fig. 2). c, NIRS3 footprint on 30 October 2018. Blue rectangles denote the NIRS3 footprints inside the crater, and yellow rectangles denote those of the outside. d, reflectance ratio of centre-to-rim of the crater. In contrast with the ID69 crater (Fig. 2d), an absorption increase at 2.72 μm does not appear in the ID46 crater.
Extended Data Fig. 5 TIR observation of SCI crater.
a, TIR image of acquired during ascending phase of the PPTD-TM1B sequence. b, Close-up of the dashed rectangle in a, showing the artificial impact crater (SCI crater) at (7.9°N, 301.3°E). The rim and pit (impact points) positions are shown. Thermal calculation results using a local digital elevation model9 with uniform thermal inertia of 50 tiu (c) and 300 tiu (d) are shown for comparison. Agreement with the observed image in b implies that the thermal inertia in the SCI crater is roughly 300 tiu including the central pit and/or impact site.
Extended Data Fig. 6 Visible, thermal, and spectral slope images acquired during the descent operations.
a-f, ONC-T v-band reflectance factor (left), TIR brightness temperature (middle) and v-to-x slope (right) image sets in the TD1-R3 descent sequence. Superimposed images of the reflectance and temperature images are also shown in Fig. 3. g, Image set for the ID69 crater. TIR images are aligned on the ONC-T images by affine transformation. The v-to-x slope image of region C was created from multi-band images acquired during the TD1-L08-E1 operation and that of region E was during the TD1-R3 ascent sequence. Blue arrows in c indicate the bright and cold spot. Red arrows in e and g indicate the hotspots HS1 and HS2, respectively.
Extended Data Fig. 7 Porosity estimate and TI and porosity distribution of boulders.
a, Models of porosity dependence of thermal conductivity. The experimental data for chondritic meteorites62, CM analogue37, Phobos simulant UTPS36 and SiO2 aggregates41 are also shown. The experimental data of the SiO2 aggregates are multiplied by 4.3/1.4 to convert the bulk thermal conductivity (at zero porosity) of SiO2 (1.4 W m−1 K−1) to that of the chondritic meteorites (4.3 W m−1 K−1). The solid and dashed curves represent Eqs. (4) and (6) in Method, respectively. b, Histogram of the thermal inertia for the boulders on Ryugu. Approximately 70% of the boulders have a TI ranging from 200 to 400 tiu, which is consistent with the global observations5,13 and local observations by the MARA instrument onboard the MASCOT lander4,63. c, Histogram of the porosity ϕ of boulders on Ryugu estimated from the thermal inertia shown in Fig. 4c and the model by Krause et al.42 (dashed curve in a). Approximately 90% of the boulders have a porosity between 30% and 60%.
Extended Data Fig. 8 Results of the thermal evolution modelling for parent bodies of Ryugu.
a-j, Radius is given by R and accretion time is t0 after CAI (calcium-aluminum-rich inclusions) formation. Left and right panels in each case show peak temperatures as a function of depth from the surface and volume fraction of the materials with different porosity ranges in the parent bodies (dark blue: ϕ < 30%, light blue: 30% < ϕ < 60%, orange: 60% < ϕ < 90%, and yellow: ϕ = 90% unconsolidated without hot pressing), respectively.
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Supplementary Discussions, Tables 1 and 2, and Figs. 1–3.
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Sakatani, N., Tanaka, S., Okada, T. et al. Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body. Nat Astron 5, 766–774 (2021). https://doi.org/10.1038/s41550-021-01371-7
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DOI: https://doi.org/10.1038/s41550-021-01371-7
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