Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Hotspot HS1 within the ID126 crater.
Fig. 2: Hotspot HS2 observed by TIR and NIRS3 observation.
Fig. 3: TIR temperature images overlain on the ONC-T images acquired during the TD1-R3 sequence.
Fig. 4: Thermal and optical properties of boulders and regolith on Ryugu.
Fig. 5: Formation process and surface behaviour of the hotspot boulders.

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.

References

  1. 1.

    Okuzumi, S., Tanaka, H., Kobayashi, H. & Wada, K. Rapid coagulation of porous dust aggregates outside the snow line: a pathway to successful icy planetesimal formation. Astrophys. J. 752, 106 (2012).

    ADS  Article  Google Scholar 

  2. 2.

    Kataoka, A., Tanaka, H., Okuzumi, S. & Wada, K. Fluffy dust forms icy planetesimals by static compression. Astron. Astrophys. 557, L4 (2013).

    ADS  Article  Google Scholar 

  3. 3.

    Neumann, W., Breuer, D. & Spohn, T. Modelling of compaction in planetesimals. Astron. Astrophys. 567, A120 (2014).

    ADS  Article  Google Scholar 

  4. 4.

    Grott, M. et al. Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu. Nat. Astron. 3, 971–976 (2019).

    ADS  Article  Google Scholar 

  5. 5.

    Okada, T. et al. Highly porous nature of a primitive asteroid revealed by thermal imaging. Nature 579, 518–522 (2020).

    ADS  Article  Google Scholar 

  6. 6.

    Sugita, S. et al. The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Science 364, eaaw0422 (2019).

    Article  Google Scholar 

  7. 7.

    Groussin, O. et al. The thermal, mechanical, structural, and dielectric properties of cometary nuclei after Rosetta. Space Sci. Rev. 215, 29 (2019).

    ADS  Article  Google Scholar 

  8. 8.

    Weissman, P., Morbidelli, A., Davidsson, B. & Blum, J. Origin and evolution of cometary nuclei. Space Sci. Rev. 216, 6 (2020).

    ADS  Article  Google Scholar 

  9. 9.

    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).

    ADS  Article  Google Scholar 

  10. 10.

    Tsuda, Y. et al. Hayabusa2 mission status: landing, roving and cratering on asteroid Ryugu. Acta Astronaut. 171, 42–54 (2020).

    ADS  Article  Google Scholar 

  11. 11.

    Morota, T. et al. Sample collection from asteroid (162173) Ryugu by Hayabusa2: implications for surface evolution. Science 368, 654–659 (2020).

    ADS  Article  Google Scholar 

  12. 12.

    Cho, Y. et al. Geologic history and crater morphology of asteroid (162173) Ryugu. Preprint at https://doi.org/10.1002/essoar.10506689.2 (2021).

  13. 13.

    Shimaki, Y. et al. Thermophysical properties of the surface of asteroid 162173 Ryugu: infrared observations and thermal inertia mapping. Icarus 348, 113835 (2020).

    Article  Google Scholar 

  14. 14.

    Sakatani, N. et al. Thermal conductivity model for powdered materials under vacuum based on experimental studies. AIP Adv. 7, 015310 (2017).

    ADS  Article  Google Scholar 

  15. 15.

    Biele, J. et al. Effects of dust layers on thermal emission from airless bodies. Prog. Earth Planet. Sci. 6, 48 (2019).

    ADS  Article  Google Scholar 

  16. 16.

    Jaumann, R. et al. Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites. Science 365, 817–820 (2019).

    ADS  Article  Google Scholar 

  17. 17.

    Noguchi, R. et al. Crater depth-to-diameter ratios on asteroid 162173 Ryugu. Icarus 354, 114016 (2021).

    Article  Google Scholar 

  18. 18.

    Roberts, J. H. et al. Origin and flatness of ponds on asteroid 433 Eros. Meteorit. Planet. Sci. 49, 1735–1748 (2014).

    ADS  Article  Google Scholar 

  19. 19.

    Rozitis, B. et al. Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator. Sci. Adv. 6, eabc3699 (2020).

    ADS  Article  Google Scholar 

  20. 20.

    Tatsumi, E. et al. Collisional history of Ryugu’s parent body from bright surface boulders. Nat. Astron. 5, 39–45 (2020).

    ADS  Article  Google Scholar 

  21. 21.

    Ryan, A. J., Pino Muñoz, D., Bernacki, M. & Delbo, M. Full-field modeling of heat transfer in asteroid regolith: radiative thermal conductivity of polydisperse particulates. J. Geophys. Res. Planets 125, e2019JE006100 (2020).

    ADS  Article  Google Scholar 

  22. 22.

    Kitazato, K. et al. The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Science 364, 272–275 (2019).

    ADS  Google Scholar 

  23. 23.

    Kitazato, K. et al. Thermally altered subsurface materials of asteroid 162173 Ryugu. Nat. Astron. 5, 246–250 (2021).

    ADS  Article  Google Scholar 

  24. 24.

    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).

    ADS  Article  Google Scholar 

  25. 25.

    Carry, B. Density of asteroids. Planet. Space Sci. 73, 98–118 (2012).

    ADS  Article  Google Scholar 

  26. 26.

    Blum, J. Dust evolution in protoplanetary discs and the formation of planetesimals: What have we learned from laboratory experiments? Space Sci. Rev. 214, 52 (2018).

    ADS  Article  Google Scholar 

  27. 27.

    Delbo, M. et al. Thermal fatigue as the origin of regolith on small asteroids. Nature 508, 233–236 (2014).

    ADS  Article  Google Scholar 

  28. 28.

    Hartzell, C. M. & Scheeres, D. J. Dynamics of levitating dust particles near asteroids and the Moon. J. Geophys. Res. Planets 118, 116–125 (2013).

    ADS  Article  Google Scholar 

  29. 29.

    Neumann, W. et al. Microporosity and parent body of the rubble-pile NEA (162173) Ryugu. Icarus 358, 114166 (2021).

    Article  Google Scholar 

  30. 30.

    Takita, J., Senshu, H. & Tanaka, S. Feasibility and accuracy of thermophysical estimation of asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 thermal infrared imager. Space Sci. Rev. 208, 287–315 (2017).

    ADS  Article  Google Scholar 

  31. 31.

    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).

    ADS  Article  Google Scholar 

  32. 32.

    Vasavada, A. R. et al. Lunar equatorial surface temperatures and regolith properties from the diviner lunar radiometer experiment. J. Geophys. Res. E 117, E00H18 (2012).

    Article  Google Scholar 

  33. 33.

    Miura, A., Sakatani, N., Yokota, Y. & Honda, R. A study on methods of generating asteroid shape models. J. Space Sci. Inform. Jpn 9, 33–44 (2020).

    Google Scholar 

  34. 34.

    Matsumoto, K. et al. Improving Hayabusa2 trajectory by combining LIDAR data and a shape model. Icarus 338, 113574 (2020).

    Article  Google Scholar 

  35. 35.

    Senshu, H. et al. Numerical simulation on the thermal moment from Ryugu-like rough surface asteroid. In Proc. 51st Lunar and Planetary Science Conference abstr. 1990 (Lunar and Planetary Institute, 2020).

  36. 36.

    Hamm, M. et al. Thermal conductivity and porosity of Ryugu’s boulders from in-situ measurements of MARA—the MASCOT radiometer. In Proc. 50th Lunar and Planetary Science Conference abstr. 1373 (Lunar and Planetary Institute, 2019).

  37. 37.

    Avdellidou, C. et al. Very weak carbonaceous asteroid simulants I: mechanical properties and response to hypervelocity impacts. Icarus 341, 113648 (2020).

    Article  Google Scholar 

  38. 38.

    Grott, M. et al. Macroporosity and grain density of rubble pile asteroid (162173) Ryugu. J. Geophys. Res. Planets 125, e2020JE006519 (2020).

    ADS  Article  Google Scholar 

  39. 39.

    Flynn, G. J., Consolmagno, G. J., Brown, P. & Macke, R. J. Physical properties of the stone meteorites: implications for the properties of their parent bodies. Chem. Erde 78, 269–298 (2018).

    Article  Google Scholar 

  40. 40.

    Wada, K. et al. Asteroid Ryugu before the Hayabusa2 encounter. Prog. Earth Planet. Sci. 5, 82 (2018).

    ADS  Article  Google Scholar 

  41. 41.

    Krause, M., Blum, J., Skorov, Y. V. & Trieloff, M. Thermal conductivity measurements of porous dust aggregates: I. Technique, model and first results. Icarus 214, 286–296 (2011).

    ADS  Article  Google Scholar 

  42. 42.

    Krause, M. et al. Modeling the early thermal evolution of meteorite parent bodies based on new thermal conductivity measurements of highly porous aggregates. In Proc. 42nd Lunar and Planetary Science Conference abstr. 2696 (Lunar and Planetary Institute, 2011).

  43. 43.

    Blum, J. & Schräpler, R. Structure and mechanical properties of high-porosity macroscopic agglomerates formed by random ballistic deposition. Phys. Rev. Lett. 93, 115503 (2004).

    ADS  Article  Google Scholar 

  44. 44.

    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).

    ADS  Article  Google Scholar 

  45. 45.

    Tatsumi, E. et al. Global photometric properties of (162173) Ryugu. Astron. Astrophys. 639, A83 (2020).

    Article  Google Scholar 

  46. 46.

    Iwata, T. et al. NIRS3: the near infrared spectrometer on Hayabusa2. Space Sci. Rev. 208, 317–337 (2017).

    ADS  Article  Google Scholar 

  47. 47.

    Riu, L. et al. Spectral characterization of the craters of Ryugu as observed by the NIRS3 instrument on-board Hayabusa2. Icarus 357, 114253 (2021).

    Article  Google Scholar 

  48. 48.

    Matsumoto, M. et al. Discovery of fossil asteroidal ice in primitive meteorite Acfer 094. Sci. Adv. 5, eaax5078 (2019).

    ADS  Article  Google Scholar 

  49. 49.

    Bischoff, A., Scott, E. R. D., Metzler, K. & Goodrich, C. A. in Meteorites and the Early Solar System II (eds Lauretta, D. & McSween, H. Y.) 679–712 (Univ. Arizona Press, 2006).

  50. 50.

    Consolmagno, G. J., Britt, D. T. & Macke, R. J. The significance of meteorite density and porosity. Chem. Erde 68, 1–29 (2008).

    Article  Google Scholar 

  51. 51.

    Macke, R. J., Consolmagno, G. J. & Britt, D. T. Density, porosity, and magnetic susceptibility of carbonaceous chondrites. Meteorit. Planet. Sci. 46, 1842–1862 (2011).

    ADS  Article  Google Scholar 

  52. 52.

    Kiefer, W. S., MacKe, R. J., Britt, D. T., Irving, A. J. & Consolmagno, G. J. The density and porosity of lunar rocks. Geophys. Res. Lett. 39, L07201 (2012).

    ADS  Article  Google Scholar 

  53. 53.

    Neumann, W. & Kruse, A. Differentiation of Enceladus and retention of a porous core. Astrophys. J. 882, 47 (2019).

    ADS  Article  Google Scholar 

  54. 54.

    Neumann, W., Jaumann, R., Castillo-Rogez, J., Raymond, C. A. & Russell, C. T. Ceres’ partial differentiation: undifferentiated crust mixing with a water-rich mantle. Astron. Astrophys. 633, A117 (2020).

    ADS  Article  Google Scholar 

  55. 55.

    Amiguet, E. et al. Creep of phyllosilicates at the onset of plate tectonics. Earth Planet. Sci. Lett. 345–348, 142–150 (2012).

    ADS  Article  Google Scholar 

  56. 56.

    Schwenn, M. B. & Goetze, C. Creep of olivine during hot-pressing. Tectonophysics 48, 41–60 (1978).

    ADS  Article  Google Scholar 

  57. 57.

    Cloutis, E. A., Hiroi, T., Gaffey, M. J., Alexander, C. M. O. D. & Mann, P. Spectral reflectance properties of carbonaceous chondrites: 1. CI chondrites. Icarus 212, 180–209 (2011).

    ADS  Article  Google Scholar 

  58. 58.

    Cloutis, E. A., Hudon, P., Hiroi, T., Gaffey, M. J. & Mann, P. Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites. Icarus 216, 309–346 (2011).

    ADS  Article  Google Scholar 

  59. 59.

    Mei, S., Bai, W., Hiraga, T. & Kohlstedt, D. L. Influence of melt on the creep behavior of olivine–basalt aggregates under hydrous conditions. Earth Planet. Sci. Lett. 201, 491–507 (2002).

    ADS  Article  Google Scholar 

  60. 60.

    Klahr, H. & Schreiber, A. Turbulence sets the length scale for planetesimal formation: local 2D simulations of streaming instability and planetesimal formation. Astrophys. J. 901, 54 (2020).

    ADS  Article  Google Scholar 

  61. 61.

    Klahr, H. & Schreiber, A. Testing the Jeans, Toomre, and Bonnor-Ebert concepts for planetesimal formation: 3D streaming instability simulations of diffusion-regulated formation of planetesimals. Astrophys. J. 911, 9 (2021).

    ADS  Article  Google Scholar 

  62. 62.

    Opeil, C. P., Consolmagno, G. J., Safarik, D. J. & Britt, D. T. Stony meteorite thermal properties and their relationship with meteorite chemical and physical states. Meteorit. Planet. Sci. 47, 319–329 (2012).

    ADS  Article  Google Scholar 

  63. 63.

    Hamm, M., Pelivan, I., Grott, M. & de Wiljes, J. Thermophysical modelling and parameter estimation of small Solar System bodies via data assimilation. Mon. Not. R. Astron. Soc. 496, 2776–2785 (2020).

    ADS  Article  Google Scholar 

Download references

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.

Author information

Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to N. Sakatani.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy Ben Rozitis, Andrew Ryan and Mario Trieloff for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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.

Supplementary information

Supplementary Information

Supplementary Discussions, Tables 1 and 2, and Figs. 1–3.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing