Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu


C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1,2,3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7.

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Fig. 1: MasCam image of the boulder observed by MASCOT indicating the MARA field of view (red shaded area).
Fig. 2: Observed and modelled surface temperatures and derived thermal inertial.
Fig. 3: Modelled temperatures for a dust-covered surface.
Fig. 4: Derived thermal conductivity and boulder porosity.

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.


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The MASCOT lander on the Hayabusa2 Mission of JAXA is a DLR/CNES cooperation. MASCOT MARA has been developed and built under the leadership of the DLR Institute of Planetary Research with contracted contributions from the Institute of Photonic Technology. The Hayabusa2 Mission is operated by JAXA. K.O. acknowledges funding by the JSPS Core-to-Core Program ‘International Network of Planetary Sciences.’ A.H. acknowledges funding by STFC under grant no. ST/S001271/1. P.M. acknowledges funding support from the French space agency CNES as well as from Academies of Excellence: Complex systems and Space, environment, risk, and resilience, part of the IDEX JEDI of the Université Côte d’Azur.

Author information




M.G. coordinated and wrote the paper. J.K. and M.G. evaluated the instrument calibration. M.H., K.O., S.M., J.-B.V., M.S. and I.P. computed illumination and thermal models. N. Schmitz, S.E.S., A.K. and F.T. contributed to camera development. K.A.O. and K.D.M. localized the MARA FoV in camera images. W.N., P.M., S. Tachibana, J.B., M.D. and M.K. provided the discussion on material parameters. H.S., T.O., E.K., J.B., H.Y., R.J., N.M., C.P., L.D., N. Sakatani, S. Tanaka, T.A. and S.S. added to the science discussion. J.H. and A.M. contributed to the radiometer development. C.K., T.-M.H. and A.M.-S. contributed to data acquisition. A.H. contributed to instrument characterization. All authors have read and approved the final manuscript.

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Correspondence to M. Grott.

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Peer review information: Nature Astronomy thanks Joshua Emery and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Grott, M., Knollenberg, J., Hamm, M. et al. Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu. Nat Astron 3, 971–976 (2019). https://doi.org/10.1038/s41550-019-0832-x

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