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Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body
Department of Physics, Rikkyo University, Toshima, Japan.
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan; The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan; University of Tokyo, Kashiwa, Japan.
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan; University of Tokyo, Bunkyo, Japan.
Department of Physics, Rikkyo University, Toshima, Japan.
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2021 (English)In: Nature Astronomy, E-ISSN 2397-3366, Vol. 5, no 8, p. 766-774Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Springer Nature, 2021. Vol. 5, no 8, p. 766-774
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Atmospheric Science
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URN: urn:nbn:se:ltu:diva-85047DOI: 10.1038/s41550-021-01371-7ISI: 000653643000001Scopus ID: 2-s2.0-85106265874OAI: oai:DiVA.org:ltu-85047DiVA, id: diva2:1561704
Note

Validerad;2021;Nivå 2;2021-08-17 (johcin)

Available from: 2021-06-07 Created: 2021-06-07 Last updated: 2024-01-22Bibliographically approved

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