4.5 Article

Microporosity and parent body of the rubble-pile NEA (162173) Ryugu

Journal

ICARUS
Volume 358, Issue -, Pages -

Publisher

ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.icarus.2020.114166

Keywords

Asteroids; Ryugu; Porosity; Rubble piles; Parent bodies

Funding

  1. Klaus Tschira foundation
  2. Geo.X, the Research Network for Geosciences in Berlin and Potsdam [SO_087_GeoX]

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Observations of C-type near-Earth asteroids and laboratory investigations of carbonaceous chondritic meteorites indicate a high microporosity of C-type asteroids. The study calculates the evolution of temperature and porosity to constrain parameters that result in microporosities compatible with Ryugu's high-porosity material and likely burial depths for observed boulders at the surface, suggesting distinct evolution paths for Ryugu's parent body compared to carbonaceous chondrites.
Both observations of C-type near-Earth asteroids and laboratory investigations of carbonaceous chondritic meteorites provide strong evidence for a high microporosity of C-type asteroids. Boulder microporosity values derived from in-situ measurements at the surface of the rubble-pile NEA (162173) Ryugu are as high as 55 %, which is substantially higher than for water-rich carbonaceous chondrite samples and could indicate distinct evolution paths for the parent body of Ryugu and parent bodies of carbonaceous chondrites, despite spectral similarities. In the present study, we calculate the evolution of the temperature and porosity for early solar system's planetesimals in order to constrain the range of parameters that result in microporosities compatible with Ryugu's high-porosity material and likely burial depths for the boulders observed at the surface. By varying key properties of the parent body, such as accretion time t(0) and radius R that have strong influence on temperature and porosity and by comparing the interior porosity distribution with the measured boulder micro-porosity, hydration, and partial dehydration of the material, we constrain a field within the (R, t(0))-diagram appropriate for bodies that are likely to have produced such material. Our calculations indicate a parent body size of only a few km and its early accretion within less than or similar to 2 - 3 Myr after the formation of Ca-Al-rich inclusions (CAIs). A gradual final porosity profile of best-fit bodies indicates production of both low- and high-density boulders from the parent body material. By contrast, parent body properties for CI and CM chondrites obtained by fitting carbonate formation data indicate a radius of approximate to 20 - 25 km and an accretion time of approximate to 3.75 Myr after CAIs. These results imply a population of km-sized early accreting highly porous planetesimals as parent bodies of the rubble-pile NEA Ryugu (and, potentially, other NEAs) and a population of larger and late accreting less porous planetesimals as parent bodies of water-rich carbonaceous chondrites.

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