10:45 AM - 11:00 AM
[PPS03-07] The Elemental Abundances of Ryugu: Assessment of Chemical Heterogeneities and the Nugget Effect
Keywords:Hayabusa2, Ryugu, Solar System abundances of elements, Sample heterogeneity, Nugget effect
Accurate determination of the Solar System composition is critical for advancing our understanding of the formation and evolution of the Solar System objects including planets, moons, asteroids, and comets. The elemental abundances of the Sun have been used to represent the chemical composition of the Solar System, since more than 99% of the mass of the Solar System is locked up in the Sun. Another robust method for determining the chemical composition of the Solar System is the measurement of chondritic meteorites. Of all the chondrites, the Ivuna-type (CI) carbonaceous chondrites have been perceived as a unique group of meteorites with a chemical composition similar to that of the solar photosphere except for highly volatile elements (noble gases, H, C, N, and O) and Li. In fact, direct comparisons of relative elemental abundances between solar photosphere and CI chondrites showed excellent agreement within ±10% difference for nearly 40 elements, regardless of their volatility and geochemical character [1].
Sample return missions are superior to meteorite analysis in that samples can be collected with no or a minimum of contamination from well-documented extraterrestrial objects. The Hayabusa 2 spacecraft, targeting the Cb-type asteroid (162173) Ryugu, sampled ~5.4 g of asteroidal material and returned the samples to Earth in December 2020 [2]. These samples were collected during the two landing sequences on the asteroid Ryugu. During the first touchdown operation (TD1), samples were collected from the asteroid surface and stored in sample Chamber A, while the other samples stored in sample Chamber C were collected from the vicinity of an artificial crater created by the small carryon impactor during the second touchdown operation (TD2). Initial analyses of the Ryugu materials in both chambers revealed a mineralogical and chemical kinship to the CI chondrites [e.g., 3-5] with a composition similar to the solar photosphere except for highly volatile elements.
This study summarizes the elemental abundances of Ryugu bulk samples published to date, evaluates the compositional variability, and compares the results with those of CI chondrites. Figure 1 shows the estimated elemental abundances of bulk Ryugu samples normalized to CI chondrite values [1], which show close agreement with CI chondrites from refractory to volatile elements, with slight excesses in most elements [3-5]. Figure 2 shows the CI-normalized abundances of 18 selected elements in the Ryugu samples. The abundances of some elements (e.g., P, Ca, Mn, REEs) in individual Ryugu particles were found to show large relative dispersions compared to the other elements. The observed variations in some elements for the measurements of relatively small samples (< 3.3 mg) most likely stem from the presence of aqueously formed secondary minerals in Ryugu (e.g., carbonate and phosphate), in which some specific elements, including P, Mn, Sr, and REEs, are strongly partitioned when these minerals precipitate. Consequently, the mean abundances of Ryugu for these elements, calculated using currently available Ryugu data, are accompanied by a certain degree of uncertainties. We thus suggest establishing a consortium to determine the representative elemental abundances of Ryugu by measuring aliquots from a large homogenized powder sample that can mitigate the nugget effect. Our statistical calculation shows that at least 750 and 400 mg of homogenized samples from Chambers A and C, respectively, are needed to achieve within ±5% compositional heterogeneity. The data obtained throughout the consortium activity complement the scientific objectives of the Hayabusa2 mission. Moreover, we anticipate that the obtained Ryugu data, coupled with the elemental abundances of CI chondrites, provide new insights into the chemical composition of the Solar System, which will be used by multidisciplinary communities, including Earth and planetary sciences, astronomy, physics, and chemistry.
References
[1] Lodders, K. (2021) Space Sci. Rev. 217, 1-33. [2] Yada, T. et al. (2022) Nat. Astron. 6, 214-220. [3] Yokoyama, T. et al. (2023) Science 379, eabn7850. [4] Nakamura, E. et al. (2022) Proc. J. Acad. B 98, 227-282. [5] Ito, M. et al. (2022) Nat. Astron. 6, 1163–1171.
Sample return missions are superior to meteorite analysis in that samples can be collected with no or a minimum of contamination from well-documented extraterrestrial objects. The Hayabusa 2 spacecraft, targeting the Cb-type asteroid (162173) Ryugu, sampled ~5.4 g of asteroidal material and returned the samples to Earth in December 2020 [2]. These samples were collected during the two landing sequences on the asteroid Ryugu. During the first touchdown operation (TD1), samples were collected from the asteroid surface and stored in sample Chamber A, while the other samples stored in sample Chamber C were collected from the vicinity of an artificial crater created by the small carryon impactor during the second touchdown operation (TD2). Initial analyses of the Ryugu materials in both chambers revealed a mineralogical and chemical kinship to the CI chondrites [e.g., 3-5] with a composition similar to the solar photosphere except for highly volatile elements.
This study summarizes the elemental abundances of Ryugu bulk samples published to date, evaluates the compositional variability, and compares the results with those of CI chondrites. Figure 1 shows the estimated elemental abundances of bulk Ryugu samples normalized to CI chondrite values [1], which show close agreement with CI chondrites from refractory to volatile elements, with slight excesses in most elements [3-5]. Figure 2 shows the CI-normalized abundances of 18 selected elements in the Ryugu samples. The abundances of some elements (e.g., P, Ca, Mn, REEs) in individual Ryugu particles were found to show large relative dispersions compared to the other elements. The observed variations in some elements for the measurements of relatively small samples (< 3.3 mg) most likely stem from the presence of aqueously formed secondary minerals in Ryugu (e.g., carbonate and phosphate), in which some specific elements, including P, Mn, Sr, and REEs, are strongly partitioned when these minerals precipitate. Consequently, the mean abundances of Ryugu for these elements, calculated using currently available Ryugu data, are accompanied by a certain degree of uncertainties. We thus suggest establishing a consortium to determine the representative elemental abundances of Ryugu by measuring aliquots from a large homogenized powder sample that can mitigate the nugget effect. Our statistical calculation shows that at least 750 and 400 mg of homogenized samples from Chambers A and C, respectively, are needed to achieve within ±5% compositional heterogeneity. The data obtained throughout the consortium activity complement the scientific objectives of the Hayabusa2 mission. Moreover, we anticipate that the obtained Ryugu data, coupled with the elemental abundances of CI chondrites, provide new insights into the chemical composition of the Solar System, which will be used by multidisciplinary communities, including Earth and planetary sciences, astronomy, physics, and chemistry.
References
[1] Lodders, K. (2021) Space Sci. Rev. 217, 1-33. [2] Yada, T. et al. (2022) Nat. Astron. 6, 214-220. [3] Yokoyama, T. et al. (2023) Science 379, eabn7850. [4] Nakamura, E. et al. (2022) Proc. J. Acad. B 98, 227-282. [5] Ito, M. et al. (2022) Nat. Astron. 6, 1163–1171.