11:00 〜 13:00
[PPS03-P15] Chemical characteristics of Ryugu particles returned by the Hayabusa2 spacecraft
キーワード：リュウグウ、化学組成、機器中性子放射化分析法
Introduction
The Hayabusa2 spacecraft collected surface and subsurface materials from the C-type asteroid Ryugu and successfully returned these materials to the Earth on December 6th, 2020. The initial examinations of Ryugu particles at the JAXA curation facility showed these materials petrologically resemble CI-chondrites [1]. Eight Ryugu particles, four from Chamber A and four from Chamber C, were allocated to the Phase2 curation Kochi team for in-depth investigation [2]. In this study, the bulk abundances of major, minor and trace elements of several allocated Ryugu particles were determined by using INAA. Based on the analytical data, we aim to characterize Ryugu particles in chemical composition and compare them with those of CI, CY and CM carbonaceous chondrites.
Experiments
Five Ryugu particles (C0068, A0002, A0029, A0037 and A0098) weighing about 8 mg in total were analyzed by using INAA at the Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS). Sample preparations for these Ryugu particles were performed in a glove box at SPring-8. First, samples and reference standards were irradiated for 30 s at the pn-3 facility, and gamma rays emitted from the samples were immediately measured by using Ge detectors to determine the concentrations of Mg, Al, Cl, Ca, Ti, V and Mn. The same samples were reirradiated for 4 hrs at the pn-2 facility, and the emitted gamma rays from the samples were measured three times after different cooling intervals over a one-month period KURNS and the RI Research Center, Tokyo Metropolitan University for the determinations of Na, K, Ca, Sc, Cr, Fe, Co, Ni, Zn, Ga, As, Se, Br, Ru, Sb, La, Sm, Eu, Yb, Lu, Ta, Os, Ir, Au and Hg.
Results and discussion
Ryugu particles from Chamber A and Chamber C have similar elemental abundances. Among five particles, A0029 has a significantly high Ta abundance (7.9 ppm) while Ta concentrations for the other four samples were less than 0.4 ppm. This higher abundance for Ta could be explained by contamination with projectile materials during sample collections. Calcium, Mn, REEs and Au show significantly variable concentrations. Calcium and Mn are positively correlated among the five Ryugu particles. Considering the presence of carbonate in Ryugu particles [1], these large variations could be caused by the heterogeneous distributions of carbonates in Ryugu particles analyzed in INAA.
Groups of carbonaceous chondrites can be distinguished from each other based on elemental ratios of refractory elements to moderately volatile and/or volatile elements. Elemental ratios (Ga/Cr, Zn/Cr, and Se/Cr) for the five Ryugu particles are similar to those of CI chondrite, and some CY chondrites such as Y-82162, and higher than those of CM chondrites. This indicates that the chemical characteristics of Ryugu particles point to genetic relationships with CI and CY chondrites. The solar abundances of elements were estimated by either spectroscopic measurements of solar photosphere or elemental abundances of CI chondrites. In the case of Hg, as identification of the absorption line of Hg was problematic, theoretical models of stellar nucleosynthesis or analyses of CI chondrites were used to estimate the solar abundance of Hg [3,4]. A large variation in Hg abundance of CI chondrites was reported to be in the range of 0.18 to 500 ppm and considered to reflect the effect of terrestrial contamination [5]. As Ryugu particles used in this study have not interacted with the terrestrial atmosphere, it is the most suitable sample for the determination of Hg content in CI-like materials. The five Ryugu particles have similar Hg abundances to each other, and their mean value is about three times higher than the solar abundances [3,4].
References
[1] Yada et al. 2021. Nature Astronomy. https://doi.org/10.1038/s41550-021-01550-6. [2] Ito et al. 2022. 53rd LPSC #1601. [2] Anders and Grevesse 1989. GCA 53:197. [3] Lodders 2021. SSR 217:44. [4] Lauretta et al. 1999. EPSL 171:35.
The Hayabusa2 spacecraft collected surface and subsurface materials from the C-type asteroid Ryugu and successfully returned these materials to the Earth on December 6th, 2020. The initial examinations of Ryugu particles at the JAXA curation facility showed these materials petrologically resemble CI-chondrites [1]. Eight Ryugu particles, four from Chamber A and four from Chamber C, were allocated to the Phase2 curation Kochi team for in-depth investigation [2]. In this study, the bulk abundances of major, minor and trace elements of several allocated Ryugu particles were determined by using INAA. Based on the analytical data, we aim to characterize Ryugu particles in chemical composition and compare them with those of CI, CY and CM carbonaceous chondrites.
Experiments
Five Ryugu particles (C0068, A0002, A0029, A0037 and A0098) weighing about 8 mg in total were analyzed by using INAA at the Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS). Sample preparations for these Ryugu particles were performed in a glove box at SPring-8. First, samples and reference standards were irradiated for 30 s at the pn-3 facility, and gamma rays emitted from the samples were immediately measured by using Ge detectors to determine the concentrations of Mg, Al, Cl, Ca, Ti, V and Mn. The same samples were reirradiated for 4 hrs at the pn-2 facility, and the emitted gamma rays from the samples were measured three times after different cooling intervals over a one-month period KURNS and the RI Research Center, Tokyo Metropolitan University for the determinations of Na, K, Ca, Sc, Cr, Fe, Co, Ni, Zn, Ga, As, Se, Br, Ru, Sb, La, Sm, Eu, Yb, Lu, Ta, Os, Ir, Au and Hg.
Results and discussion
Ryugu particles from Chamber A and Chamber C have similar elemental abundances. Among five particles, A0029 has a significantly high Ta abundance (7.9 ppm) while Ta concentrations for the other four samples were less than 0.4 ppm. This higher abundance for Ta could be explained by contamination with projectile materials during sample collections. Calcium, Mn, REEs and Au show significantly variable concentrations. Calcium and Mn are positively correlated among the five Ryugu particles. Considering the presence of carbonate in Ryugu particles [1], these large variations could be caused by the heterogeneous distributions of carbonates in Ryugu particles analyzed in INAA.
Groups of carbonaceous chondrites can be distinguished from each other based on elemental ratios of refractory elements to moderately volatile and/or volatile elements. Elemental ratios (Ga/Cr, Zn/Cr, and Se/Cr) for the five Ryugu particles are similar to those of CI chondrite, and some CY chondrites such as Y-82162, and higher than those of CM chondrites. This indicates that the chemical characteristics of Ryugu particles point to genetic relationships with CI and CY chondrites. The solar abundances of elements were estimated by either spectroscopic measurements of solar photosphere or elemental abundances of CI chondrites. In the case of Hg, as identification of the absorption line of Hg was problematic, theoretical models of stellar nucleosynthesis or analyses of CI chondrites were used to estimate the solar abundance of Hg [3,4]. A large variation in Hg abundance of CI chondrites was reported to be in the range of 0.18 to 500 ppm and considered to reflect the effect of terrestrial contamination [5]. As Ryugu particles used in this study have not interacted with the terrestrial atmosphere, it is the most suitable sample for the determination of Hg content in CI-like materials. The five Ryugu particles have similar Hg abundances to each other, and their mean value is about three times higher than the solar abundances [3,4].
References
[1] Yada et al. 2021. Nature Astronomy. https://doi.org/10.1038/s41550-021-01550-6. [2] Ito et al. 2022. 53rd LPSC #1601. [2] Anders and Grevesse 1989. GCA 53:197. [3] Lodders 2021. SSR 217:44. [4] Lauretta et al. 1999. EPSL 171:35.