講演情報
[PPS12-P13]Characterization of Category 2 Ryugu Samples: Implications for Impactor-Based Sampling Techniques on Small Asteroids
*金丸 礼1、橘 省吾2,1、坂本 佳奈子1、矢田 達1、高野 淑識3、岡崎 隆司4、臼井 寛裕1、安部 正真1、岡田 達明1、- 地球外物質研究グループ1、- はやぶさ２サンプラーチーム、澤田 弘崇1 (1.宇宙航空研究開発機構 宇宙科学研究所、2.東京大学、3.海洋研究開発機構、4.九州大学)
キーワード：
リュウグウ
Introduction: In December 2020, the Hayabusa2 spacecraft returned samples from asteroid Ryugu, revealing mineralogical and chemical characteristics closely similar to CI chondrites [e.g., 1]. The Sampling operation used an impactor-based technique in which a sampling horn touched the surface and a Ta projectile (5 g, 8 mm) was fired at ~300 m/s. Lofted materials were guided upward and captured through multiple reflections inside the horn [2, 3]. This technique is highly adaptable to variable microgravity surface conditions and enabled efficient collection while minimizing contact time. The two touchdowns yielded ~5.4 g of material— far exceeding the 0.1 g mission requirement—demonstrating the effectiveness of this technique. Verification tests and molecular-level analyses of returned samples [4] confirmed the authenticity of primary information preserved in both TD1 and TD2 materials. Sample-return missions enable high-sensitivity laboratory analyses not possible by remote sensing. From collection onward, Ryugu samples were stored in vacuum or purified N2 and handled only with designated materials inside controlled containers, preventing contact with Earth’s atmosphere or foreign matter. However, when the sample catcher was first opened, metallic fragments were found. During initial curation, nine particles clearly distinct from Ryugu regolith (Category 1)—such as metal and polyimide-like fragments—were identified and classified as Category 2 samples. These particles are likely derived from spacecraft materials, but their specific identity remains unclear. Therefore, this study aims to identify these Category 2 particles and determine their sources.
Methods: In this study, we performed optical microscopic observation, SEM–EDS analyses (SU6600, Hitachi High-Tech), and XCT (SkyScan 1272, Bruker) 3D shape analysis on five Category 2 particles (C0088, C0110-1, C0111, C0112, C0215) with metallic appearances.
Results and Discussion: Optical microscopy showed four morphological types: foil-like (C0110-1, C0215), plate-like (C0088), screw-like (C0111), and wire-like (C0112). SEM–EDS revealed that all particles consist of pure aluminum. Pure aluminum is used in several sampler components—including the sabot that holds the Ta projectile, the M2 screws that secure it, and the protective aluminum film on the impactor—indicating these as plausible sources. XCT analysis of C0111 revealed a diameter of ~2 mm, a thread pitch of ~0.275 mm, and a thread angle near 60º, matching M2 screw specifications. The foil-like particles have thicknesses of ~18 μm (2 μm/pixel), consistent with the 15 μm aluminum film used on impactor. SEM observations also showed that four particles attached Ta metal. These Ta fragments likely originated from partial fragmentation of the Ta projectile. Such attachment is an engineering response inherent to the impactor-based mechanism and does not indicate any malfunction. Previous studies reported anomalously high Ta concentrations in Ryugu bulk composition compared with CI chondrites [5]. Our results suggest that part of this Ta enrichment may reflect engineering-derived contributions at the trace-element level, indicating that geological interpretation of Ta requires caution. Nevertheless, these metallic fragments represent extremely small amounts of material generated even during normal sampler operation and do not compromise the scientific value of the samples. Identifying spacecraft-derived particles and clarifying their introduction mechanisms provides essential baseline information for improving authenticity assessment of chemistry data.
References: [1] Yada et al., 2022, Nature Astron.; [2] Fujiwara et al., 2000, International Journal of Microgravity Science and Application. [3] Sawada et al., 2017, Space Science Review. [4] Takano et al., 2020, Earth Planets Space. [5] Yokoyama et al., 2025, Geochemical Journal.
Methods: In this study, we performed optical microscopic observation, SEM–EDS analyses (SU6600, Hitachi High-Tech), and XCT (SkyScan 1272, Bruker) 3D shape analysis on five Category 2 particles (C0088, C0110-1, C0111, C0112, C0215) with metallic appearances.
Results and Discussion: Optical microscopy showed four morphological types: foil-like (C0110-1, C0215), plate-like (C0088), screw-like (C0111), and wire-like (C0112). SEM–EDS revealed that all particles consist of pure aluminum. Pure aluminum is used in several sampler components—including the sabot that holds the Ta projectile, the M2 screws that secure it, and the protective aluminum film on the impactor—indicating these as plausible sources. XCT analysis of C0111 revealed a diameter of ~2 mm, a thread pitch of ~0.275 mm, and a thread angle near 60º, matching M2 screw specifications. The foil-like particles have thicknesses of ~18 μm (2 μm/pixel), consistent with the 15 μm aluminum film used on impactor. SEM observations also showed that four particles attached Ta metal. These Ta fragments likely originated from partial fragmentation of the Ta projectile. Such attachment is an engineering response inherent to the impactor-based mechanism and does not indicate any malfunction. Previous studies reported anomalously high Ta concentrations in Ryugu bulk composition compared with CI chondrites [5]. Our results suggest that part of this Ta enrichment may reflect engineering-derived contributions at the trace-element level, indicating that geological interpretation of Ta requires caution. Nevertheless, these metallic fragments represent extremely small amounts of material generated even during normal sampler operation and do not compromise the scientific value of the samples. Identifying spacecraft-derived particles and clarifying their introduction mechanisms provides essential baseline information for improving authenticity assessment of chemistry data.
References: [1] Yada et al., 2022, Nature Astron.; [2] Fujiwara et al., 2000, International Journal of Microgravity Science and Application. [3] Sawada et al., 2017, Space Science Review. [4] Takano et al., 2020, Earth Planets Space. [5] Yokoyama et al., 2025, Geochemical Journal.