講演情報
[R5-13]ベンヌ粒子マトリクスのバルク密度、空隙率、Fe/Mg比：リュウグウおよび炭素質コンドライトとの比較
*土`山 明1、松本 恵2、安武 正展3、松野 淳也4、上杉 健太朗3、竹内 晃久3、松本 徹5、大野 遼6、スン ミンチ7、森田 朋代2、橘 省吾8、コノリー ハロルド9、ローレッタ ダンテ10 (1. 立命大・総研、2. 東北大・院理、3. スプリング８、4. 京大・化研、5. 京大・白眉、6. 京大・院理、7. 広州地球化学研究所、8. 東大・院理、9. ローワン大、10. アリゾナ大)
キーワード：
ナノX線CT、スプリング８、オリリス・レックス計画、はやぶさ２計画、CIコンドライト
We determined the matrix bulk density, porosity, and Mg# [Mg/(Mg + Fe) atom ratio] of particles returned from asteroid Bennu by the OSIRIS-REx mission [1], and we compared them with those of other aqueously altered carbonaceous astromaterials, including asteroid Ryugu and chondrites. The analyzed Bennu particles (5–50 μm) were picked from unsorted aggregate samples (OREX-803143-0, OREX-803118-0), either directly or by FIB extraction from larger particles. At SPring-8 beamline BL47XU, 3D X-ray computed tomography images were acquired at 7 and 7.35 keV (absorption contrast) and 8 keV (phase contrast), with a spatial resolution of ~200 nm [2]. Our calculations assumed that the matrix is composed of phyllosilicates, organic matter, and pores based on the Bennu matrix mineralogy [1], using the linear attenuation coefficients and refractive index decrement (proportional to density). The matrix properties of the aqueously altered samples were as follows:
Bennu (N= 16): bulk density = 1.6 ± 0.3 (1.2–2.3) g/cm³, porosity = 39 ± 12 (16–57) %, Mg# = 0.77 ± 0.11 (0.48–0.91)
Ryugu (N = 72):8 ± 0.2 (1.4–2.2) g/cm³, 32 ± 9 (18–48) %, 0.83 ± 0.06 (0.66–0.93)
CI (N = 16):1 ± 0.1 (1.9–2.3) g/cm³, 22 ± 5 (12–31) %, 0.79 ± 0.06 (0.64–0.89)
CM (N = 7):2 ± 0.2 (1.9–2.4) g/cm³, 20 ± 6 (13–28) %, 0.63 ± 0.11 (0.44–0.77)
Tagish Lake (N = 5):1 ± 0.0 (2.1–2.2) g/cm³, 19 ± 1 (19–21) %, 0.84 ± 0.06 (0.76–0.89)
Although Bennu and Ryugu samples are similar to CIs and each other [1,2], some Bennu matrices exhibited lower density, higher porosity, and lower Mg# than Ryugu matrices. CI matrices had generally higher density, lower porosity, and lower Mg# than Bennu and Ryugu matrices, likely due to terrestrial weathering. CM and Tagish Lake matrices were similar to CIs’, except for the CMs’ more Fe-rich composition.
[1] Lauretta & Connolly et al. (2024) MAPS 59, 2453–2486. [2] Tsuchiyama et al. (2024) GCA 375, 146–172.
Bennu (N= 16): bulk density = 1.6 ± 0.3 (1.2–2.3) g/cm³, porosity = 39 ± 12 (16–57) %, Mg# = 0.77 ± 0.11 (0.48–0.91)
Ryugu (N = 72):8 ± 0.2 (1.4–2.2) g/cm³, 32 ± 9 (18–48) %, 0.83 ± 0.06 (0.66–0.93)
CI (N = 16):1 ± 0.1 (1.9–2.3) g/cm³, 22 ± 5 (12–31) %, 0.79 ± 0.06 (0.64–0.89)
CM (N = 7):2 ± 0.2 (1.9–2.4) g/cm³, 20 ± 6 (13–28) %, 0.63 ± 0.11 (0.44–0.77)
Tagish Lake (N = 5):1 ± 0.0 (2.1–2.2) g/cm³, 19 ± 1 (19–21) %, 0.84 ± 0.06 (0.76–0.89)
Although Bennu and Ryugu samples are similar to CIs and each other [1,2], some Bennu matrices exhibited lower density, higher porosity, and lower Mg# than Ryugu matrices. CI matrices had generally higher density, lower porosity, and lower Mg# than Bennu and Ryugu matrices, likely due to terrestrial weathering. CM and Tagish Lake matrices were similar to CIs’, except for the CMs’ more Fe-rich composition.
[1] Lauretta & Connolly et al. (2024) MAPS 59, 2453–2486. [2] Tsuchiyama et al. (2024) GCA 375, 146–172.