10:45 〜 12:15
[PPS08-P11] Microstructural and chemical analyses of impact melt splashes on Ryugu particle surfaces
キーワード：小惑星リュウグウ、小惑星表層、x-ray CT
Hayabusa2 spacecraft delivered the C-type asteroid Ryugu samples to the earth in 2020. Initial analyses of the returned samples revealed that the Ryugu samples consist mainly of Mg-rich phyllosilicates, Fe-Ni-sulfides, magnetite, carbonates and other minor minerals, which are most similar to CI chondrites among known meteorite groups [e.g., 1,2]. In this study, we investigated two Ryugu samples, A0067 and A0094, which show evidence of exposure to space and have flat sample surfaces on which a lot of microcraters and impact melt splashes were observed [2,3]. One relatively large crater (A0067-crater#1) and two melt splashes (A0067-melt#1 and A0094-melt#1) were analyzed by scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray nano-tomography (XnCT), and scanning transmission electron microscope (STEM) equipped with EDS to investigate the nature of the impactors hit on the asteroid Ryugu.
A0067-melt#1 shows round shape and is ~30 µm in diameter. It is composed of a Mg-Fe-rich glassy silicate main body and an Fe-rich opaque drop (~10 µm) attaching on the glassy silicate. It was extracted from the A0067 sample using focused ion beam technique and analyzed by XnCT and STEM-EDS. The analyses showed that the glassy silicate has a smooth boundary with the Fe-rich opaque drop. The glassy silicate has homogeneous Mg-Fe-rich composition with the ratio Mg/(Mg+Fe) in atom (hereafter Mg#) of ~0.64 and contains small amounts of Fe-Ni metal–sulfide spherules (<100 nm). The Fe-rich opaque drop consists of flower-like crystals of kamacite (~200–300 nm) embedded in the troilite matrix with pentlandite veins (~20 nm in width).
A0094-melt#1 shows hourglass-like morphology (~15 × 5 µm) and is probably made of two Mg-Fe-rich glassy silicate drops connected to each other. XnCT–STEM-EDS analyses revealed that the glassy silicate is compositionally inhomogeneous and shows patchy structure with Fe-rich (Mg# 0.52–0.55) and Fe-poor (Mg#~0.79) glassy silicate regions (2–5 µm in size). The boundaries between the regions are unclear. Spherical voids (a few tens of nanometers to ~2 µm) are abundant both in the Fe-rich and the Fe-poor regions. The Fe-rich region contains spherical and irregular-shaped Fe-Ni sulfides (<500 nm) and olivine grains (1–2 µm). Some carbonaceous aggregates (0.3–1 µm) consisting mainly of spongy carbonaceous material, irregular-shaped Fe-Ni sulfides, and Mg-rich silicates were also observed in A0094-melt#1.
A0067-crater#1 is ~5 µm in diameter. XnCT–STEM-EDS analysis revealed that A0067-crater#1 is ~4 µm in depth and traps small amount of mixture of glassy silicate and Fe-Ni sulfides. The mixture should be an impact melt and shows flow structure consisting of glassy silicate and Fe-Ni sulfide layers (30–250 nm in thickness) stacking with each other. The glassy silicate layer is compositionally inhomogeneous and separated into Mg-Fe-rich (Mg#~0.72) and Si-rich (Mg#~0.39) glasses. The Mg-Fe-rich glass is abundant compared to the Si-rich glass. Both the silicate glasses contain spherical voids (<200 nm) and Fe-Ni sulfide spherules (<100 nm).
The impact melts studied contain abundant Mg-Fe-rich glassy silicates. The major element compositions of the Mg-Fe-rich glassy silicates (including Fe-Ni sulfide and olivine grains) are plotted along with an extension of a line connecting the CI (solar) composition [4] and the Fe-vertex in a (Si+Al)–Mg–Fe ternary diagram. The compositional trend seems to represent a mixing line of the impactors and the Ryugu surface materials. As some of the impact melts studied contain large amounts of Fe-Ni sulfides, some large Fe-Ni sulfide grains should be included in the impactors and/or the Ryugu surface materials. In the meeting, the formation process of each impact melt and the nature of the impactors will be discussed.
[1] Yokoyama et al. (2022) Science. [2] Nakamura et al. (2022) Science. [3] Noguchi et al. (2022) Nat. Astronom. [4] Lodders (2021) Space Sci. Rev.
A0067-melt#1 shows round shape and is ~30 µm in diameter. It is composed of a Mg-Fe-rich glassy silicate main body and an Fe-rich opaque drop (~10 µm) attaching on the glassy silicate. It was extracted from the A0067 sample using focused ion beam technique and analyzed by XnCT and STEM-EDS. The analyses showed that the glassy silicate has a smooth boundary with the Fe-rich opaque drop. The glassy silicate has homogeneous Mg-Fe-rich composition with the ratio Mg/(Mg+Fe) in atom (hereafter Mg#) of ~0.64 and contains small amounts of Fe-Ni metal–sulfide spherules (<100 nm). The Fe-rich opaque drop consists of flower-like crystals of kamacite (~200–300 nm) embedded in the troilite matrix with pentlandite veins (~20 nm in width).
A0094-melt#1 shows hourglass-like morphology (~15 × 5 µm) and is probably made of two Mg-Fe-rich glassy silicate drops connected to each other. XnCT–STEM-EDS analyses revealed that the glassy silicate is compositionally inhomogeneous and shows patchy structure with Fe-rich (Mg# 0.52–0.55) and Fe-poor (Mg#~0.79) glassy silicate regions (2–5 µm in size). The boundaries between the regions are unclear. Spherical voids (a few tens of nanometers to ~2 µm) are abundant both in the Fe-rich and the Fe-poor regions. The Fe-rich region contains spherical and irregular-shaped Fe-Ni sulfides (<500 nm) and olivine grains (1–2 µm). Some carbonaceous aggregates (0.3–1 µm) consisting mainly of spongy carbonaceous material, irregular-shaped Fe-Ni sulfides, and Mg-rich silicates were also observed in A0094-melt#1.
A0067-crater#1 is ~5 µm in diameter. XnCT–STEM-EDS analysis revealed that A0067-crater#1 is ~4 µm in depth and traps small amount of mixture of glassy silicate and Fe-Ni sulfides. The mixture should be an impact melt and shows flow structure consisting of glassy silicate and Fe-Ni sulfide layers (30–250 nm in thickness) stacking with each other. The glassy silicate layer is compositionally inhomogeneous and separated into Mg-Fe-rich (Mg#~0.72) and Si-rich (Mg#~0.39) glasses. The Mg-Fe-rich glass is abundant compared to the Si-rich glass. Both the silicate glasses contain spherical voids (<200 nm) and Fe-Ni sulfide spherules (<100 nm).
The impact melts studied contain abundant Mg-Fe-rich glassy silicates. The major element compositions of the Mg-Fe-rich glassy silicates (including Fe-Ni sulfide and olivine grains) are plotted along with an extension of a line connecting the CI (solar) composition [4] and the Fe-vertex in a (Si+Al)–Mg–Fe ternary diagram. The compositional trend seems to represent a mixing line of the impactors and the Ryugu surface materials. As some of the impact melts studied contain large amounts of Fe-Ni sulfides, some large Fe-Ni sulfide grains should be included in the impactors and/or the Ryugu surface materials. In the meeting, the formation process of each impact melt and the nature of the impactors will be discussed.
[1] Yokoyama et al. (2022) Science. [2] Nakamura et al. (2022) Science. [3] Noguchi et al. (2022) Nat. Astronom. [4] Lodders (2021) Space Sci. Rev.