JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Oral

P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS10] [JJ] Formation and evolution of planetary materials in the solar system

Mon. May 22, 2017 3:30 PM - 5:00 PM Convention Hall A (International Conference Hall 2F)

convener:Tomohiro Usui(Earth-Life Science Institute, Tokyo Institute of Technology ), Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Akira Yamaguchi(National Institute of Polar Research), Yoko Kebukawa(Faculty of Engineering, Yokohama National University), Chairperson:Tomohiro Usui(Earth-Life Science Institute, Tokyo Institute of Technology )

3:45 PM - 4:00 PM

[PPS10-02] Irradiation experiments on CM chondrites: To estimate surface textures of the returned samples by Hayabusa 2

*Takaaki Noguchi1, Yuji Miyake2, Ryuji Okazaki2, Takahito Osawa3, Hiroyuki Serizawa3, Hikaru Yabuta4, Tomoki Nakamura5 (1.Faculty of Arts and Science, Kyushu University, 2.Department of Earth and Planetary Science, Kyushu University, 3.Japan Atomic Energy Agency, 4.Department of Earth and Planetary System Science, Hiroshima University, 5.Department of Earth Sciences, Tohoku University)

Keywords:irradiation experiment, CM chondrites, TEM

Introduction: In 2020, Hayabusa 2 spacecraft will return the surface and sub-surface samples from the asteroid (162173) Ryugu, a C-type asteroid. We will have an opportunity to investigate pristine materials from a C-type asteroid. Because CM chondrites contain solar gases and because most of they contain abundant subangular mineral and lithic fragments, they are regolith breccias (e.g. [1], [2], [3] and references therein). Although solar noble gases are restricted to the clastic matrix [1], [2], textures related to the solar wind irradiation and/or micrometeoroid impacts have not been identified among CM chondrites. Although there are many spectroscopic studies of CM chondrites (e.g. [4]), only a few studies are focused on the textural changes related to the micrometeoroid impacts and solar wind irradiation on CM chondrites (e.g. [5], [6]). In this study, we performed spectrum measurements, micro-petrographic study, and C Kα X-ray absorption near-edge structure measurement of irradiated CM chondrites. These studies will serve to understand the space weathering on the surface of fine-grained Ryugu grains because it is highly likely that space weathering will be found on the surface of Ryugu grains.
Samples and methods: We performed irradiation of 4 keV He+ ions on Murchison CM chondrites at Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency (TARRI, JAEA). The fluences are 5 × 1016 and 5 × 1017 He+/cm2, which correspond to ~102- and ~103-year irradiation at 1.1 AU (the averaged orbital radius of Ryugu). Reflectance spectra of the irradiated surface were measured at JASCO Co. Ltd. by using JASCO V-670 absorption spectrometer with an integrating sphere. The irradiated samples were observed by field-emission scanning electron microscope (FE-SEM) at JAEA and Kyushu University. We observed the samples by using 2 or 3 kV acceleration voltage to avoid structural changes during observation. Thin samples were prepared by using scanning electron microscope-focused ion beam sample preparation machine and low acceleration voltage Ar milling machine at Kyushu University. They were observed by transmission electron microscope (TEM) at Kyushu University.
Results and discussion: Reflectance spectrum of the sample irradiated by a fluence of 5 × 1016 He+ does not show remarkable difference from the spectra of an un-irradiated sample. By contrast, a broad absorption from 0.7 to 1.4 μm, related to the absorption by Fe-rich serpentine group minerals, is disappeared in the case of the sample irradiated with 5 × 1017 He+. These data suggest that 1000-year equivalent solar wind irradiation gives an effect on the shape of reflectance spectra, which is similar to the effect by dehydration [4]. There is no remarkable difference in surface morphology of the sample irradiated by a fluence of 5 × 1016 He+ from those of un-irradiated sample. On the other hand, the sample irradiated with 5 × 1017 He+ shows blistering on both matrix and chondrules. The surface of fine-grained matrix has a ~30-nm thick amorphous layer. In the amorphous layer, a small amount of nanoparticles is observed. Their 0.2-nm lattice fringes suggest that they are nanophase Fe0. In the case of the sample irradiated with 1017 He+ has ~60-nm amorphous rim containing abundant bubbles (blistering), which is especially remarkable in cronstedtite-tochilinite intergrowth. Just below the amorphous layer, both cronstedtite and tochilinite show sharp lattice fringes. The amorphous rim contains abundant nanoparticles is observed. They also show 0.2-nm lattice fringes, suggestive of nanophase Fe0. This result is consistent with [5].
References: [1] Nakamura T. et al. (1999a) GCA 63, 241-255. [2] Nakamura T. et al. (1999b) GCA 63, 257-273. [3] Krot A. et al. In: Meteorites and the early solar system II, pp. 679-712. [4] Hiroi T. et al. (1993) Science 261, 1016-1018. [5] Matsuoka M. et al. (2015) Icarus 254, 135-143. [5] Keller L. P. et al. (2015) LPSC 46, Abstract #1913.