Planetary Science, Mineralogy, Meteoritics
My current major research topic is mineralogical study of extraterrestrial materials by micro-area X-ray and electron beam instruments (e.g., X-ray diffractometer, electron microprobe, scanning and transmission electron microscopes). My research goal is to clarify material evolution in the solar system by studying diverse planetary materials from primitive meteorites recording early solar system and cometary and asteroidal dust particles returned by the NASA Stardust and JAXA Hayabusa missions to evolved materials such as lunar and Martian meteorites. I am also involved in meteorite search in the Antarctica.
Recently, I have found the following important aspects of planetary materials:
1. As a part of preliminary examination team of the NASA Stardust mission, I found that comet Wild 2 particles are mainly composed of olivine and pyroxene by electron microscopy and synchrotron X-ray diffraction (SR-XRD). I also worked on Itokawa asteroidal particles returned by the Hayabusa mission and showed that they are minealogically similar to equilibrated ordinary chondrite (LL5-6), experiencing thermal metamorphism reaching 800 degrees Celsius.
2. I studied Almahata Sitta meteorite which is the first example of meteorite recovery after it was detected as an asteroid in space. Pyroxene microstructures show quench from high temperature similar to other ureilites, but Fe metal suggests super-rapid cooling event at low temperature. The analysis of many brecciated meteorites including Almahata Sitta revealed that break-up events of meteorite parent bodies were common and they were re-accumulated into daughter bodies.
3. Detailed mineralogical investigations on nakhlite Martian meteorites revealed that all the samples of this group originated from the same igneous body on Mars. Then, I proposed a model that the igneous body shows stratigraphical distribution from shallow areas to deep areas. Mineralogical analysis of heavily shocked shergottites showed that olivine darkening is a common shock features of Martian meteorites and the darkening is responsible for Fe-rich nano-particles in olivine.
4. I found high Fe valence of lunar anorthosite samples, suggesting that crystallization of lunar magma ocean took place under an oxidizing condition
1. Deng Z. et al. (2020) Early oxidation of the martian crust triggered by impacts. Sci. Adv., 6, eabc4941, DOI: 10.1126/sciadv.abc4941.
2. Mikouchi T. et al. (2014) Mineralogy and crystallography of some Itokawa particles returned by the Hayabusa asteroidal sample return mission. Earth, Planets and Space, 66, 82, doi.org/10.1186/1880-5981-66-82.
3. Mikouchi T. et al. (2010) Electron microscopy of pyroxenes in the Almahata Sitta ureilite. Meteoritics and Planet. Sci., 45, 1812-1820.