[M-IS25] 惑星火山学

[MIS25-P03] A preliminary study of reddish rock at a Mars analog sites on the Earth with a focus on opaque minerals

*金丸 龍夫¹、藁谷 哲也¹、壷井 基裕³、菅沼 悠介²、大岩根 尚⁴、岡 祥司³、三浦 真⁵ (1.日本大学文理学部、2.国立極地研究所、3.関西学院大学、4.株式会社musuhi、5.GIA Tokyo 合同会社)

キーワード：火星、酸化、風化、不透明鉱物、火星アナログ

It is widely known that Mars is characterized by a reddish surface, which is also shown in the field photos sent by NASA's Mars Rover clearly. There are reasons for the reddish color of the rocks, and it is certainly affected by the surface environment of Mars. However, there are various factors that cause rocks surface to turn reddish, and it is difficult to simply identify the constituent materials from the color tone. In the near future, sample returns from Mars are expected to reveal some of these factors. While, in order to extract more information from a small amount of samples brought back from Mars, it is important to accumulate basic research data in areas on the Earth that can serve as Mars analog sites. In particular, opaque minerals (e.g., iron oxides, iron hydroxides) are sensitive to the surrounding environment and are expected to record changes in the surface environment, but studies conducted from this perspective are rare. In order to contribute to the accumulation of basic data for this kind of study, we focused on the opaque minerals near the surface of igneous rocks that show significant reddening from inland Antarctica and Australia, and described their characteristics by microscopic observation, microprobe analysis, and Raman spectroscopy. For Antarctic rocks, reddish-brown translucent filmy material with a thickness of about a few micrometers develops at the mineral grain boundaries under microscope, which causes the rocks to become reddish. Raman spectroscopy revealed that this material is goethite. It is thought that this ferric material was formed mainly by progressive oxidation into the rock due to physical weathering, and the resulting breakdown of the original magnetite and ilmenite and the migration of iron (Kanamaru et al., 2018, Geomorphology, https://doi.org/10.1016/j.geomorph.2018.05.015). On the other hand, opaque minerals ranging in size from a few micrometers to several tens of micrometers have developed in Australian rocks, showing amoeboid morphology from the surface to the interior. Electron probe and Raman spectroscopy revealed that these were homogeneous hematite. This is thought to be the cause of the reddening of the rocks in this area. As described above, there are various factors that cause rocks to turn reddish and redding of rocks should vary depending on the surrounding environment. Detailed research on the reddening of rocks on Earth, considering climatic conditions and rock types, may help us to understand the changes in the Martian surface environment.