Surface microstructures of lunar soil returned by Chang'e-5 mission reveal an intermediate stage in space weathering process

Abstract

The lunar soils evolution over time is mainly caused by space weathering that includes the impacts of varying-sized meteoroids and charged particles implantation of solar/cosmic winds as well. It has long been established that space weathering leads to the formation of outmost amorphous layers (50-200 nm in thickness) embedded nanophase iron (npFe⁰) around the mineral fragments, albeit the origin of the npFe⁰ remains controversial . The Chang'e-5 (CE-5) mission returned samples feature the youngest mare basalt and the highest latitude sampling site , providing an opportunity to seek the critical clues for understanding the evolution of soils under space weathering. Here, we report the surface microstructures of the major minerals including olivine, pyroxene, anorthite, and glassy beads in the lunar soil of CE-5. Unlike the previous observations, only olivine in all crystals is surrounded by a thinner outmost amorphous SiO₂ layer (∼10 nm thick) and embedded wüstite nanoparticles FeO (np-FeO, 3-12 nm in size) instead of npFe⁰. No foreign volatile elements deposition layer and solar flare tracks can be found on the surface or inside the olivine and other minerals. This unique rim structure has not been reported for any other lunar, terrestrial, Martian, or meteorite samples so far. The observation of wüstite FeO and the microstructures support the existence of an intermediate stage in space weathering for lunar minerals by thermal decomposition.

Keywords: Amorphous rim; Lunar soil; Nanoparticle; Nanophase Fe; Wüstite FeO.