¹Chengxiang Yin,^1,2Xiaohui Fu,¹Haijun Cao,¹Xuejin Lu,¹Jian Chen,¹Jiang Zhang,^1,2Zongcheng Ling,³Xiaochao Che
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008733]
¹Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Technology, Institute of Space Sciences, Shandong University, Weihai, China
²CAS Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei, China
³Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China
Published by arrangement with John Wiley & Sons

Submicroscopic metallic iron particles (SMFe) are unique components of lunar soil produced during long-term exposure on the Moon’s surface. They can significantly alter the optical properties of lunar soil and this alteration is crucial for the interpretation of remote sensing data. The origin and formation of SMFe remain a subject of controversy, with multiple competing mechanisms coexisting. The newly returned Chang’e-5 (CE-5) samples provide a new opportunity to elucidate the formation of SMFe. Here, we conducted a systematical study on the morphology and chemical characteristics of metal globules in CE-5 impact melt splash. A total of 30,630 metal globules were identified with an average diameter of 222.87 nm. Most of them are nearly/perfectly spherical, but the others are irregular in shape. Three types of irregular metal globules have been found: Spindle type, deformation type, and coalescence type. Spindle and deformation types were formed under the influence of local thermal disequilibrium and/or differences in wettability, while the coalescence type reflects the growth of metal globules driven by the Oswald ripening. A series of metal globules at different coalescence stages were found, providing conclusive petrographic evidence for the long-term hypothesis of SMFe growth (e.g., Pieters & Noble, 2016, https://doi.org/10.1002/2016je005128). Geochemical analysis shows that meteoritic Fe-Ni metals (like iron meteorite) made a significant contribution to the formation of metal globules. This further indicates the contribution of exotic meteoroid materials to the CE-5 lunar soil.