Separate effects of irradiation and impacts on lunar metallic iron formation observed in Chang’e-5 samples

Abstract

Nanophase iron particles (npFe0) are generated on the surface of airless bodies by space weathering and can alter surficial optical properties substantially. However, the details of their formation pathways are still unclear. Here we use impact glasses returned from the Moon by Chang’e-5 to distinguish the relative contributions of solar wind irradiation and (micro)meteorites impacts to the production of different-sized npFe0. We show that solar wind irradiation can solely produce small npFe0, via implantation of solar wind ions into the topmost grain surfaces. On the other hand, (micro)meteorite impacts produce directly large npFe0 in melts, through impact-triggered disproportionation reaction or thermal decomposition. These nanoparticles are also capable to further coalesce into micrometre-sized Fe0 particles during impacts. These findings can help in predicting the space-weathering behaviour of regions exposed to different space environments. A lunar glass bead can preserve nanophase iron (npFe0) of varying sizes via multiple mechanisms. The formation of small and large npFe0 with distinct weathering effects is independently governed by solar wind irradiation and micrometeorite impacts.