A close look at microcraters on a lunar grain from a Chang'e-5 returned sample shows evidence of the formation of nanophase metallic iron (np-Fe-0) via disproportionation of Fe2+ triggered by secondary impacts, with no space weathering contribution from the solar wind.

 

Abundant nanophase iron (np-Fe-0) is ubiquitously formed on the surface of the Moon and other airless bodies through space weathering processes, and plays a dominant role in transforming the optical properties of the lunar surface. The main sources of np-Fe-0 are usually considered to be evaporative deposition or ion-reduction processes. Here we show that disproportionation reactions triggered by micrometeorite impacts can be the main contributor to np-Fe-0 formation. We measured the valence states of iron in the microcraters on a fine-grained Chang'e-5 sample (number CE5C0200YJFM00302) and found the presence of np-Fe-0 and associated Fe3+ in the amorphous mixture of olivine, which can be explained by the disproportionation reaction of Fe2+ during microimpacts. The chemical composition of the residual impactor suggested that it was formed by a secondary low-velocity impact on lunar anorthite. As the whole process was dominated by impact events without the contribution from the solar wind, these findings inform us on weathering mechanisms on regions or bodies that do not experience a strong solar wind component, such as permanently shadowed areas or outer Solar System bodies.