Coupled Space Weathering: Nanophase Iron Formation by Micrometeoroid Impact and Solar Wind Sputtering

Understanding the interplay between micrometeoroid impacts and solar wind irradiation is crucial for interpreting lunar surface evolution. Using reactive molecular dynamics simulations and surface binding energy (SBE) analyses, this study investigates the coupled effects of these two dominant space weathering processes on lunar regolith composed of Fe$_2$SiO$_4$. Our simulations reveal that micrometeoroid impacts significantly modify the lunar surface, creating structurally heterogeneous zones with varying SBEs across microcrater morphologies. Specifically, microcrater floors exhibit enhanced surface cohesion due to high-density compaction, whereas microcrater walls and ejecta show weakened structures. Applying Sigmund's sputtering theory with these SBEs indicates differential sputtering yields for Fe, Si, and O suggesting preferential retention of heavier elements like Fe. This selective sputtering mechanism supports the formation and growth of nanophase metallic iron (npFe$^0$) clusters, influencing the optical and compositional maturation of the lunar surface. These findings advance our understanding of lunar space weathering processes.