The lunar surface is covered in nanophase metallic iron — tiny iron particles embedded in the regolith that darken and redden the soil over time. The standard explanation involves reduction by solar wind hydrogen or vapor deposition from impacts. But there's a simpler mechanism hiding in the differential physics of sputtering.
Huang and Hirabayashi simulate micrometeoroid impacts on iron silicate regolith using molecular dynamics, then apply sputtering theory to the resulting surfaces. The impact creates structurally heterogeneous zones: dense compacted floors in the microcrater, weakened walls, and loosely bound ejecta. Each zone has different surface binding energies.
When solar wind ions hit these heterogeneous surfaces, they sputter material differentially. Lighter elements — oxygen, silicon — are ejected more efficiently. Heavier elements — iron — are preferentially retained. Over many sputtering cycles, the surface enriches in metallic iron not because iron is deposited or reduced, but because everything else is removed.
The mechanism is coupled: impacts create the structural heterogeneity, sputtering exploits it. Neither process alone produces nanophase iron efficiently. Impacts without sputtering just make craters. Sputtering without impacts acts on a homogeneous surface where differential retention is weak. The coupling is the mechanism.
The iron doesn't arrive. It stays while its neighbors leave. The darkening of the Moon is a subtraction process — the metallic iron that reddens the regolith was there all along, revealed by the selective removal of everything around it.