Micrometeoroid Impacts: Dual Pathways for Iron Reduction and Oxidation on Lunar and Asteroidal Surfaces

TL;DR

This study uses molecular dynamics simulations to show that micrometeoroid impacts create both reducing and oxidizing environments on lunar surfaces, explaining the coexistence of metallic iron and hematite observed in lunar samples and remote sensing.

Contribution

It reveals atomistic mechanisms by which impacts produce both reduced and oxidized iron species, bridging previous observations and advancing understanding of space weathering processes.

Findings

Impact craters generate transient oxygen-rich environments.

Nanophase metallic iron (npFe^0) forms in high-temperature impact zones.

Hematite presence is explained by impact-driven oxidation processes.

Abstract

Nanophase metallic iron ( $\mathrm{npFe}^0$ ) is a key indicator of space weathering on the lunar surface, primarily attributed to solar wind irradiation and micrometeoroid impacts. Recent discoveries of hematite ( $\mathrm{Fe}_2 \mathrm{O}_3$ ), a highly oxidized form of iron, in the lunar polar regions challenge the prevailing understanding of the Moon's reducing environment. This study, using ReaxFF molecular dynamics simulations of micrometeoroid impacts on fayalite ( $\mathrm{Fe}_2 \mathrm{SiO}_4$ ), investigates the atomistic mechanisms leading to both reduced and oxidized iron species. Our simulations reveals that the high-temperature and pressure conditions at the impact crater surface produces a reduced iron environment while providing a transient oxygen-rich environment in the expanding plume. Our findings bridge previously disparate observations-linking impact-driven $\mathrm{npFe}^0$ formation to the puzzling presence of oxidized iron phases on the Moon, completing the observed strong dichotomous distribution of hematite between the nearside and farside of the Moon. These findings highlight that micrometeoroid impacts, by simultaneously generating spatially distinct redox environments, provide a formation mechanism that reconciles the ubiquitous identification of nanophase metallic iron ( $\mathrm{npFe}^0$ ) in returned lunar samples with $\mathrm{Fe}^{3+}$ signatures detected by remote sensing. This underscores the dynamic nature of space weathering processes. For a more nuanced understanding of regolith evolution, we should also consider the presence of different generations or types of $\mathrm{npFe}{ }^0$, such as those formed from solar wind reduction versus impact disproportionation.