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

TL;DR

This work addresses how micrometeoroid impacts on Fe-rich lunar materials can simultaneously drive reduction and oxidation, reconciling the ubiquitous presence of nanophase metallic iron $npFe^0$ with remote-sensing signatures of oxidized iron such as hematite. Using ReaxFF molecular dynamics, the authors model hypervelocity impacts on fayalite $Fe_2SiO_4$ to capture atomistic redox processes during crater formation and plume expansion, tracking cluster formation and oxidation states. They find a dual redox environment: a reducing crater core that forms $npFe^0$ and an oxygen-rich ejecta plume that yields $Fe^{3+}$-bearing species, without extensive crystalline hematite formation on the simulated 100 ps timescale. These results bridge previous observations and suggest that multiple generations of $npFe^0$ and post-impact oxidation contribute to regolith evolution, with implications for interpreting lunar samples and remote sensing data; they also propose an intrinsic mechanism for hematite formation that operates alongside, or in competition with, Earth-derived oxygen delivery. The study highlights the dynamic, non-equilibrium nature of space weathering and underscores the need to consider temporal evolution and mineral-specific redox pathways when modeling regolith maturation on airless bodies.

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.

Figures (4)

• Figure 1: Snapshots of the micrometeoroid impact simulation : (a) T = 0 fs – initial configuration, we use Fe$_2$SiO$_4$ unit cell to make the impactor and the substrate. (b) T = 20 fs – plume expansion, the developing amorphous structure is formed due to the impact. (c) T = 100 fs – post‐impact steady state, showing the local impact site and emergence of Fe–Fe bonds.
• Figure 2: A comprehensive view of the system during the ejecta expansion phase following micrometeoroid impact. Panels (a)–(f) show the most frequent chemical species identified by cluster analysis: (a) O$_2$, (b) Fe$_2$O$_3$, (c) FeO, (d) FeSiO$_2$, (e) SiO, and (f) SiO$_2$. The corresponding cluster counts for each species are plotted in Figure \ref{['fig:VDF_ejecta']}.
• Figure 3: Ten most frequently observed elemental compositions of clusters ejected during the impact event. Each horizontal bar represents a unique cluster composition identified through spatial clustering and subsequent elemental analysis, with the length of the bar directly correlating to the number of clusters found for that specific composition. The compositions are ordered from most to least frequent, allowing for immediate identification of the dominant ejected species.
• Figure 4: Oxidation state distribution of iron in impact ejecta. By analyzing the composition of ejected species such as Fe$_2$SiO$_4$ and assuming fixed oxidation states for O (-2) and Si (+4), the oxidation state of Fe was calculated.