Revealing Exotic Nanophase Iron in Lunar Samples Through Impact-Driven Spatial Fingerprints

TL;DR

The paper addresses the origin of nanophase metallic iron (npFe) on airless bodies by comparing in-situ formation from native Fe-bearing minerals against exotic npFe delivered by micrometeoroid impacts. Using ReaxFF molecular dynamics with a DBSCAN-inspired clustering analysis, it simulates two paired impact scenarios to quantify Fe delivery, retention, and npFe nucleation under hypervelocity conditions, including redox chemistry such as $\mathrm{Fe^{2+}}$ reduction to $\mathrm{Fe^{0}}$ and disproportionation $3\mathrm{Fe^{2+}} \rightarrow \mathrm{Fe^{0}}+2\mathrm{Fe^{3+}}$. Results show exotic Fe is efficiently retained and forms localized, trajectory-imprinted clusters (up to 4-atom Fe assemblies) aligned with the impact, whereas in-situ npFe forms more isotropic, radial distributions. These distinct spatial fingerprints provide a practical diagnostic for identifying npFe origins in lunar soils, aligning with Chang'e-5 observations and informing space weathering interpretations and regolith evolution on the Moon and other airless bodies.

Abstract

Nanophase iron (npFe) plays a crucial role in controlling the optical, chemical, and physical evolution of lunar regolith grains. While in-situ formation of npFe via reduction of native Fe-bearing minerals has long been considered a dominant pathway, recent mineralogical evidence from X.Zeng et al. (2025) reveals that the source of a significant fraction of npFe may be delivered directly by exotic micrometeoroid impacts (exotic npFe). Yet the atomic-scale processes governing how exotic np-Fe forms and survives during hypervelocity impacts remain largely unknown. To quantitatively compare in-situ and exotic delivery and formation of npFe, we perform a series of innovative atomistic modeling of micrometeoroid impacts with distinct projectile target compositions: (1) SiO$_2$ projectiles on Fe$_2$SiO$_4$ targets (in-situ formation), (2) Fe$_2$SiO$_4$ projectiles on SiO$_2$ targets (exotic delivery). Our results reveal distinct mechanistic fingerprints: in-situ np-Fe forms diffusely and radially around the impact site, whereas exotic np-Fe is efficiently retained and concentrated in asymmetric, momentum-aligned clusters. These contrasting spatial signatures provide a potential diagnostic criterion for distinguishing exotic versus in-situ np-Fe in returned lunar soils. In agreement with Chang'e-5 observations, our simulations demonstrate that exotic np-Fe production can be substantial, particularly in Fe-poor terrains such as highland regions. These findings highlight the need to account for exotic np-Fe when interpreting space weathering processes and remote-sensing data for the Moon and other airless bodies.

Figures (4)

• Figure 1: Snapshots of the micrometeoroid impact simulations: (a) SiO$_2$ impacting Fe$_2$SiO$_4$ (Case I), (b) Fe$_2$SiO$_4$ impacting SiO$_2$ (Case II). Red particles represent oxygen, orange particles represent iron, and tan particles represent silicon.
• Figure 2: Velocity distribution function (VDF) of Fe atoms classified as ejected (above $z=220\,\text{\AA}$) in the exotic Fe delivery case. A pronounced Maxwellian-like peak between 0 and 10 m/s corresponds to Fe atoms bound within two large clusters, snapshots of which are shown in the inset. The sporadic velocity tail from 20 to 60 m/s represents individual atomic and molecular fragments such as Fe$_2$O$_2$, Fe$_2$SiO$_2$, and Fe$_2$O generated during the impact. This distribution suggests that most ejected Fe is locally confined and likely to re-deposit, while a smaller fraction forms transient high-velocity species. Further discussion of these molecular species is provided in huang2025micrometeoroid.
• Figure 3: Post-impact atomic configuration for the exotic Fe delivery case. (a) Full atom snapshot overlaid with a surface mesh outlining the crater morphology. (b) Surface mesh with atoms removed, showing Fe–Fe bonds constructed using a 2.48 Å cutoff threshold. Fe atoms and Fe–Fe bonds are concentrated primarily on the right side of the crater, corresponding to the downstream direction of the left-to-right impact, illustrating directional aggregation of exotic iron.
• Figure 4: Post-impact atomic configuration for the in-situ Fe formation case. Fe atoms and Fe–Fe bonds are more diffusely distributed without clear clustering near the impact site. The spatial distribution is approximately radially symmetric, indicating npFe formation driven primarily by local thermal conditions radiating outward from the impact location.