The surface of the Moon and other airless planetary bodies is perpetually subjected to space weathering—the effects of the solar wind and bombardment by micrometeoroids. A principal byproduct of these processes is nanophase iron ($\text{npFe}$), which dictates the optical and chemical characteristics of the lunar regolith. Until recently, the prevailing view was that $\text{npFe}$ formed exclusively from indigenous iron-bearing minerals through exposure to high temperatures and shock waves.
However, recent analysis of samples returned by the Chang’e-5 mission uncovered the presence of exotic $\text{npFe}$ whose origin is linked to the direct delivery of iron via impacting micrometeoroids. To numerically assess the contribution of this formation pathway, the researchers conducted molecular dynamics simulations for two distinct scenarios: the conventional in-situ formation of $\text{npFe}$ resulting from a silicon-bearing micrometeoroid impacting iron-rich material, and the exotic iron delivery scenario involving an iron-rich micrometeoroid striking silicon-rich material.
The simulation outcomes indicated that exotic $\text{npFe}$ tends to aggregate in localized clusters along the impact trajectory, thereby preserving a ‘signature’ of the micrometeoroid’s motion. Conversely, in-situ $\text{npFe}$ exhibits a more homogeneous and radial distribution surrounding the impact site. This distinction in spatial patterning allows researchers to differentiate the genesis of nanophase iron in lunar samples using contemporary electron microscopy techniques.
The authors point out that the contribution of exotic $\text{npFe}$ is particularly significant in iron-poor regions of the Moon, such as the highlands. The high efficiency of iron retention (up to 91%) during micrometeoroid impacts validates that this mechanism substantially influences regolith evolution and warrants consideration when interpreting spectral data and designing forthcoming missions.
The discovery of exotic nanophase iron not only broadens our comprehension of space weathering mechanisms but also furnishes a novel analytical tool for investigating the surface history of the Moon and other solar system bodies.
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