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Coupled Space Weathering: Nanophase Iron Formation by Micrometeoroid Impact and Solar Wind Sputtering

Ziyu Huang, Masatoshi Hirabayashi

Abstract

Understanding the interplay between micrometeoroid impacts and solar wind irradiation is crucial for interpreting lunar surface evolution. Using reactive molecular dynamics simulations and surface binding energy (SBE) analyses, this study investigates the coupled effects of these two dominant space weathering processes on lunar regolith composed of Fe$_2$SiO$_4$. Our simulations reveal that micrometeoroid impacts significantly modify the lunar surface, creating structurally heterogeneous zones with varying SBEs across microcrater morphologies. Specifically, microcrater floors exhibit enhanced surface cohesion due to high-density compaction, whereas microcrater walls and ejecta show weakened structures. Applying Sigmund's sputtering theory with these SBEs indicates differential sputtering yields for Fe, Si, and O suggesting preferential retention of heavier elements like Fe. This selective sputtering mechanism supports the formation and growth of nanophase metallic iron (npFe$^0$) clusters, influencing the optical and compositional maturation of the lunar surface. These findings advance our understanding of lunar space weathering processes.

Coupled Space Weathering: Nanophase Iron Formation by Micrometeoroid Impact and Solar Wind Sputtering

Abstract

Understanding the interplay between micrometeoroid impacts and solar wind irradiation is crucial for interpreting lunar surface evolution. Using reactive molecular dynamics simulations and surface binding energy (SBE) analyses, this study investigates the coupled effects of these two dominant space weathering processes on lunar regolith composed of FeSiO. Our simulations reveal that micrometeoroid impacts significantly modify the lunar surface, creating structurally heterogeneous zones with varying SBEs across microcrater morphologies. Specifically, microcrater floors exhibit enhanced surface cohesion due to high-density compaction, whereas microcrater walls and ejecta show weakened structures. Applying Sigmund's sputtering theory with these SBEs indicates differential sputtering yields for Fe, Si, and O suggesting preferential retention of heavier elements like Fe. This selective sputtering mechanism supports the formation and growth of nanophase metallic iron (npFe) clusters, influencing the optical and compositional maturation of the lunar surface. These findings advance our understanding of lunar space weathering processes.
Paper Structure (10 sections, 6 equations, 2 figures, 2 tables)

This paper contains 10 sections, 6 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Method
  3. Impact Simulation by Reactive Molecular Dynamics
  4. Surface Binding Energy Calculation
  5. Sputtering Yield from TRIM Simulation
  6. Results and Discussions
  7. Micrometeoroid Impact Setup and Surface Amorphization
  8. Surface Binding Energy across Microcrater Morphologies
  9. Sputtering Yield
  10. Conclusion

Figures (2)

  • Figure 1: Schematic illustration of a micrometeoroid impact sequence: (a) Initial setup showing a 4 nm micrometeoroid approaching a pristine $Fe_2SiO_4$ mineral surface; (b) Moment of contact between the micrometeoroid and the surface; (c) Formation of ejecta as atomic structures are disrupted; (d) Post-impact state depicting the amorphized $Fe_2SiO_4$ surface. Atom colors represent Oxygen (red), Iron (orange), and Silicon (tan).
  • Figure 2: Left panel: Sampling locations for surface binding energy calculations, with selected atoms colored in yellow. Locations include the pristine surface (undisturbed crystalline region), crater wall (compressed and sheared zone), crater floor (heavily amorphized region), and ejecta zone (displaced and re-deposited material). Upper right panel: Close-up view of the amorphous impact site on the regolith testbed surface; Lower right panel: illustration of atoms being artificially given kinetic energy until they detach, demonstrating the procedure used to measure surface binding energy