^1,2Pei Ma,³Hao Zhang
Journal of Geopyhsical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2026JE009863]
¹Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat‐senUniversity, Zhuhai, China
²Now at Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy ofSciences, Guiyang, China
³School of Earth Sciences and Hubei Key Laboratory of Planetary Geology and Deep SpaceExplorations, China University of Geosciences, Wuhan, China
Published by arrangement with John Wiley & Sons

Lunar-like space weathering causes spectral darkening, reddening, and the attenuation of absorption bands due to metallic iron particles. The commonly cited iron size boundary, particles smaller than 40–50 nm redden spectra, while larger ones only darken, comes mainly from measurements of silica gel powders, and does not reflect the true particle-size effects. Using rigorous Mie theory to calculate absorption efficiencies of metallic iron spheres, we find that particles smaller than 80 nm primarily induce reddening with moderate darkening across the 0.4–2.6 μ⁢m$$ wavelengths, and the Hapke and Lucey‒Riner space weathering models are equivalent in this size range. Particles larger than ∼2 μ⁢m$$ cause darkening with only minor reddening. The upper size limit for reddening is wavelength-dependent; for 0.5–2.6 μ⁢m$$ wavelengths, particles <120 nm always redden the spectrum. Accounting for the polydisperse nature of metallic iron in lunar and laboratory samples, we incorporate the size distribution into the Lucey‒Riner space weathering model and validate it with laboratory data. We also discuss UV bluing, NIR reddening, and spectral brightness induced by metallic iron particles.