^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.

^1,2S. Ottaviani,^1,3L. Folco,^1,4M. D. Suttle,⁵R. Repič,⁵L. Mancini,⁵T. Battiston,^1,6S. Iannini Lelarge,^1,3M. Masotta
Journal of Geophysical Research: Planets (in Press) Open Access Link to Article [https://doi.org/10.1029/2026JE009650]
¹Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
²Dipartimento di Fisica e Geologia, Università degliStudi di Perugia, Perugia, Italy
³CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Pisa, Italy
⁴School of Physical Sciences, The Open University, Milton Keynes, UK
⁵Slovenian National Building and CivilEngineering Institute ‐ ZAG, Ljubljana, Slovenia
⁶Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse,Pisa, Italy
Published by arrangement with John Wiley & Sons

We estimate the cosmic dust flux to Earth through the study of rare micrometeorites that preserve part of their precursor features during atmospheric entry heating, namely unmelted and scoriaceous subtypes. Combining high-precision mass balance measurements, X-ray computed microtomography and scanning electron microscopy, we studied mass, size and petrography of 207 micrometeorites recovered from sediment traps in the Transantarctic Mountains (TAM), ranging from ∼170 to ∼1650 µm. Chondrules were identified in ∼14% of the micrometeorites, particularly among coarse-grained and composite particles. The analysed population shows a bimodal size-frequency distribution, with peaks at ∼305 µm and ∼470 µm. A similar bimodal distribution was previously reported from the TAM cosmic spherule population, yet shifted towards lower sizes. This size-shift is consistent with an average mass loss of ∼87% during atmospheric entry heating. The mass-size relationship follows a power-law, where the spherical equivalent diameter (d_µm) and mass (m_µg) of the micrometeorite are related by: 𝑚 =1.07 ×10−6 𝑑2.96$$. The size-frequency distribution of the fine- and coarse-grained micrometeorites reveals two well-separated clusters, with peaks at ∼315 µm and ∼550 µm, respectively. These observations suggest that the bimodal distribution in the micrometeorite flux reflects contrasting lithological end-member components with different physical properties and fragmentation behaviours during dust production in space. Based on earlier mass flux estimates from TAM melted micrometeorites and accounting for the mass loss due to atmospheric entry derived here, we calculate a time-averaged pre-atmospheric mass flux of ∼12,000 (±6,000) t/yr over the Quaternary, suggesting that the influx has remained stable over the last few million years.