Formation of np-Fe⁰ particles by H⁺ irradiation: Insight into space weathering on the moon and other airless bodies

¹ Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang (Guizhou) 550081, PR China
² College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
³ Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei (Anhui) 230026, PR China

^★ Corresponding authors: tanghong@vip.gyig.cn; lixiongyao@vip.skleg.cn

Received: 22 April 2025
Accepted: 10 October 2025

Abstract

Context. Space weathering on airless bodies results in the formation of nanophase metallic iron (np-Fe⁰) particles, which will cause spectra darkening and reddening. However, the effects of temperatures and iron contents on np-Fe⁰ particles formation by H⁺ irradiation have not yet been well understood.

Aims. This research focuses on revealing how temperatures and iron contents affect the formation of solar wind-derived np-Fe⁰ particles.

Methods. We selected Chang’e 5 (CE-5) olivine and pyroxene grains with different iron contents and removed their native space weathering rim. H⁺ irradiation experiments were conducted at 87±2°C and room temperature, with an energy of 1.5 keV and a fluence of 1.0 × 10¹⁸ ion/cm². Raman spectroscopy (Raman) was used to determine the mineral phases and the changes of chemical bonds before and after H⁺ irradiation. Fourier transform infrared spectroscopy (FTIR) was used to observe the Christiansen features (CFs), Reststrahlen bands (RBs), and the formation of water (OH/H₂O) before and after H⁺ irradiation. Transmission electron microscopy (TEM) and dispersive X-ray spectroscopy (EDS) were used to observe the microstructure and composition of np-Fe⁰ particles, while electron energy loss spectroscopy (EELS) was used to analyze the valence states of iron.

Results. Np-Fe⁰ particles are only produced in the minerals with high iron contents after H⁺ irradiation at 87±2°C, rather than minerals with high iron contents but irradiated at room temperature or minerals with low iron content irradiated at 87±2°C. Compared with the np-Fe⁰ particles that are formed by impact melting, solar wind irradiation can effectively lower the formation temperature by about one magnitude.

Conclusions. This research provides direct evidence of np-Fe⁰ particle formation by H⁺ irradiation experiments and clarifies the necessary conditions, indicating solar wind irradiation is an effective way for np-Fe⁰ particles formation. The variation in iron contents and temperatures will help us to recognize the formation of np-Fe⁰ particles at different lunar regions and other airless bodies such as Mercury and asteroids, evaluating the effect of space weathering on remote sensing more precisely.