Formation of np-Fe ⁰ particles by H ⁺ irradiation: Insight into space weathering on the moon and other airless bodies

Context . Space weathering on airless bodies results in the formation of nanophase metallic iron (np-Fe 0 ) particles, which will cause spectra darkening and reddening. However, the effects of temperatures and iron contents on np-Fe 0 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-Fe 0 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 × 10 18 ion/cm 2 . 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/H 2 O) 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-Fe 0 particles, while electron energy loss spectroscopy (EELS) was used to analyze the valence states of iron. Results . Np-Fe 0 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-Fe 0 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-Fe 0 particle formation by H + irradiation experiments and clarifies the necessary conditions, indicating solar wind irradiation is an effective way for np-Fe 0 particles formation. The variation in iron contents and temperatures will help us to recognize the formation of np-Fe 0 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.