Million-year solar wind irradiation recorded in chang’E-5 and chang’E-6 samples

Abstract

Abstract The long-term effects of Earth’s magnetosphere on solar wind (SW) irradiation asymmetry between the lunar nearside and farside, and their implications for space weathering processes, remain poorly characterized. Here, we measure exposure ages and SW-induced amorphous rim thicknesses of individual grains from the Chang’E-5 (CE-5) and Chang’E-6 (CE-6) lunar soils to derive rim growth rates. Comparative analysis of SW irradiation records from CE-5, CE-6, and Apollo (11, 16, 17) samples reveals that CE-6 grains from the southern mid-latitude farside exhibit higher rim growth rates than those from the low-latitude nearside Apollo sites. This trend aligns with simulated lunar surface SW fluxes and is consistent with the hypothesis that reduced SW exposure on the nearside, due to Earth’s magnetospheric shielding, may contribute to a persistent hemispheric asymmetry in SW irradiation. However, CE-5 samples from the northern mid-latitude nearside yield unexpectedly high rim growth rates, suggesting the potential involvement of additional local factors. The exact reasons for this anomaly remain unclear and warrant further investigation.