Space-weathered rims on lunar ilmenite as an indicator for relative exposure ages of regolith

Solar wind irradiation, as a crucial space weathering mechanism, alters the microscopic characteristics and reflectance spectrum of the lunar regolith, and its cumulative effect is strongly related to the exposure time. Ilmenite is highly resistant to solar wind irradiation. Here, we combined transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy to systematically illustrate the diverse space-weathered rims on the shoveled and drilled Chang’e-5 ilmenites resulting from varying degrees of solar wind irradiation, revealing that solar wind plays a key role in the early-stage alteration of exposed lunar soil. In addition, the space weathering microstructure observations are consistent with the model-predicted exposure history at the Chang’e-5 landing site and prove that the Chang’e-5 deep-layered drilled (~65 cm) and surface samples (<3 cm) have been exposed for a longer time than that of the intermediate-layered drilled samples. We conclude that the ilmenite rims have the potential to be an effective indicator for comparing the relative exposure ages of regolith on airless bodies. Ilmenite grains collected in lunar regolith sampled by the Chang’e-5 sample return mission reveal that the 1 m deep core contains a mixture of space-weathered material, buried ejecta from the nearby Xu Guangqi crater, and a lunar paleoregolith.