Impact Gardening Affects the Composition of Chang'e-5 Lunar Soils

The composition of lunar samples sheds light on the Moon's evolutional history. Analyses of Chang'e-5 (CE-5) lunar soils showed <5% of foreign materials, significantly less than numerical predictions (similar to 10%). To address this inconsistency, we simulated the impact gardening process, accounting for distal ejecta, and tracked the compositional changes in the top 1 m layer at CE-5 landing area over time. Our results show that impact gardening brings deeper local materials to the surface, leading to a mixture that reduces the distal ejecta proportion within the top 1 m layer from which the soils were collected. After 2.0 Gyr of impact gardening, most materials of the top 1 m layer originate from the upper layer (depth <30 m) of local basalts, with distal ejecta as a minor component (similar to 2.7 vol.%), consistent with CE-5 soils analyses. Our results emphasize the profound influence of impact gardening on the composition of lunar soils. Plain Language Summary The analyses of the composition of lunar soil are crucial for unraveling the Moon's complicated history. China's Chang'e-5 (CE-5) mission collected 1.731 kg lunar soil from a depth of 1 m within a relatively young region. The detailed examinations of the CE-5 soils showed that less than 5% of the components originated from distal locations, possibly as ejecta from remote craters. This value contrasted sharply with early predictions (similar to 10%) from numerical models, which accounted for all potential ejecta contributions from distant craters formed after the CE-5 target basalts. The discrepancy between the laboratory analyses and numerical models suggests a more intricate evolutional history for the CE-5 sampling area than previously understood. To reconcile this inconsistency, we simulated the impact gardening process at the CE-5 landing area and found that impact gardening continuous churning and overturning of the lunar surface that brought deeper local materials to the surface. This process diluted and decreased the proportion of distal ejecta within the top 1 m layer, from which CE-5 soils were collected. Our findings indicate that the unexpectedly low presence of foreign materials in CE-5 soils is due to the impact gardening process, which is crucial for understanding the composition of lunar soils.