Recently, the Planetary Science Group of the Space Science Climbing Team of Shandong University researched China’s Chang’e-6 lunar soil samples and achieved new understandings of the space-weathering characteristics of young maria and the evolution mechanism of lunar soils. The related findings have been published in The Astrophysical Journal Letters. This paper features Lu Xuejin (Ph.D. candidate) as first author, with Prof. Ling Zongcheng and Dr. Chen Jian as corresponding authors.

Fig 1 China Chang’e-6 lander on the lunar surface and returned soil sample in the laboratory

Solar wind implantation and micrometeoroid bombardment are two main mechanisms acting in lunar space weathering, however, their relative contribution remained unknown. The Chang’e-6 mission returned the first soil/regolith samples from the lunar farside (Fig 1), and the dating results indicate that the local lava plain is about 2.8 billion years old (Ga). Therefore, the Chang’e-6 lunar soil sample preserves the evolution history of the regolith at the landing region since 2.8 Ga, which is a key anchor point between the Chang’e-5 landing region (~2.0 Ga) and the Apollo sampling regions (>3.2 Ga), which provides an essential sample for studying the source and mechanism of lunar space weathering.

The Chang’e-6 lunar soil has low reflectance values, reddened spectral slopes, and shallow mineral absorption bands, consistent with mature lunar soils' spectral characteristics (I_s/FeO=65±10 and OMAT=0.187±0.004). Orbital spectral data also reveal that the Chang’e-6 landing site exhibits relatively mature soil properties (I_s/FeO=80±10; OMAT=0.14±0.02). The Chang’e-6 returned samples were approximately 20% less mature compared to the remote sensing results. In contrast, in previous Chang’e-5 maturity studies, the maturity of the landing site was reduced by ~50% after rocket plume disturbance, suggesting that the degree of maturity reduction due to rocket exhaust may be inversely correlated to the geologic age of the landing regions.

The Chang’e-6 soil samples contain less than 30% glassy agglutinates compared to the average contents (50-70%) in Apollo mature soils. The high spectroscopic maturity and low glassy agglutinate content of Chang’e-6 soil represents a unique space-weathering characteristic in which solar wind implantation may have played a dominant role and micrometeoroid bombardment produces less glassy content.

Fig 2 A cartoon illustration comparing lunar regolith formation and evolution process at the younger (<3.0 Ga) and older maria (>3.0 Ga)

In younger lunar mare units, such as the Chang’e-6 and Chang’e-5 landing sites, the relatively weaker impact processes led to a reduction in the gardening efficiency and mixing of the regolith, a decrease in the production of glassy agglutinates, and the formation of a thinner optically mature layer on the surface by solar wind implantation (Fig 2). Rocket exhaust led to the disturbance and disruption of the optically mature layer, and the subsurface material buried beneath the optically mature layer was more easily exposed and returned by the Chang’e missions. The relative contribution of solar wind and micrometeoroid impacts changed due to the weakening of the impact and the change in the impactor properties, with solar wind implantation dominating the space weathering of the young basalts.

The study carried out a spectroscopic characterization of the Chang’e-6 lunar soil, which, together with the glassy agglutinate content of the lunar soil and the thin optically mature layer observed at the landing site, revealed the unique space weathering characteristics of high spectroscopic maturity and weak impact modification in the weathering layer at the Chang’e-6 landing site. The results indicate the change in the micro-impactor properties after 3.0 Ga, revealing the relative contributions of solar wind effects and micro-meteoroid impacts and their evolution over time. The results are of great value to the understanding of the origin and mechanism of lunar space weathering.