Thin basaltic regolith at the Chang'e-6 landing site

Abstract

The Chang'e-6 (CE6) mission is the first probe that has returned samples from the lunar farside. The sampling site is located on the southern mare basalt within the Apollo peak-ring impact basin. The Apollo basin itself lies within the clearly-recognized, largest and oldest South Pole–Aitken (SPA) basin on the Moon. Samples from the SPA are believed to be of great implications for multiple critical lunar science questions, such as impact chronology, impact basin formation processes, as well as stratigraphy and composition of lunar lower crust/upper mantle. Using available data sets from previous lunar missions, this study proposes a subsurface stratigraphy for the CE6 landing area based on crater morphologies and their excavation depths. The result revealed the thickness of the basaltic regolith at CE6 landing site is as thin as ∼1.6 m. Compositional data suggested the basalt-dominant samples (Mg#: ∼40) actually contain both intermediate-Ti (∼5.6 wt.% TiO₂) and low-Ti (∼3.3 wt.% TiO₂) basalts, with exotic Mg-suite clasts (Mg#: ∼70) as ejecta mainly from craters inside the Apollo basin. The remotely sensed compositional signatures indicated that the CE6 probe had sampled KREEP (an acronym for potassium, rare earth elements, and phosphorus)-poor (Th: ∼2 ppm) high-Al (∼14 wt. % Al₂O₃) materials. Since the mare basalts were products of thermal evolution and partial melting of the mantle, they are appropriate agents to study lunar farside thermal and geochemical evolution, and examine the nearside-farside crustal and volcanic dichotomies. Given that the sampling site is located in the margin of the proposed SPA transient cavity and melt pool, the exotic Mg-suite clasts in the CE6 samples may represent shallower crustal materials, which have the potential to provide valuable insights into lunar crustal composition and stratigraphy.