image: Schematic illustration of how the SPA basin-forming impact modified the lunar crust and mantle and led to volatile loss during the event.
Credit: Image by Prof. TIAN Hengci.
Since the Moon's formation, asteroid impacts have been the dominant exogenous geological process, creating craters and basins across its surface and profoundly altering its topography and geochemical properties. However, the extent to which large-scale impacts affected the Moon's deep interior has been poorly understood.
To address this problem, a research team led by Prof. TIAN Hengci from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) conducted extensive analysis of lunar basalts collected by Chang'e-6 (CE6) from the South Pole–Aitken (SPA) Basin. These samples exhibit significantly heavier potassium (K) isotopic compositions than all previously documented lunar basalts from the Apollo missions and lunar meteorites.
The team recognized that moderately volatile elements, such as K, are susceptible to volatilization and isotopic fractionation under impact-generated high temperatures. As a result, the isotopic composition of such an element can record information such as the temperature, pressure, and material sources of impact events, thus revealing the scale of an impact, its thermal history, and how it modified the lunar crust and mantle materials.
Based on this understanding, the team focused on examining the isotopic composition of K in the sample.
The study, which was published in Proceedings of the National Academy of Sciences (PNAS) on January 12, attributes this isotopic signature to the giant impact that formed the SPA Basin.
The team first conducted high-precision K isotope measurements of four basalt clasts using sapphire collision-cell multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The CE6 samples consistently showed elevated δ41K values, ranging from 0.001 ± 0.028‰ to 0.093 ± 0.014‰ (mean: 0.038 ± 0.044‰, 2SE). This average is approximately 0.16‰ higher than that of the Apollo lunar basalts (-0.13 ± 0.06‰, 2SE), which are widely regarded as representative of the composition of the lunar mantle and the Bulk Silicate Moon.
To clarify the cause of the unusual K isotopic composition, the team systematically assessed three potential modifying mechanisms—long-term cosmic-ray irradiation, magmatic differentiation, and meteoritic contamination. Their analysis showed these processes exert only minor effects, well within analytical uncertainties, and cannot explain the observed enrichment of heavy K isotopes.
Further research showed that extensive loss of volatile elements (specifically K evaporation) was associated with the SPA-forming giant impact. Such volatile depletion may have suppressed magma generation and volcanic activity on the lunar far side, potentially accounting for the long-recognized asymmetry in volcanic activity between the Moon's near and far sides.
Numerical simulations further confirmed that the giant impact not only excavated deep crustal and possibly mantle materials but also generated substantial thermal energy capable of inducing mantle convection.
Overall, the findings confirm that the SPA-forming giant impact exerted a profound influence on the Moon's deep interior. They also highlight the fundamental role of large-scale impact events in shaping the chemical evolution of planetary mantles and crusts.
This research was supported by the National Natural Science Foundation of China, the CAS Youth Innovation Promotion Association, and other sources.
Journal
Proceedings of the National Academy of Sciences