The Moon has long served as a silent witness to the cataclysmic events that shaped the early inner Solar System. Particularly, the epoch dominated by the formation of enormous basins, spanning the first billion years of lunar history, holds vital clues to understanding planetary evolution, impact fluxes, and the conditions that may have influenced the emergence of life on Earth. Despite decades of research and sample returns, a persistent veil of uncertainty shadows this formative era due to the scarcity of lunar materials with a definitive origin tied to specific ancient impact basins. However, a breakthrough study led by Chen, Zhang, Cui, and colleagues, recently published in Nature Astronomy, offers groundbreaking insights by examining hitherto unstudied impact-melt clasts recovered by the Chinese Chang’e-6 mission.
These researchers focused their investigation on three intriguing impact-melt clasts found within the regolith of the Apollo basin, which itself is nested inside the gargantuan South Pole–Aitken (SPA) basin — the largest and oldest recognized impact feature on the Moon. Through rigorous geochemical and isotopic analyses, the team discovered that these rocks exhibit KREEP-like (potassium, rare earth elements, and phosphorus) compositional signatures, a distinctive geochemical fingerprint typically associated with the lunar crust’s incompatible-element-enriched reservoir. Such a finding markedly challenges earlier interpretations of the basin-forming epoch and carries profound implications for the Moon’s geological history.
By carefully unravelling the complex petrogenesis of these impact-melt clasts, the scientists hypothesize that they originated from differentiates formed within the SPA basin’s own colossal impact-melt sheet or pool, which subsequently suffered reworking during the formation of the younger Apollo basin roughly 4.16 billion years ago. This narrative suggests a far more intricate overlapping timeline of basin formation events than conventionally accepted, which often placed the majority of basin-forming impacts within a narrow scope of about 3.8 to 4.0 billion years ago, coincident with the much-debated Late Heavy Bombardment (LHB).
The Late Heavy Bombardment hypothesis has dominated the discourse of lunar and terrestrial planetary evolution for decades, positing a sudden spike in impact frequency that almost “reset” the inner Solar System’s surface chronologies. However, the new evidence reveals that basin formation and major impact processes might have been extended and more staggered in time, with basin-forming impacts taking place significantly earlier and not confined to the previously assumed window. This revelation forces planetary scientists to reconsider the nature and cadence of early impact events, and their consequent thermal and compositional imprint on planetary crusts.
Central to the study are the geochemical signatures of the clasts, which bear the hallmark of KREEP-like lithologies. KREEP components are crucial reservoirs enriched in incompatible elements such as potassium (K), rare-earth elements (REE), and phosphorus (P), thought to represent residual melts from the final stages of the lunar magma ocean’s crystallization. These materials provide an unparalleled proxy for constraining the timescales of lunar crust differentiation and subsequent impact-related reprocessing. The identification of KREEP-like material in these impact-melt clasts is thus a gateway to decoding the chronology and mechanics of multi-stage basin formation within the Moon’s early history.
Analytical techniques employed involve a suite of state-of-the-art geochemical and isotopic tools designed to pinpoint age, mineralogy, and elemental composition with high precision. Using isotopic dating of radiogenic systems, particularly uranium-lead (U-Pb) and argon-argon (Ar-Ar) methods, the researchers determined the timing of rock formation and subsequent resetting events. These time-stamped records exhibit a complex yet resolvable history of impact-melting, crystallization, and secondary reworking, providing a detailed sequence of lunar surface processes.
The Apollo basin, although younger than the SPA basin, proved instrumental as a lens to peer into this intricate history, allowing the researchers to observe the mixing and metamorphism resulting from overprinting impact processes. The evidence that the Apollo impact event reworked melt products from an older SPA melt sheet provides compelling proof that lunar basin formation was not a set of isolated, discrete episodes but part of a protracted and overlapping series of catastrophic events that reshaped the Moon’s crust over hundreds of millions of years.
The implications of this study extend well beyond lunar sciences. If such a protracted basin-forming epoch holds true for the Moon, a corollary possibility exists for Earth and other terrestrial planets, which share a similar bombardment environment. The timing, intensity, and duration of impacts have direct bearings on crustal evolution, atmospheric conditions, and potentially the early biosphere’s habitability. Revised lunar chronology necessitates revisiting models of early terrestrial bombardment, planetary differentiation, and volatile delivery across the inner Solar System.
Moreover, the Chang’e-6 mission’s ability to return pristine regolith samples from such a strategically significant location underlines the critical role of ongoing and future lunar exploration in advancing planetary science. Accessing and analyzing materials from within ancient impact basins provides an unprecedented window into the timing, compositional diversity, and thermal history of the lunar crust. Such exploration efforts promise to fill persistent gaps in our understanding of planetary impact dynamics and surface evolution.
This study also exemplifies how combining geochemical fingerprints, precise isotopic dating, and sample provenance analysis can unravel the tangled history of planetary surfaces that have witnessed repeated reshaping events. The synergy between sample analysis and remote sensing data paves the way for more focused hypotheses and mission planning targeting the Moon’s complex geological past.
In essence, the findings of Chen and colleagues reshape the timeline of lunar basin formation, challenging the prevailing dogma of a sharp spike in impact flux centered around 3.8 to 4.0 billion years ago. Instead, the data support a more nuanced narrative of prolonged and overlapping basin-forming impacts, with significant events recorded as far back as 4.16 billion years ago. This represents a paradigm shift that will influence future interpretations of the Moon’s chronology and its role as a recorder of Solar System evolution.
The research also highlights the importance of identifying and characterizing KREEP-like lithologies within impact melts as they serve as critical tracers of lunar geological processes. Their presence within the melt sheets of major basins sheds light on differentiation processes within massive impact events, magma ocean crystallization, and subsequent crustal reworking due to younger basin-forming impacts.
Furthermore, this study underscores the limitations of previous lunar sample collections and the value of targeted sampling missions like Chang’e-6 in accessing previously unexplored geological contexts. With fresh samples sourced from within the SPA basin’s environment, the lunar chronology can be refined with unprecedented confidence and precision.
As planetary scientists incorporate these new findings into broader models, questions about the frequency, magnitude, and global consequences of ancient impacts can be re-examined with much-needed clarity. In turn, this could reshape our understanding of the conditions prevalent in the early Solar System and provide critical insights into the processes that influenced planetary crustal dynamics, volatile reservoirs, and early habitability.
Eventually, these revelations contribute to a growing consensus that the Moon carries a multistage record of impact events that are far more complex than hitherto assumed. Understanding these complexities not only enriches lunar science but also serves as a comparative baseline for evaluating other terrestrial planets and their geological histories.
In conclusion, through meticulous study of KREEP-like impact melts within the Apollo basin’s regolith, Chen et al. provide compelling evidence for a broad and enduring basin-forming epoch on the Moon. This finding challenges long-standing theories about the Late Heavy Bombardment, emphasizing a more drawn-out sequence of massive impacts that shaped the lunar surface and offer profound insights into the early evolution of our planetary neighborhood.
Subject of Research: Lunar impact basin chronology and early Solar System impact fluxes
Article Title: KREEP-like lithologies in the South Pole–Aitken basin reworked by the Apollo basin impact at 4.16 Ga
Article References:
Chen, J., Zhang, L., Cui, Z. et al. KREEP-like lithologies in the South Pole–Aitken basin reworked by the Apollo basin impact at 4.16 Ga. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02640-5
Image Credits: AI Generated
