Scientists from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (CAS), the CAS Aerospace Information Research Institute, and other institutions, have revised the decades-old lunar crater chronology model, using samples collected from the far side of the Moon by China's Chang'e-6 mission and complementary remote sensing imagery.
For the first time, the team confirmed that meteorite impact fluxes on the Moon's near and far sides are essentially consistent. Their findings show that the early lunar impact flux declined steadily over time, offering no support for the Late Heavy Bombardment (LHB) hypothesis.
The study was published in Science Advances on February 4.
Impact craters are the dominant geomorphic features on the lunar surface and record the cumulative effects of meteorite bombardment since the Moon's formation. Galileo's early telescopic observations first identified these features, and later advances in imaging resolution revealed a systematic relationship between crater density and surface age.
Following the return of lunar samples by the Apollo and Luna missions, researchers quantitatively calibrated this relationship by developing a lunar cratering chronology function (CF), which connects crater density to absolute radiometric ages. This function has since become the foundation of lunar geological research, allowing age estimates for regions without returned samples.
However, before the Chang'e-6 mission, all samples used to calibrate the lunar CF came exclusively from the lunar near side. This limitation raised questions about the model's global applicability, as some studies suggested impact fluxes might differ between the near and far sides.
Additionally, the nature of the early lunar impact history has long been debated. Many Apollo samples contain impact-related materials with ages clustered around 3.9 billion years, which led to the proposal of the LHB hypothesis. This theory posits a brief, intense spike in impact activity across the inner Solar System. Alternative interpretations argue that this apparent age clustering reflects sampling bias, with most dated materials representing ejecta from the Imbrium impact rather than a global bombardment event.
The Chang'e-6 mission provided an opportunity to address both questions. The spacecraft landed in the Apollo Basin within the South Pole-Aitken (SPA) Basin on the lunar far side - the largest and oldest known impact basin on the Moon and a critical archive of its earliest history. The returned samples are dominated by local basalt dated to approximately 2.807 billion years, serving as an independent far-side calibration point for assessing near-side-far-side consistency in impact flux. The samples also include noritic lithologies dated to 4.247 billion years. Integrated petrological, mineralogical, and remote-sensing analyses indicate these norites represent crystallized impact melts formed during the creation of the SPA Basin, thus constraining the timing of this major early impact event.
Using well-established near-side calibration points and their corresponding crater densities, the team first constructed a lunar cratering chronology curve and quantified its uncertainty. The Chang'e-6 far-side ages and associated crater densities fall within the 95% confidence interval of the near-side-derived curve, indicating no measurable difference in impact flux between the two hemispheres. This result supports the use of a unified global cratering chronology and contradicts earlier suggestions of intensified far-side bombardment.
The researchers then established an updated lunar cratering chronology function by incorporating all reliable control points, including those from Chang'e-6. The reconstructed impact flux evolution shows a smooth and rapid decline during the early lunar period, rather than a prominent spike at approximately 3.9 billion years ago. Notably, the 4.247-billion-year-old samples from Chang'e-6 are inconsistent with both the LHB model and alternative sawtooth impact-flux scenarios. These observations indicate that the Chang'e-6 samples do not support the existence of a Late Heavy Bombardment at around 3.9 billion years ago - either on the Moon or across the inner Solar System - and instead favor a monotonic decrease in impact activity following planetary accretion.
Beyond resolving long-standing debates about lunar impact history, the revised cratering chronology based on Chang'e-6 data provides an improved framework for dating unsampled regions of the Moon.
Topographic map of the lunar farside based on LRO LOLA Data. (Image by YUE Zongyu's team)
