A significant debate continues between scholars in the astronomical community regarding the early history of our solar system. It’s been hypothesised that the solar system experienced a dramatic cataclysm a short while —in cosmological terms— after the Earth and Moon solidified. Dubbed the Late Heavy Bombardment theory, disturbances in gas giant orbits may have caused a sudden hailstorm of comets and asteroids to be hurled towards the inner planets. Lunar rock samples collected from craters during Apollo missions seemed to support the idea, but new evidence is demanding a rethink.

Professor Yi-Gang Xu and his team examined samples recovered from the Chang’e-6 mission in 2024, and they suggest a more gradual change in the frequency of asteroid impacts, rather than a sudden, violent cataclysm.

The Evidence for a Lunar Cataclysm

The Moon is like a crime scene; its barren surface, pock-marked by impact craters, lays bare the cosmic assaults it has suffered over its history, like fingerprints or DNA samples. That makes it our best window into the early history of the solar system. On Earth scars heal quickly; craters caused by asteroid impacts billions of years ago have eroded away without a trace. But they’re preserved on the moon, and tell a curious story.

Rock samples collected during the Apollo missions suggest that many of the Moon’s impact craters date from around the same time. Rather than being spread through the Moon’s 4.5-billion year history, radiometric analyses suggest there’s a sharp spike in their frequency about 3.9-billion years ago. This finding led scientists to believe that a lunar cataclysm occurred, where the Moon was subjected to a sudden, intense barrage of asteroid impacts. At the time it was a controversial claim, but over the last 50 years additional scraps of evidence have supported the hypothesis. The “Nice” model postulates that changes in the orbits of the gas giants scattered objects in the asteroid belt, causing them to rain down in a short burst upon the inner planets and our moon. If it exists, this period is called Late Heavy Bombardment, and it’s been suggested that it might have contributed to the development of life on Earth; an abundance of icy comets may have transported water to our newly formed planet.

Chang’e-6 took several images of its leg and surroundings, which were stitched together to make this panorama. Credit: CNSA via Xinhua/Alamy

Difficulties with Dating Lunar Impact Sites

However, evidence has never conclusively favoured the lunar cataclysm scenario, and many scientists remain sceptical. One reason is that impact sites on the Moon’s surface are notoriously difficult to date. The Apollo missions collected rock samples from nearby basins –what we call the craters resulting from asteroid impacts– and assumed they originated from the basins they were closest to. But when an asteroid slams into the Moon it’s a messy affair. Fragments and debris from the impact can be scattered vast distances, making it hard to know whether a certain rock results from one impact or another. Many argue that the rock samples brought back from the Apollo missions might originate from the same event: one that formed a young impact crater called Imbrium.

In 2024, the Chang’e-6 lunar mission became the first to successfully retrieve rock samples from the dark side of the Moon. It landed in the Apollo basin within the South Pole-Aitken (SPA) basin: a colossal crater that’s one of the largest in the entire solar system and the oldest known on the Moon. This offered an unprecedented opportunity to collect specimens from an impact site other than those explored by the Apollo missions. An international team led by Professor Yi-Gang Xu, a renowned geochemist working within the Chinese Academy of Sciences, conducted a study of three of the samples which had intriguing features. What they found calls for a rethink of solar system history.

KREEPy Signatures

The team discovered that their rock samples contained KREEP elements, where K is the chemical symbol for potassium, REE stands for rare earth elements, and P is the chemical symbol for phosphorus. These elements are found in a thin layer between the Moon’s crust and mantle; they’re remnants from when the magma ocean of the young, molten Moon cooled and crystallised. KREEP signatures are usually detected on the Moon’s nearside, where more frequent volcanic activity spews these buried elements across the surface. But when asteroids gouge the Moon’s surface, KREEP elements are uncovered, remelted, and then recrystallised into scattered fragments. A possible scenario is that the giant impact leading to the SPA could have excavated the deeply buried KREEP layer to the surface, or produced a melt pool that is rich in KREEP. So, on the Moon’s farside, KREEP-bearing rocks can indicate an asteroid impact.

With the Chang’e-6 samples likely formed from an impact event, Prof Xu and his team began a detailed analysis of their composition, to ascertain exactly how and when they originated. Several features (including the fine-grained textures, the two-stage cooling rate, and the meteoritic Ni/Co ratios of Fe-Ni metal) supported an impact origin of the studied samples. Most interestingly, the team found that the crystalline structure contained two different crystals that had clearly cooled and solidified at different rates. This indicated that the sample had formed not from a single asteroid collision, but from two.

Dating Apollo and Overturning Lunar Cataclysm

Prof Xu and his team hypothesised that their samples had been shaped by two impact events, in line with the location of CE-6 landing site—in the Apollo basin, within the much bigger SPA (not to be confused with the Apollo landing sites which were on the Moon’s nearside). While the older event could have been responsible for the colossal SPA crater formed 4.33–4.25 billion years ago, the younger one could be related to the Apollo basin formation. It would make sense if whatever created the Apollo basin re-melted and re-solidified the pre-existing rock fragments formed by the earlier SPA event.

With the age of the SPA crater already known, the team used radiometric dating techniques to work out when the Apollo-forming event might have occurred. What they found was startling. Results showed that the impact rock fragments were formed at ~4.16 billion years ago, younger than the SPA basin – vindicating their successive impact event hypothesis – but significantly older than the proposed lunar cataclysm that was supposed to have occurred 3.9 billion years ago. In their paper to Nature Astronomy, Prof Xu and his colleagues argue that this sheds new light on our solar system’s history. Rather than the sudden relentless bombardment assumed by the lunar cataclysm hypothesis, they suggest that the Late Heavy Bombardment era consisted of a much more gradual decline in asteroid impacts over an extended period of time.

This doesn’t conclusively settle the debate, but it’s a major development standing firmly against the cataclysmic version of Late Heavy Bombardment theory. More evidence is needed. It’s hoped that if samples are collected from other key impact craters, such as Nectaris and Orientale, they will allow for a more complete picture of the Moon’s, and our solar system’s, history.