Asteroid shower on the Earth-Moon system immediately before the Cryogenian period revealed by KAGUYA

Understanding meteoroid bombardment of the Earth system is crucial due to its potential hazard. Mass extinction events, such as the Big Five, have occurred since the Cambrian explosion (541 Ma), with extraterrestrial impacts considered a potential cause for some, alongside flood basalt eruptions. Evidence from fossil L-chondrites, chromite grains, and iridium enrichment in rocks dating back 470–480 million years (Ma) points to a major impact event around that time, disrupting the L-chondrite parent body and leading to a significant meteoroid shower. This shower is believed to have cooled the Earth, triggering Ordovician icehouse conditions, sea-level fall, and faunal turnovers. However, studying ancient impacts on Earth is challenging due to erosion and resurfacing. The Moon, with less weathering, provides a valuable alternative for investigating ancient impact events. The lunar orbiter Kaguya's data reveals a disruption of an asteroid that formed multiple craters larger than 20 km simultaneously on the Moon approximately 800 Ma ago. The study uses this data, along with crater scaling laws and collision probabilities, to estimate the scale of the meteoroid shower that impacted the Earth-Moon system.

Previous research highlights the link between extraterrestrial impacts and mass extinction events. The Late Triassic and Cretaceous-Paleogene extinctions are prime examples where impact events are considered potential triggers, although flood basalt eruptions also play a significant role. The discovery of fossil L-chondrites and other indicators in Ordovician limestones suggests a major asteroid disruption event around 470 Ma, leading to a prolonged meteoroid shower impacting Earth. This event is linked to the Great Ordovician Biodiversification Event and associated environmental changes such as glaciation. Studies of lunar crater records offer a less eroded record of impact events, providing a valuable complement to terrestrial studies. Previous lunar impact studies have established methods for deriving the relative and absolute ages of planetary surfaces using crater size-frequency distributions. These methods, coupled with radiometric dating of lunar samples, offer a way to better understand the history of asteroid impacts on the Moon and indirectly, Earth.

The study focuses on 59 lunar craters with fresh morphologies and diameters exceeding 20 km. The researchers utilized data from the Kaguya mission's Terrain Camera (TC) to measure the crater size-frequency distributions (CSFDs) in the ejecta blankets of these craters. Crater counting was performed on the ejecta, avoiding impact melt regions to minimize bias. The formation ages of individual craters were initially estimated using the conventional constant flux model over 3 billion years. A key finding was the simultaneous formation of eight craters around 660 Ma. To evaluate the significance of this clustering, a Monte Carlo simulation was performed, demonstrating that the probability of such a concentration occurring by chance is extremely low (0.69%). The masses of the impactors were estimated using a formula derived from previous research, considering the density of the impactor (C-type, S-type, and Eros-type asteroids were considered), the density of the lunar crust, the impact velocity, and crater diameter. The initial analysis suggested an age around 660 Ma, but this was reconciled with the 800 Ma age derived from radiometric dating of samples from the Copernicus crater ejecta and lunar impact spherules from Apollo missions. A new model that incorporates a spike in impact flux at ~800 Ma is proposed to account for this. This model maintains a constant flux before and after the 30 Ma spike (830 Ma to 800 Ma), reflecting the break-up age of the Eulalia asteroid family. This model was used to re-evaluate the crater ages, confirming the concentration of impact events around 800 Ma. The collision probability ratio of 23:1 between Earth and the Moon was used to extrapolate the impactor masses from lunar data to estimate the total mass of meteoroids that impacted the Earth at ~800 Ma. The software tool craterstats was used for analyzing crater size distributions and error calculations.

The analysis revealed a statistically significant concentration of eight lunar crater formations at approximately 660 Ma, with a weighted mean age of 658 ± 16 Ma. This clustering, based on a conventional constant flux model, has an extremely low probability of occurring randomly (0.69%). Considering the previously established radiometric ages (around 800 Ma) of ejecta from Copernicus crater and impact glass spherules from Apollo samples, the researchers proposed a modified model with a spike in impact flux at ~800 Ma. This 800 Ma spike model better accounts for the radiometric ages and the observed crater age distribution. Using the estimated masses of the impactors for the eight simultaneously formed craters (around 1.3-1.6 × 10^15 kg), and considering the Earth-Moon collision probability ratio, the study estimates that at least (4–5) × 10^16 kg of meteoroids impacted the Earth around 800 Ma. This is approximately 30–60 times the mass of the Chicxulub impactor. The absence of significant geological or geochemical evidence of such a massive impact event on Earth is attributed to the subsequent large-scale Neoproterozoic glaciations, which may have erased much of the evidence. The study also notes the potential influence of this asteroid shower on Earth's climate, biogeochemical cycles, and the emergence of animals. The Eulalia family is identified as a possible source of the asteroid shower.

The findings suggest a significant asteroid shower impacted the Earth-Moon system immediately before the Cryogenian period (720–635 Ma). The magnitude of this event, considerably larger than the Chicxulub impact, highlights its potential influence on the environment and biological evolution. The lack of direct geological evidence on Earth is explained by the subsequent Neoproterozoic glaciations that likely erased much of the impact-related geological record. The study proposes that the impacts could have affected the global phosphorus cycle, marine redox states, and climate system, contributing to the emergence of animals. The identified potential source of this shower, the Eulalia family, a carbonaceous chondrite family, supports the idea of volatile-rich impactors supplying carbon and water to the Moon, adding to the understanding of lunar volatile history. Future research should focus on better understanding the interaction between asteroid impacts, glaciation events, and environmental change during the Neoproterozoic.

This study presents compelling evidence for a large-scale asteroid shower impacting the Earth-Moon system around 800 Ma, just prior to the Cryogenian period. The findings highlight the significance of such events in shaping Earth's environment and influencing the course of biological evolution. Further research should explore the detailed geochemical and geological consequences of this event, focusing on the potential link between the impact and environmental changes during the Cryogenian. Refining the models used to estimate impactor mass and integrating diverse geological and geochemical data is crucial for a more complete understanding of this significant event.

The study relies on indirect evidence from lunar crater analysis and extrapolation to Earth. Direct geological evidence of the impact on Earth remains scarce due to subsequent glaciation events. The estimated mass of the impactor depends on assumptions about the density and velocity of the impactors, which may introduce uncertainty. The models used for crater age determination rely on several assumptions regarding the constant flux of impacts and the duration of the spike. These factors might introduce limitations on the precision of the age estimates.