Onset of a Heavy Bombardment 1.4Ga ago Triggered by the Flora Asteroid Family and Its Implications for Earth–Moon Evolution

The collisional evolution of asteroids in the main belt between Mars and Jupiter has governed the flux of impactors delivered to the Earth–Moon system throughout much of Solar System history. The impact flux over the last ~3 billion years in the inner Solar System has generally been considered approximately constant [1-3], with crater chronologies on the Moon and terrestrial planets failing to reveal any statistically robust long-term variations from steady-state cratering rates [4, 5]. However, the incompleteness of crater catalogs, particularly for small impact structures, has limited our ability to detect temporal variations in bombardment history. Here we combine machine-learning-based crater detection [6] with lunar crater chronology analyses and dynamical modeling [Broz] to identify a previously unrecognized episode of enhanced bombardment that affected the Earth–Moon system during the Proterozoic.

Using a convolutional neural network trained to identify impact structures on high-resolution Kaguya/SELENE Terrain Camera imagery, we detect and characterize more 250 million small lunar craters across the Moon’s surface. This large and internally consistent crater inventory allows us to refine crater production statistics and estimate model ages for 211 lunar craters with diameters ≥20 km. Our analysis reveals a statistically significant and long-lived increase in lunar cratering between approximately 1.4 and 0.9 Ga. During this interval, the production rate of 20–40 km lunar craters increased by a factor of 4.0 ± 0.8 relative to the background flux of impactors. The enhancement persisted for several hundred million years, distinguishing it from transient spike-like bombardment episodes previously proposed in planetary chronology studies.

To determine the origin of this bombardment episode, we performed dynamical simulations tracing the orbital evolution and delivery efficiency of asteroid fragments from major main-belt families. The simulations reconstruct the temporal evolution of both large and small impactor populations reaching near-Earth space and indicate that the Flora asteroid family was the unique contributor to the enhanced Proterozoic flux. The Flora family occupies a dynamically favorable region of the inner asteroid belt adjacent to resonances capable of efficiently transporting large fragments into Earth-crossing orbits. Our results show that a major collisional disruption within the Flora parent population [7, 8] could generate a sustained increase in terrestrial and lunar impact rates lasting several hundred million years.

Independent support for the timing of this breakup event is provided by meteoritic and sample-return evidence: shock-reset ages measured in LL chondrite meteorites cluster within the inferred bombardment interval, impact-reset grains returned from asteroid Itokawa show an age distribution consistent with a major collisional event occurring near the onset of the enhanced cratering episode [9-11]. Together, these constraints strengthen the interpretation that the Flora family breakup initiated a prolonged delivery of debris into the inner Solar System.

Scaling the reconstructed lunar impact flux to the Earth suggests that our planet experienced a substantial increase in large impacts during this Proterozoic interval. We estimate that Earth was struck by approximately 1,900 impactors larger than 1 km in diameter between ~1.4 and 0.9 Ga, corresponding to an average frequency of roughly one such impact every 255 kyr. This impact frequency is close to biological recovery timescales inferred for the aftermath of the Chicxulub impact event, which caused the end-Cretaceous mass extinction and required on the order of 200–300 kyr for ecological recovery [12]. Although most Proterozoic impacts would not individually have caused global sterilization, repeated large impacts over hundreds of millions of years could have imposed persistent environmental stress through atmospheric perturbations, oceanic disruption, nutrient cycling, and climate forcing [13,14].

The termination of this bombardment episode broadly coincides with the emergence and diversification of multiple eukaryotic clades during the late Proterozoic. This temporal relationship raises the possibility that asteroid-driven environmental disturbances influenced evolutionary trajectories on Earth, either by periodically suppressing ecological complexity or by generating selective pressures. While a direct causal link remains speculative, our findings suggest that impact flux variations should be considered as a potentially important environmental factor in models of Earth’s biological and geochemical evolution.

More broadly, this work demonstrates that planetary bombardment histories may preserve signatures of discrete asteroid-family breakup events long after the early Solar System. The combination of machine-learning crater detection, improved lunar chronology, and dynamical modeling opens a new avenue for reconstructing the temporal evolution of impact fluxes with respect to impactor sizes in the inner Solar System. Rather than a monotonic decline punctuated only by the earliest heavy bombardment phases, the Earth–Moon impact record may contain multiple prolonged episodes of enhanced cratering tied to the collisional evolution of specific asteroid families. These results therefore have implications not only for planetary surface dating and Solar System dynamics, but also for understanding the environmental context in which life evolved on Earth.

[1] Morbidelli, A., et al. In: Asteroids III, pp. 409–422 (2002). [2] Marsset, M., et al. Nature 634(8034), 561–565 (2024). [3] Brož, M., et al. Nature 634(8034), 566–571 (2024). [4] Neukum, G., et al.  Space Science Reviews 96(1), 55–86 (2001). [5] Lagain, A., et al. Earth and Planetary Science Letters 579, 117362 (2022). [6] Fairweather, J.H., et al. Earth and Space Science 10(7), 2023–002865 (2023). [7] Vernazza, P., et al. Nature 454(7206), 858–860 (2008). [8] Vokrouhlický, D., et al. The Astronomical Journal 153(4), 172 (2017). [9] Swindle, T.D., et al. Geological Society of London Special Publications 378(1), 333–347 (2014). [10] Park, J., et al. Meteor. Planet. Sci. 50(12), 2087–2098 (2015). [11] Terada, K., et al.  Scientific Reports 8(1), 11806 (2018). [12] Lowery, C.M., et al. Nature 558(7709), 288–291 (2018). [13] Anderson, R.P., et al. Royal Society Open Science 11(8) (2024). [14] Brocks, J.J., et al. Nature 618(7966), 767–773 (2023).