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An Archaean heavy bombardment from a destabilized extension of the asteroid belt


The barrage of comets and asteroids that produced many young lunar basins (craters over 300 kilometres in diameter) has frequently been called the Late Heavy Bombardment1 (LHB). Many assume the LHB ended about 3.7 to 3.8 billion years (Gyr) ago with the formation of Orientale basin2,3. Evidence for LHB-sized blasts on Earth, however, extend into the Archaean and early Proterozoic eons, in the form of impact spherule beds: globally distributed ejecta layers created by Chicxulub-sized or larger cratering events4. At least seven spherule beds have been found that formed between 3.23 and 3.47 Gyr ago, four between 2.49 and 2.63 Gyr ago, and one between 1.7 and 2.1 Gyr ago5,6,7,8,9. Here we report that the LHB lasted much longer than previously thought, with most late impactors coming from the E belt, an extended and now largely extinct portion of the asteroid belt between 1.7 and 2.1 astronomical units from Earth. This region was destabilized by late giant planet migration10,11,12,13. E-belt survivors now make up the high-inclination Hungaria asteroids14,15. Scaling from the observed Hungaria asteroids, we find that E-belt projectiles made about ten lunar basins between 3.7 and 4.1 Gyr ago. They also produced about 15 terrestrial basins between 2.5 and 3.7 Gyr ago, as well as around 70 and four Chicxulub-sized or larger craters on the Earth and Moon, respectively, between 1.7 and 3.7 Gyr ago. These rates reproduce impact spherule bed and lunar crater constraints.

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Figure 1: Snapshots of the evolution of the E-belt population over time.
Figure 2: Decay curves for our E-belt runs before and after the LHB.
Figure 3: The E-belt impactor flux on the Earth and Moon.


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We thank B. Cohen, C. Chapman, M. Ćuk, L. Dones, D. Kring, S. Marchi, R. Malhotra, M. Norman, E. Scott, J. Taylor and K. Walsh for discussions and comments. We also thank the University of Hawaii for sponsoring W.F.B.’s recent visit, during which these ideas were first developed. This project was supported by NASA’s Lunar Science Institute (Center for Lunar Origin and Evolution, grant number NNA09DB32A). D.V.’s contribution was supported by the Grant Agency of the Czech Republic. A.M. and R.B. thank Germany’s Helmholtz Alliance for providing support through their “Planetary Evolution and Life” programme. B.S.’s contribution was supported by NASA grant NNX08AI29G. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center.

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W.F.B. developed the scenario of the E belt. The numerical runs were constructed by W.F.B. and D.V., with input from D.M., D.N. and H.F.L. Information on the initial conditions of the planets and the nature of late giant planet migration within the Nice model was provided by A.M., R.B., H.F.L. and D.N. Information on the nature of sweeping resonances, how they affected the main belt, and what happened afterwards was provided by D.M., A.M. and R.B. Additional numerical runs not shown here were performed by A.M. and R.B. Information on the nature of impact spherules and their context was provided by B.S. All authors participated in numerous discussions.

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Correspondence to William F. Bottke.

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Bottke, W., Vokrouhlický, D., Minton, D. et al. An Archaean heavy bombardment from a destabilized extension of the asteroid belt. Nature 485, 78–81 (2012).

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