Extraterrestrial craters and lunar samples suggest that the inner Solar System was pummelled by asteroids about 3.9 billion years ago, but the scarcity of contemporaneous rocks on Earth has limited our study of the terrestrial effects of the so-called Late Heavy Bombardment. An analysis of ancient zircons that were incorporated and preserved in younger sedimentary rocks suggests an intense thermal event on Earth at this time.
Elizabeth Bell and Mark Harrison from the University of California, Los Angeles, analysed the geochemistry of 4–3.6 billion-year-old detrital zircons from Western Australia. They found that the zircons from approximately 3.9 billion years ago were unique in showing geochemical characteristics that could be explained by recrystallization following exposure to extremely high temperatures.
The Late Heavy Bombardment is thought to have caused widespread heating of the Earth's ancient crust and provides a plausible — although not the only — mechanism to explain the evidence of thermal metamorphism preserved in the 3.9 billion-year-old zircons.
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Goldin, T. Earth bombarded. Nature Geosci 6, 158 (2013). https://doi.org/10.1038/ngeo1760
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DOI: https://doi.org/10.1038/ngeo1760
