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      Impact spherules as a record of an ancient heavy bombardment of Earth.

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          Abstract

          Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes. Earth’s impact history is inferred therefore either from estimates of the present-day impactor flux as determined by observations of near-Earth asteroids, or from the Moon’s incomplete impact chronology. Asteroids hitting Earth typically vaporize a mass of target rock comparable to the projectile’s mass. As this vapour expands in a large plume or fireball, it cools and condenses into molten droplets called spherules. For asteroids larger than about ten kilometres in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found. Here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers. The impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3.5 billion years ago than it is now. This conclusion is consistent with a gradual decline of the impactor flux after the Late Heavy Bombardment.

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          Author and article information

          Journal
          Nature
          Nature
          1476-4687
          0028-0836
          May 3 2012
          : 485
          : 7396
          Affiliations
          [1 ] Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, Indiana 47907, USA. johns477@purdue.edu
          Article
          nature10982
          10.1038/nature10982
          22535246

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