Late Heavy Bombardment

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Period between ~4.0 to 3.8 Ga ago when the Moon and other objects in the Solar System were pounded heavily by wayward asteroids. The evidence for the Late Heavy Bombardment (LHB) includes the lunar maria basins and similar structures elsewhere, such as the Caloris Basin on Mercury and the great craters in the southern hemisphere of Mars. On Earth, LHB would have produced 22,000 or more impact craters with diameters >20 km, ~40 impact basins with diameters ~1000 km, and several impact basins with diameters ~5,000 km with a serious environmental damage event occurring about every 100 years. However, plate tectonics and erosion have erased the evidence from Earth’s surface. Conclusive evidence that Earth experienced LHB was not discovered until 2002, when British and Australian researchers announced they had found Tungsten (W) isotopes in 3.7 Ga rocks from Greenland and Canada that can only be extraterrestrial. Recent computer models also suggest that resonances and perturbations caused by the four large outer planets settling into their current orbital configurations would cascade large volumes of asteroidal material into the inner solar system.

Some or all content above used with permission from J. H. Wittke.