The Late Heavy Bombardment was a time about 500 million years after the planets formed (planets formed approximately 4.5 billion years ago; the Late Heavy Bombardment took place 4.0 to 3.9 billion years before present) when many asteroids and comets smashed into the Earth and moon. Why was there a spike in collisions between planetesimals and planets so long after the formation of planets and the solar system? Possibly, the large planets suddenly changed orbits, and this caused a gravitational disturbance for all the rocks and asteroids and comets out beyond the large planets, and as the orbits were disturbed, many of the planetesimals came zooming in to smash against the surface of Mars, the Moon, and the Earth. There are other theories to explain the LHB, such as the idea that Neptune and Uranus were slow to form, and when they finally became established planets, this caused the disturbances in orbits of asteroids and comets that created the LHB.

We know there was a late heavy bombardment mainly from samples of rocks taken from the moon. The rocks that seem to have been formed from impact melting are all about 3.8 to 4.1 billion years old. Other evidence for the Late Heavy Bombardment includes the impact craters in the Caloris Basin on Mercury and the huge craters in the southern hemisphere of Mars. There are also extraterrestrial tungsten isotopes in 3.7 billion year old rocks.

Also, the fact that so few rocks or minerals on earth are older than 4.1 billion years in age may indicate that the LHB pretty much destroyed the existing crust and rocks on Earth. In fact, we might think of the Hadean Era as concluding with the Late Heavy Bombardment when the surface was pretty much destroyed and re-formed. The LHB was a cataclysmic finale to a period from the formation of the earth (4.7-4.5 billion years ago) and its cooling (the molten crust probably cooled and solidified by 4.4 billion years ago), through the formation of oceans and the origins of life (before the LHB). After the LHB we have the beginning of the Isuan Era, and then the Swazian Era and the Randian Eras to finish the Archean Eon around 2.5 billion years ago. During these eras the planet experienecd the formation (or re-formation) of continental crust, continental drift, and the time of prokaryotes (archea, bacteria, and primitive intermediate forms of prokaryotes from which bacteria and archea evolved).

Some of the impact craters formed during the LHB would have been the size of continents. These massive collisions would have occurred frequently, perhaps every century, for 200 million years during the peak of the LHB. Many of the impacts would have been from icy comets, so the LHB probably added a significant amount of water to the earth’s surface. If so, the oceans after the LHB would have been larger than the oceans that condensed earlier in the Hadean Era.

1. David Darling's Late Heavy Bombardment description in his Encyclopedia of Science.
2. The BBC has a little video you can watch about the LHB.
3. Nature geoscience articles with references to the late heavy bombardment. Check out this article about orbit migrations of giant planets as the likely cause.
4. NASA has a page about the late heavy bombardment.

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This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.