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Late Heavy Bombardment
c. 4.1–3.8 billion years ago (debated)· Event· Earth, climate & extinctions
DebatedScientists disagree about when, how or why.
A proposed spike of asteroid and comet impacts that pummelled Earth and the Moon about 4.1–3.8 billion years ago.
See it on the Life map →What happened
Apollo moon rocks melted by impacts gave ages clustering near 3.9 billion years, suggesting a late 'cataclysm' rather than a smooth decline. A popular explanation, the 2005 Nice model, has the giant planets shifting orbits and scattering asteroids inward; the Curious Science map covers this. Big impacts could boil off ocean surfaces and sterilize land. But newer analyses suggest the cluster may be a sampling quirk: many Apollo rocks came from one huge basin's debris.
Why it matters
If a spike happened, any early life faced repeated near-sterilization, which affects when life could have begun and whether it had to start over.
The evidence (fossils, genes, rocks)
Ages of impact-melted rock from Apollo missions and lunar meteorites; crater counts on the Moon, Mercury and Mars; computer models of planet orbits. Earth has no surviving craters this old.
Still debated
Was it a true spike, a long declining tail of impacts, or a mix? Many researchers now lean toward the tail.
Connections (2)
▸Late Heavy Bombardment may have delayed or disrupted Origin of lifeDebated
The connection
Large impacts could have boiled the top of the oceans and sterilized the surface, perhaps wiping out any early life. But impacts also delivered organic molecules and created hot, mineral-rich crater systems that some see as good places for life to start. Models suggest microbes underground or deep at sea could have survived.
When and how
About 4.1–3.8 billion years ago, if the spike happened; survival models by Oleg Abramov and Stephen Mojzsis at the University of Colorado, 2009.
How sure are we?
Mostly modeling. Crater counts and moon rock ages show heavy bombardment, but there is no direct record of life before or during it.
Debates and open questions
Whether the bombardment was a short spike or a long, declining tail, and whether life began once or was wiped out and restarted.
▸Late Heavy Bombardment may have topped up Water & the first oceansDebated
The connection
Comets and water-rich asteroids striking Earth would have added some water. But oceans already existed by about 4.4 billion years ago, and seawater's hydrogen 'fingerprint' matches certain asteroids better than most comets. So late impacts probably added only a small share, and the biggest may have partly boiled the seas.
When and how
About 4.1–3.8 billion years ago; the key comet test came from the European Rosetta probe at comet 67P in 2014.
How sure are we?
Ratios of heavy hydrogen (deuterium) in seawater, meteorites and comets; Rosetta found comet 67P's water about three times richer in deuterium than Earth's.
Debates and open questions
What share of Earth's water arrived late versus with its original building blocks; some recent studies argue Earth's own rock held most of it.