The Late Heavy Bombardment, and why it is disputed
What happened
Around 4 billion years ago the young Earth and Moon may have been hit by a spike of asteroid and comet impacts, an episode called the Late Heavy Bombardment. The idea came from Apollo Moon rocks: many impact-melted samples from different landing sites clustered near the same age of roughly 4 billion years, hinting at a single violent pulse. NASA is blunt that the idea 'was and remains fairly controversial.' Later work showed lunar basins are very hard to date and that the Apollo samples may all be contaminated by debris from one or two big impacts, so the apparent cluster could be an artifact of where the astronauts happened to land.
Why it matters
Whether the bombardment was a real spike bears directly on life's timeline. A late cataclysm could have sterilized or reset the surface just before the earliest signs of life; a gradual tapering of impacts would instead have left early life a calmer window to take hold.
How we know
The evidence is radiometric ages of impact-melt rocks from the Apollo missions. The dispute is a sampling problem: lunar meteorites from a wider spread of the Moon show impact ages more spread out in time, which weakens the case for a tight 4-billion-year spike. Researchers now argue variously that the bombardment was gradual, came in segments, or never happened as a distinct event.
Sources
- NASA (Moon: Lunar Craters). What is the Late Heavy Bombardment? · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Bottke & Norman. The Late Heavy Bombardment (2017) · Peer-reviewed (author-declared)doi.org · Cited as a "journal" source (no stronger domain match).
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Related timelines
- Big Bang to Now → · Just before the spine's 'Life begins on Earth'