Late Heavy Bombardment
Apollo moon rocks suggest a burst of large impacts on the inner planets around 3.9 billion years ago. Whether it was a true spike or the tail of a steady decline is debated.
The story
The Late Heavy Bombardment is the proposed time, around 3.9 billion years ago, when the inner solar system was hit by an unusually intense wave of large impacts. The idea comes from the Moon. Rocks from the Apollo missions between 1969 and 1972, which brought back about 382 kilograms, include rocks melted by impacts, and many of them seemed to date to between 3.8 and 4.0 billion years ago. In 1974 scientists proposed that this showed a 'lunar cataclysm', a spike in impacts that carved basins such as Imbrium and Orientale, more than half a billion years after the planets formed.
If real, the spike might have been triggered when the giant planets shifted orbits and flung asteroids and comets inward, an idea in the 'Nice model' proposed in 2005. It would also have battered the young Earth during the era when life began. But the evidence is contested. Most Apollo samples came from near the Imbrium basin and may be contaminated by its debris, giving a false cluster of dates. Lunar meteorites and other samples do not clearly show a spike, so some scientists see only a steady decline in impacts from earlier times. The question is unresolved.
Why it mattered
- It set limits on when life could survive on early Earth, if impacts repeatedly sterilized the surface.
- It inspired models of how the giant planets' orbits changed in the early solar system.
- It showed how rocks from one body, the Moon, can be used to date events on another.
Contested history
Consensus: Apollo samples cluster around 3.9 billion years, which some read as a burst of impacts.
Community notes
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