Did a cataclysm of asteroids batter the Moon 3.9 billion years ago?

Maybe, and scientists disagree. The Late Heavy Bombardment, or lunar cataclysm, is a hypothesized event about 4.1 to 3.8 billion years ago, when an unusually large number of asteroids and comets hit the inner planets and the Moon. Evidence comes from the Apollo samples: isotopic dating showed that the rocks were last molten in impacts within a narrow interval of time, suggesting that many craters formed then. The idea was once widely accepted but is now debated, and definitive evidence either way remains elusive.

The case is a lesson in how samples can mislead. Doubters argue that the clustering of impact-melt ages is a statistical artifact produced by sampling rocks scattered from a single large impact, and that bombardment may instead have lasted from about 4.2 to 3.5 billion years ago. The huge Imbrium basin's ejecta should be present at all Apollo landing sites, so the age cluster may reflect one impact rather than several.

Craters help, but with care. Because craters accumulate at a nearly constant rate, counting craters per unit area can estimate a surface's age. But secondary craters can resemble primary ones and even dominate small crater populations, so unidentified ones can distort age estimates.

If a cataclysm did happen, what caused it? The Nice model, popular among planetary scientists, says the giant planets migrated and scattered objects into the paths of the inner planets. Several hypotheses explain the spike, but no consensus exists. So the best summary is honest: a dramatic story, still a hypothesis.