Late Heavy Bombardment

The Late Heavy Bombardment is a hypothesized period roughly 4.1 to 3.8 billion years ago during which the inner Solar System, including Earth, the Moon and Mars, experienced a markedly elevated rate of asteroid and comet impacts compared to both earlier and later periods. The main evidence comes from the ages of impact melt rocks collected from the Moon during the Apollo missions, which cluster suspiciously close together in time rather than declining smoothly from the era of planet formation. The reality, timing and even the existence of a distinct spike remain debated among planetary scientists, with some proposing instead a more gradual decline that only appears spike-like because of sampling bias in the lunar samples available.