Late Heavy Bombardment

The Late Heavy Bombardment (commonly referred to as the lunar cataclysm, or LHB) is a period of time approximately 4.1 to 3.8 billion years ago (Ga) during which a large number of impact craters were formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well. The LHB is "late" only in relation to the main period of accretion, when the Earth and the other three rocky planets first formed and gained most of their mass; in relation to Earth or Solar System history as a whole, it is still a fairly early phase. The evidence for this event comes primarily from the dating of lunar samples, which indicates that most impact melt rocks formed in this rather narrow interval of time. While many hypotheses have been put forth to explain a spike in the flux of either asteroidal or cometary materials in the inner Solar System, no consensus yet exists as to its cause. The Nice model, popular among planetary scientists, postulates that the gas giant planets underwent orbital migration at this time, scattering objects in the asteroid belt and/or Kuiper belt on eccentric orbits that crossed those of the terrestrial planets. Nevertheless, some researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact melt ages near this time is an artifact of sampling material affected by a single large impact basin.