The Late Heavy Bombardment

Heavily cratered surfaces on the Moon, Mars, and Mercury show that the terrestrial planets were battered by an intense bombardment during their first billion years or more, but the timing, sources, and dynamical implications of these impacts are controversial. The Late Heavy Bombardment refers to impact events that occurred after stabilization of the planetary lithospheres such that they could be preserved as craters and basins. Lunar melt rocks and meteorite shock ages point toward a discrete episode of elevated impact flux between ∼3.5 and ∼4.0-4.2 Ga, and a relative quiescence between ∼4.0-4.2 and ∼4.4 Ga. Evidence from Precambrian impact spherule layers suggests that a long-lived tail of terrestrial impactors lasted to ∼2.0-2.5 Ga. Dynamical models that include populations residual from primary accretion and destabilized by giant planet migration can potentially account for the available observations, although all have pros and cons. The most parsimonious solution to match constraints is a hybrid model with discrete early, post-accretion and later, planetary instability-driven populations of impactors.