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The Late Heavy Bombardment (LHB) was an intense period of asteroid and comet impacts ~3.8–4.1 billion years ago that dramatically reshaped the inner solar system. The Nice model explains this event as a consequence of dynamical instability in the giant planet configuration, driving rapid migration and scattering of the primordial planetesimal disk. The LHB had profound effects on planetary surfaces, delivering water and organic compounds to terrestrial worlds.
After the initial burst of planetary formation wound down roughly 4.5 billion years ago, the solar system did not settle into a calm state immediately. The evidence — primarily from lunar samples returned by the Apollo missions — points to a dramatic spike in impact rates about 3.8 to 4.1 billion years ago, several hundred million years after the planets had already assembled. This delayed cataclysm is what planetary scientists call the Late Heavy Bombardment (LHB). The puzzle is straightforward: if the leftover debris from planetary formation should have been swept up quickly, what caused this second wave of devastating impacts long after the main construction phase ended?
The leading explanation comes from the Nice model, named after the French city where it was developed. Building on the planetary migration mechanisms you already know, the Nice model proposes that the four giant planets initially orbited in a much more compact configuration than they do today. For hundreds of millions of years, gravitational interactions with a dense outer disk of planetesimals gradually nudged the giant planets' orbits. Eventually, Jupiter and Saturn crossed a critical orbital resonance — likely the 2:1 mean-motion resonance, where Saturn's orbital period was exactly twice Jupiter's. This resonance crossing destabilized the entire outer solar system in a geologically brief episode. Uranus and Neptune were flung outward into the planetesimal disk, scattering trillions of icy bodies inward toward the terrestrial planets.
The consequences were catastrophic for the inner solar system. The Moon's heavily cratered highlands preserve a record of this bombardment — enormous impact basins like Imbrium and Orientale date to this narrow time window. Earth, with its larger gravitational cross-section, would have been hit even more intensely, though plate tectonics and erosion have erased most of the direct evidence. Mars and Mercury also bear scars consistent with this timing. The impactors were a mix of rocky asteroids, scattered inward as Jupiter's migration disrupted the asteroid belt, and icy comets from the outer disk that Neptune plowed through.
Beyond the destruction, the LHB may have played a constructive role in planetary evolution. Cometary and asteroidal impacts delivered significant quantities of volatile compounds — water, carbon dioxide, nitrogen, and organic molecules — to the inner planets. Some models suggest that a substantial fraction of Earth's ocean water arrived during this period. The bombardment also would have repeatedly sterilized surface environments, meaning that any life existing before the LHB would have faced severe bottlenecks. The end of the bombardment around 3.8 billion years ago roughly coincides with the earliest isotopic evidence for life on Earth, suggesting that persistent biology could only take hold once impact rates dropped to manageable levels.