TheLate Heavy Bombardment (LHB) orlunar cataclysm happened about 4.1 to 3.8billion years ago. During this time, a large number of impact craters formed on theMoon. Byinference, the same thing happened onEarth,Mercury,Venus, andMars as well.[1]
The evidence for this event comes from thedating of lunarsamples. This shows that most impact melt rocks formed at that time. Manyhypotheses have been suggested to explain howasteroidal orcometary material got into the innersolar system, but no consensus yet exists.[2]
One popular theory is that thegas giant planets (or at leastJupiter) migrated inorbit at this time: seeGrand tack hypothesis. This caused objects in theasteroid belt and/orKuiper belt to be forced into orbits which reached theterrestrial planets.
Some think that the apparent clustering of impact melt ages near this time is an artifact of sampling.[3][4]
This topic is part of theformation and evolution of the Solar System.
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