Late Heavy Bombardment The Late Heavy Bombardment (LHB) or lunar
cataclysm, is a period of time about 4.1 to 3.8 billion years
ago. During this time, a large number of impact craters formed
on the Moon. By inference the same thing happened on Earth,
Mercury, Venus, and Mars as well. The evidence for this event
comes from the dating of lunar samples, which indicates that
most impact melt rocks formed at that time. Many hypotheses have
been suggested to explain how asteroidal or cometary material
got into the inner solar system, but no consensus yet exists.
One popular theory is that the gas giant planets migrated in
orbit at this time, causing objects in the asteroid belt and/or
Kuiper belt to be forced into orbits which reached the
terrestrial planets.