What was the Late Heavy Bombardment?

FullMoon2010

The cratered face of the Moon. Image: Wikipedia

The term “Late Heavy Bombardment”, also known as the “Lunar Cataclism”, refers to the possibility that the inner solar system received an enhanced rate of bombardment early in its history. The period  in which this took place is considered to be between  4.1 and 3.8 billion years ago. The concept was originally formulated with reference to the heavily cratered terrain of the Moon’s near side. However, it has been subsequently extended to the inner solar system as a whole on the basis of the evidence provided by the  highly cratered surfaces of other bodies, such as Mercury and asteroid Vesta. The reason why this enhanced flux took place is poorly understood, but may be related to the early migration of the outer planets.