Scientists have often wondered how a planet such as the Earth accumulated the amount of water it currently has. Around 70% of the Earth's surface is covered by water and yet there is no obvious explanation as to where it comes from. This question can be asked of other planets in our solar system that appear to have minerals and substances that couldn't have been made there. There is however a theory currently thought of as being correct amongst the scientific community, the Late Heavy Bombardment (LHB).

The LHB is thought to have occured somewhere between 3.8 and 4.1 billion years ago, shortly after the rocky planets had been formed in the Solar System. During this period a large number of asteroids entered the inner Solar System colliding with all the rocky planets, forming huge craters and depositing foreign material such as water on Earth.

One of the main areas of interest for the lunar landings was to collect impact material to bring back to Earth in order that isotopic aging could take place. Apollo 15, 16 and 17 all brough back samples dating to the postulated time of 3.8-4.1 billion years, suggesting the theory was correct. Some scientists however do not believe this evidence as all the sample could have come from a single large collision with the moon which spread out rock of a similar age. Studies in Earth of impact melts also suggest that the Earth was subjected to an intense period of asteroid impacts around the same time. The melts, found in the mid-70's by Fouad Tera, Dimitri Papanastassiou, and Gerald Wasserburg were part of what they named the 'Lunar Cataclysm' proposing these samples were from the same time as the collisions on the moon.

At the time the theory was considered contraversial as little other evidence could be found, however as more data became available the idea gained momentum. Since then many more meteorites have been surveyed giving a much better approximation of the overall composition of the Moon's surface and as yet no meteorites have been found with an age more than 3.9 billion years.

With this evidence of a LHB the next question is why it happened, during this period of the Solar System most of the inner planets had been formed and the majority of asteroids were in stable orbits. There are many theories as to why a Late Heavy Bombardment occured and each have credible arguments.

The first possible cause is as a result of giant planet migration. This theory suggests that early on in the lifetime of the Solar System the gas giants were much closer to the sun than they are now, however over a large period of time they were knocked out of thier stable orbits by other planets or large asteroids. This forced them into a more chaotic orbit and therefore disrupted the orbits of other asteroids, sending them into the inner Solar System, creating the LHB.