The moon is thought to have formed about 4.5 billion years ago, from the debris of a collision between Earth and a Mars-sized body. Its pockmarked surface records a lifetime of impacts, the rate of which, researchers agree, has fallen over time.
The first complete topographic map of the moon and its craters has revealed details of billions of years of bombardment by asteroids, and the early history of our solar system. Among other things, the map confirms theories of an onslaught of massive asteroids around 3.9 billion years ago that likely evaporated any water present on Earth at the time.
The accurate map of the moon was also used to confirm that the oldest regions on the moon are the southern near side and the north-central far side. The moon was volcanic for about half of its history, until it cooled to the point where the volcanism shut down around 2 billion years ago. Read more here or here.