The History of the Moon!

The Moon may seem like a lifeless and unchanging place, but it actually has a fascinating history. The driving factor behind the lunar history is the fact that because the moon is small, it cooled quickly and formed a thick, rigid crust.

The Moon solidified about 4.1 to 4.6 billion years ago. Though there is little iron in the lunar composition, what there was sunk to the core and solidified. The lower density material floated to the top and formed the crust.

The next stage in the moon's history involved extensive cratering. Cratering in the highlands shows that impacts were at a peak during the moon's first 0.5 billion years. At this time, there was a great deal of debris floating around in the solar system that was left over from planet formation.

Giant crater basins were formed by the largest impacts, some of the craters being hundreds of kilometers in diameter. The Imbrium basin was formed by an impact from a body the size of the state of Delaware. The impact threw ejecta 1400 km away from the site. This site is about 4 billion years old.

The third stage in lunar history is lava floods. By this time, the anorthosite crust had been cracked many times from impacts. The subsurface material had been heated, probably by radioactive decay in the rocks. Lava then flowed up through the cracks and flooded the basins with successive lava flows. Flooding ceased about 3.2 billion years ago.