ABSTRACT
The Earth has a common feature with all the planets and satellites in the Solar System, in that it has endured extensive meteoritic and cometary bombardment. In the first half of the 20th century this ‘Earth bombardment’ was either given relatively little attention or relegated to the period prior to about 3800 Ma. However, in the second half of the 20th century, there has been a growing recognition of the importance of this process to more recent geological evolution. The circular features of the Moon have interested astronomers and philosophers for centuries. Smaller circular features on the Moon, it was initially thought, could be equated to volcanoes or calderas on Earth. The dark and apparently level areas of the Moon were likened to seas and oceans on Earth, and termed Maria. For a long period, the larger circular features were a complete mystery. However, it was eventually recognised that major circular features on the Moon could be attributed to impacts.
