The geologic history of Earth's Moon has been divided into five broad time periods, or systems. From oldest to youngest, these are pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican. These systems are defined by the number of impact craters on the ground surface (more craters = older surface) correlated to the absolute ages determined for the lunar rock samples returned to Earth by the U. S. Apollo missions and unpiloted Soviet Luna missions. The pre-Nectarian system extends from 4. 5 billion to 3. 9 billion years ago.