Figure 5-8 RIVUXG Our Moon This photograph, taken by astronauts in 1972, shows thousands of craters produced by impacts of leftover rocky debris from the formation of the solar system. Age-dating of lunar rocks brought back by the astronauts indicates that the Moon is about 4.5 billion years old. Most of the lunar craters were formed during the Moon’s first 700 million years of existence, when the rate of bombardment was much greater than it is now. The large dark regions are the maria.