Apollo 17 mission at the Taurus-Littrow landing site in this photograph from Dec. 13, 1972. Rock samples collected during the Apollo missions provided evidence that the Moon resulted from an object crashing into Earth in the early history of the solar system. Credit: NASA/Eugene Cernan/Universal Images Group via Getty ImagesFrom Earth, our inner solar system seems to work like clockwork. Mercury, Venus, our Earth-Moon system, and Mars all orbit the Sun in very orderly fashion.
Most early bombardment of the Earth, Moon, and inner solar system planets came from planetesimals that were left over from terrestrial planet formation, planetary scientist William Bottke, co-author of a comprehensive recent study of early planetary bombardment published in the journal, told me. The bombardment declined as time proceeded, with a long tail responsible for large relatively late impacts on the Earth and Moon, Bottke, a researcher at the Southwest Research Institute, told me.
For this model to work, some process must have eliminated the earliest history of the Moon, says Bottke. That means the craters and basins there either do not go back to the Moon's origin, or the Moon formed later than many argue, he says. The leftover planetesimals produced most lunar impacts within the first 1.1 billion years of our solar system, with roughly 50 known lunar basins formed between 4.36 to 4.41 billion years ago, the authors note.