Noachian

Noachian is a geological period and system of the Moon. It is the earliest of the four periods of the Lunar geologic timescale, following the pre-Nectarian and preceding the Eratosthenian. The Noachian epoch is defined as the time during which the youngest large impact basins formed. Its beginning is marked by the formation of the Mare Imbrium basin, and its end is defined by the formation of the Oceanus Procellarum basin. This period is characterized by intense bombardment by asteroids and comets, leading to the formation of many of the Moon's large impact craters and basins.

The Noachian period is estimated to have lasted from approximately 4.1 billion to 3.8 billion years ago. During this time, the Moon experienced a significantly higher rate of impacts than it does today. These impacts played a crucial role in shaping the lunar surface, creating vast plains of ejected material and contributing to the differentiation of the lunar crust and mantle. Evidence for the Noachian period is primarily found in the heavily cratered highlands of the Moon, which are the oldest features visible. The composition of these ancient rocks provides insights into the early solar system.

The term "Noachian" is derived from Noah, the biblical figure associated with a great flood, reflecting the idea that the Moon was subjected to a deluge of impacts during this time. The study of Noachian geology helps scientists understand the early history of the Moon and the processes that affected terrestrial planets in the early solar system, including Earth.