4.36 billion - The approximate age of the moon in years (in 2013, new analysis of rock samples gathered in 1972 (by the Apollo 16 mission) led scientists to conclude that the moon is around 100 million years younger than the previous given age of 4.567 billion years).
30 - 100 million - The number of years, after the formation of the earth, it is thought that the moon formed. This, so one theory goes, occurred as a result of a large 'Mars' size object (specifically a 'proto-planet', a young planet which has not fully formed) striking the earth. This is known as the Impact hypothesis. Other theories include:
According to the fission theory, the Moon was once a part of the earth which became separated, possibly from the basin of the present day Pacific Ocean. Supporting this now largely discredited theory, it was suggested that similarities in the composition of the Earth and the Moon were indicative of the Moons earthly origins early in the history of the solar system.
Similarly discredited like the fission theory, the capture theory posited that the Moon was, as the name suggests, captured by the Earth. Formed in an entirely different part of the solar system, it was previously thought that the Moon became trapped by the gravitational pull of the Earth as it travelled through the solar system as a large, and fully formed, celestial body.
The lunar geological timescale (the Selenological Timescale) divides the Moon's history into five broad time periods:
Pre-Nectarian - 4.4 to approximately 3.9 billion years ago.
Nectarian - 3.9 to 3.8 billion years ago (some recent research suggests the Nectarian period could in fact stretch back 4.1 billion years).
Imbrian - Approximately 3.8 to 3.2 billion years ago.
Eratosthenian - 3.2 to 1.1 billion years ago.
Copernican - 1.1 billion years ago to the present day.
The boundaries of this time scale are related to large impact events that have modified the lunar surface, changes in crater form that occur through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.
© 2013 - Dave Fowler, History in Numbers.
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