Moon: Encyclopedia II - Moon - Origin and history

Moon - Origin and history

The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate.

Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin (presumed to be the Pacific) behind as a scar. This concept requires too great an initial spin of the Earth and the presumption of a Pacific origin is not compatible with the geological standard model, the theory of plate tectonics. Others speculated the Moon formed elsewhere and was captured into its orbit. Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal.

The Coformation or Condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon.

Recently, the giant impact hypothesis has been considered a more viable scientific theory for the moon's origin than the coformation or condensation theory. The Giant Impact theory holds that the Moon formed from the ejecta resulting from a collision between a very early, semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia. The material ejected from this impact would have gathered in orbit around earth and formed the moon. This hypothesis is bolstered by two main observations: First, the composition of the moon resembles that of the earth crust, whereas it has relatively few heavy elements that would have been present if it formed by itself out of the same material from which earth formed. Second, through radiometric dating, it has been determined that the moon's crust formed between 20 and 30 million years after that of the earth, despite its smallness and associated larger loss of internal heat.

The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. Analysis of craters and Moon rocks show that there was a late heavy bombardment by asteroids around the period 4.0 to 3.8 billion years ago.

Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.

In 2005, scientists from Germany and Oxford University measured the Moon's age at 4.527 ± 0.010 billion years, which would imply that it was formed only 30–50 million years after the origin of the solar system. ^{[citation needed]}

Adapted from the Wikipedia article "Origin and history", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki