How Was the Moon Formed?
Earth’s moon is larger relative to the size of its primary planet than any other moon in the solar system. The most common theory of its origin is that the moon was formed soon after Earth when a large body of some kind collided with Earth, causing the ejection of a large amount of matter that then formed the moon through mutual gravitation. (This is the “giant impact theory.”) Although the question has not been settled conclusively, this theory is currently favored over several other candidate theories. These include (1) the possibility that the moon simply formed at the same time as Earth, from the primordial solar system; and a separate theory (2) that the moon came from elsewhere inside or outside the solar system and was captured by Earth’s gravity. However, theory (1) has difficulty explaining the difference in the average densities of Earth and the moon. Earth’s overall density (about 5500 kg/m3) results from the fact that it is a mixture of rock (average density of about 3000 kg/m3) and iron (density of 7900 kg/m3). The moon’s average density is only 3300 kg/m3, indicating nearly all rock with very little iron. If the moon formed in place near Earth, its density should be closer to that of Earth. Theory (2) has trouble explaining another chemical clue to formation: the ratios of the stable isotopes of oxygen 16O, 17O, and 18O. Isotopic analysis of meteorites shows that the oxygen ratios varied with location in the solar nebula.
However, the ratios of 17O/16O and 18O/16O indicate that the moon and Earth formed near each other in space. The giant impact theory explains these chemical and isotopic paradoxes in the following way. After the formation of Earth and the separation of the iron core (in the center) from the rocky mantle (above the core), any impact would eject rocky, iron-poor material into space, which could explain why the moon is iron poor and maintained the same oxygen isotope ratios as Earth.
However, the ratios of 17O/16O and 18O/16O indicate that the moon and Earth formed near each other in space. The giant impact theory explains these chemical and isotopic paradoxes in the following way. After the formation of Earth and the separation of the iron core (in the center) from the rocky mantle (above the core), any impact would eject rocky, iron-poor material into space, which could explain why the moon is iron poor and maintained the same oxygen isotope ratios as Earth.
ΑΝΑΔΗΜΟΣΙΕΥΣΗ ΑΠΟ ANDREW REX , "COMMON ASKING QUESTIONS IN PHYSICS", 3/7/2017
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