Where did that Moon of ours come from?

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An early theory explained the moon by saying it formed from the same gas cloud from which the Earth and other planets formed. This explanation doesn’t fly because the moon contains very little iron. The Earth has a lot, mostly at its molten core. If the moon formed out of the same stuff as the Earth, the moon ought to have more iron.

A second theory explained away the Moon’s small amount of iron by suggesting the Moon formed somewhere else, where there wasn’t much of that pesky iron, and was captured by the Earth’s gravitation as it swung by. This one didn’t work out either. When samples of the moon were brought back by Apollo astronauts and analyzed, it was found that they had a chemical composition almost identical to Earth’s making it unlikely that it came from somewhere else.

A third idea tried to explain away the Moon’s ‘iron deficiency anemia’ by suggesting the early Earth spun so fast that some of its surface was flung off and went into orbit. The hole it left was the Pacific Ocean basin. Since most of the Earth’s iron is at its center, the flung off material would not have a lot of iron in it. But when they crunched the numbers relating to the Moon’s orbit, they found the math didn’t work out.

The latest theory is that a chunk of debris, leftover from the formation of the Solar System, slammed into the Earth and knocked a large amount of the iron poor crust loose. Scientists estimate that the piece that hit us was a third to half the size of the Earth. The material blasted loose formed a ring around the Earth which clumped together to form the moon. This is the currently accepted theory.

It’s interesting that even after decades of head-scratching, scientists are still not really sure how the moon came to be.