The Genetics of the Earth and Moon – Facts So Romantic

Imagine that two very similar-looking neighbors undergo a genetic test. The exam shows that the pair’s genetic fingerprints are virtually identical. They feel a flash of shock and excitement. What does this mean? Could they be long-lost twins, separated in a hospital mixup?

The Earth and Moon share a similar issue, one that poses a major scientific puzzle. At the isotopic level—the “genetic” level for rocks—the Earth and Moon are essentially identical. While they have different relative amounts of rock and iron, the Earth and Moon’s rocks share the same isotopic signatures in several different elements; oxygen is the most studied. This isotopic similarity has stumped scientists for decades. It must mean something about the Moon’s origin, but exactly what?

Over the past century astronomers have come up with several theories for how the Moon formed. Perhaps the Moon was a wandering planet that was gravitationally captured by Earth (the capture theory). Perhaps the nascent Earth was spinning so fast that it barfed out the Moon (the fission theory). Or perhaps the young Earth was hit by another growing planet and this created the Moon (the giant impact theory). In recent years the third of these ideas has taken over as the…

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