When astronauts brought back moon rocks, scientists discovered they were similar to the rocks here on Earth, meaning that the moon must be the result of an impact that knocked a piece of the Earth into orbit. The isotope match is too close to support the theory that the moon could have been made partly of material from another planet.
The Daily Galaxy reports that around 4,500 billion years ago, "according to the giant impact hypothesis, there was once a Mars-sized body referred to as Theia orbiting in our solar system. Theia would have formed in about the same orbit as Earth, but about 60 degrees ahead or behind. When the protoplanet had grown to be about the size of Mars, its size made it too heavy for its orbit to remain stable. As a result, its angular distance from Earth varied increasingly, until it finally it crashed into the Earth."
However, the Earth and the moon both have iron cores, despite the fact that the moon does not have enough mass to generate the pressure necessary to form the same core as found in the Earth--and no one can explain how it got there.
The Daily Galaxy says, "Most scientists agree that if a planet had smacked into Earth and the moon came about as a result, than the moon ought to be made of some of that other planet as well. Some say the laws of physics suggest it would be somewhere in the neighborhood of 40%. If that’s the case, why don't studies of rocks brought back by the Apollo missions show evidence of this other planet?"
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