How was the moon formed?
The moon is a bit of a mystery. It has a mantle like the Earth's, but no iron core. The minerals on the moon have the exact same oxygen isotope composition, while minerals on Mars and on asteroids that formed in other areas of the solar system have different oxygen isotopes--strongly suggesting that the moon was formed in the vicinity of Earth, out of the same materials. One of the long-standing theories about the moon states that it was formed somewhere else in the solar system, and was captured by Earth's gravity during its early formative years--but this is unlikely, due to the similar mineral composition on the Earth and the moon.
The most cutting-edge theory on the moon's formation suggests that a planet-sized body actually collided with the Earth at some point in its early history, causing vast amounts of molten material to be ejected into Earth's orbit. These materials eventually coalesced into the moon. This theory explains many of the mysteries surrounding the moon, including why its mineral composition is so similar. The theory also states that there is no iron in the moon because the Earth's iron core had already collected to the center by the time this cataclysmic event happened, and the material that eventually became the moon was ejected from the Earth's surface.
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