Giant impact hypothesis
The giant impact hypothesis says the Moon formed from debris after a Mars-sized body struck the young Earth.
The giant impact hypothesis is the leading explanation for the Moon's origin. About 4.5 billion years ago, a planet-sized body, often called Theia and perhaps similar in size to Mars, collided with the young Earth. The impact threw out a huge cloud of molten and vaporised rock, which gathered into a disc around Earth and quickly clumped together to form the Moon.
The idea, developed in the 1970s, explains several facts. The Moon has a small iron core, because the impact mainly ejected rocky outer material. It is poor in volatile elements, which would have boiled away. It was once covered in a magma ocean, consistent with forming hot.
One puzzle is that the Moon's oxygen isotopes are almost identical to Earth's, whereas other bodies, such as Mars, differ. Possible answers include Theia having formed near Earth, or the debris being thoroughly mixed with Earth's material in an extremely energetic impact.
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