As it turns out, the Moon matches the composition of the Earth’s crust and mantle fairly well, while overall the Moon lacks the Earth’s iron content—and on Earth, the most iron-rich portion of the planet is its interior.
The most commonly accepted theory is that a hypothetical, approximately Mars-sized body called Thea struck the Earth obliquely early in the solar system’s formation, knocking off a significant portion of the mantle, while leaving the iron-rich core relatively intact. The resulting debris from the Earth’s mantle and the remnants of Thea formed the Moon and settled back on the Earth’s remaining surface, accounting for the similarity between the two.
Sounds good as far as it goes, but to stand up, the theory needs to account for the Earth’s rotation and the Moon’s orbit, as well as allow for sufficient mixing between Thea and the material blasted off the early Earth so as to eliminate any noticeable difference in composition.
Meanwhile, the Juno spacecraft has returned a new video, shown above, of the Earth-Moon system, showing both objects in motion. Juno is heading for Jupiter, but used an Earth flyby on October 9th to gain speed on its way. Read more about the video here.