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The compositional similarity of the Moon and Earth`s mantle and the recognized importance of large impacts in the early solar system, together with the large angular momentum present in the Earth-Moon system, have led to the theory that the Moon formed after a Mars-sized object collided with the growing Earth. Simulations predict that much of the preexisting layered structure of Earth was obliterated and much of the impacting material thrown back into orbit, creating a post-impact accretion disk surrounding proto-Earth. Simulations also predict that the Moon consists primarily of material from the impacting object, and not material from proto-Earth. This computational model is challenged, however, by a remarkable similarity in oxygen isotopes found between lunar and Earth rocks, raising questions about the partitioning of material during impact.