Study challenges prevailing theory of Moon’s origin

Science : The leading theory for the Moon’s formation is that Earth and a Mars-sized planet collided some 4.5 billion years ago and produced a disk of magma that orbited Earth and over time coalesced to form the Moon. If so, say researchers, the Moon’s chemical composition should reflect that of both Earth and the other planet. However, studies of lunar rocks collected by the Apollo missions in the 1970s have cast doubt on that theory. Previous research established that the Moon’s oxygen isotope composition is indistinguishable from that of Earth. Now, according to a study published yesterday in Nature Geoscience by Junjun Zhang of the University of Chicago and colleagues, another isotope ratio, that of titanium-50 to titanium-47, has been shown to be similar on both Earth and the Moon. Although the oxygen finding can be explained because oxygen vaporizes easily and could have been exchanged between Earth and the newly forming Moon, titanium does not vaporize as easily. Despite the new evidence, Zhang does not completely rule out the earlier theory: “Our study cannot provide a definite answer to the origin of the Moon yet. The message we hope to convey is that isotopic homogeneity between the Earth and Moon is a fundamental new constraint on the evolution of the Earth–Moon system.”