Over the years, there have been various hypotheses about the origin of the Moon. Historically, the major theories have been fission, capture, giant impact, and co-accretion. A chemical analysis of lunar rocks may force scientists to revise the leading theory for the Moon’s formation: that the satellite was born when a Mars-sized body smacked into the infant Earth some 4.5 billion years ago. If that were the case, the Moon ought to bear the chemical signature of both Earth and its proposed second parent. But a study published today in Nature Geoscience suggests that the Moon’s isotopic composition reflects only Earth’s contribution.