The interior of the Moon contained water early in its history, reports a study published online this week in Nature Geoscience. It had been thought that any water in lunar rocks was added after its formation by smaller meteorite impacts or solar wind, but this new finding suggests that the process that led to the formation of the Moon may require a re-think.

Hejiu Hui and colleagues analysed samples from the lunar highlands obtained during the Apollo missions. They detected a tiny but measureable amount of water in mineral grains. The lunar highlands are thought to represent the original crust that crystallized from a mostly molten early Moon. Therefore, the findings indicate that the lunar interior contained water during this molten stage, before the crust solidified.

The presence of water in the moon’s interior at such an early stage is difficult to reconcile with the theory that the Moon formed from the debris generated during a giant impact with the proto-Earth.