Water molecules detected in lunar rocks originate from various areas of the Moon's interior, and some areas are wetter than others, according to a paper published online this week in Nature Geoscience. The Moon was thought to be dry until six years ago when water was detected in lunar samples.

Katharine Robinson and G. Jeffrey Taylor compiled measurements, reported over the past six years, of water molecules trapped within various lunar samples returned by the Apollo missions. They found that the concentration of the water molecules and their chemistry varies between rock types. For example, the water concentration in volcanic glasses is consistent with an origin from magmas that were as wet as parts of the Earth’s mantle, whereas other basaltic rocks are thought to derive from much drier mantle sources.

Taken together, the measurements suggest that the distribution and chemical composition of water varies in the lunar interior-a clue to understanding how the Moon formed and evolved over time.