That's the suggestion of Jessica Barnes and colleagues at The Open University who investigated the amount of water present in the mineral apatite, a calcium phosphate mineral found in samples of the ancient lunar crust brought back to earth by the Apollo missions.
"These are some of the oldest rocks we have from the Moon and are much older than the oldest rocks found on Earth," Barnes said. "The antiquity of these rocks make them the most appropriate samples for trying to understand the water content of the Moon soon after it formed about 4.5 billion years ago and for unraveling where in the Solar System that water came from."
Barnes presented the study's finding Monday at the European Planetary Science Congress in London.
The ancient lunar rocks contain appreciable amounts of water locked into the crystal structure of apatite, the researchers said, noting they have measured the hydrogen isotopic signature of the water in these lunar rocks to identify the potential source for the water.
"The water locked into the mineral apatite in the Moon rocks studied has an isotopic signature very similar to that of the Earth and some carbonaceous chondrite meteorites," Barnes said.
"The remarkable consistency between the hydrogen composition of lunar samples and water-reservoirs of the Earth strongly suggests that there is a common origin for water in the Earth-Moon system."