Researchers from Brown and Case Western Reserve universities and Carnegie Institution of Washington have found that the water found on the moon, like that on Earth, came from small meteorites called carbonaceous chondrites in the first 100 million years or so after the solar system formed.
Evidence discovered within samples of moon dust returned by lunar crews of Apollo 15 and 17 dispels the theory that comets delivered the molecules.
The discovery's telltale sign is found in the ratio of an isotopic form of hydrogen, called deuterium, to standard hydrogen.
The ratio in the Earth's water and in water from specks of volcanic glass trapped in crystals within moon dust match the ratio found in the chondrites. The proportions are far different from those in comet water.
The moon is thought to have formed from a disc of debris left when a giant object hit the Earth 4.5 billion years ago, very early in Earth's history.
Scientists have long assumed that the heat from an impact of that size would cause hydrogen and other volatile elements to boil off into space, meaning the moon must have started off completely dry.
But recently, NASA spacecraft and new research on samples from the Apollo missions have shown that the moon actually has water, both on and beneath its surface.
By showing that water on the moon and Earth came from the same source, this new study offers yet more evidence that the moon's water has been there all along, or nearly so.
"The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact," Alberto Saal, a geochemist at Brown University and the study's lead author said.
"Some of that water survived the impact, and that's what we see in the moon," he said.
Or, the proto-moon and proto-Earth were showered by the same family of carbonaceous chondrites soon after they separated, said James Van Orman, professor of earth, environmental and planetary sciences at Case Western Reserve, and a co-author.
To find the origin of the moon's water, the researchers looked at the trapped volcanic glass, referred to as a melt inclusion. The surrounding olivine crystals prevent water form escaping during an eruption, providing researchers an idea of what the inside of the moon is like.
The research is published online in Science Express.