Backscatter electron image of a lunar melt inclusion from Apollo 17 sample 74220, enclosed within an olivine crystal. The inclusion is 30 µm in diameter. Skeletal crystals within the melt inclusion are a fine mixture of olivine and ilmenite. Dark area in the lower-left is an ion microprobe sputter crater. Credit: John Armstrong, Geophysical Laboratory, Carnegie Institution of Washington

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, researchers from Brown and Case Western Reserve universities and Carnegie Institution of Washington have found.

Evidence discovered within samples of  returned by lunar crews of Apollo 15 and 17 dispels the theory that comets delivered the molecules.

The research is published online in Science Express today.

The discovery's telltale sign is found in the ratio of an isotopic form of hydrogen, called , to standard hydrogen. The ratio in the 's water and in water from specks of  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  to boil off into space, meaning the moon must have started off completely dry. But recently,  and new research on samples from the  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.

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