

The findings, published online Sunday in the journal Nature Geoscience, support mounting evidence that the moon once contained some "native" water - throwing a wrench into current beliefs about how it formed.

Prevailing theories hold that the moon was created when a Mars-size body crashed into the young Earth and broke off debris that eventually coalesced. In the process, much of the water would have evaporated into space, leaving Earth's new satellite quite arid.

"It's thought that the moon's formation involved the materials getting very hot," said Paul Warren, a University of California, Los Angeles cosmochemist who was not involved in the new study. "It's usually assumed that little water would have survived through that."

Indeed, the samples returned by the Apollo missions that visited the lunar highlands seemed to confirm that Earth's cold, rocky companion was bone-dry, said University of Notre Dame geologist Hejiu Hui, Ph.D., who led the new analysis.

But work in the past five years has challenged that notion, as scientists have used more advanced methods to look for increasingly tiny concentrations of water in glass beads that are thought to have been formed by volcanic eruptions in the moon's early days.

Some experts have argued that those glass beads could have been exposed to alien water sources after they had been ejected from the moon's interior. So Dr. Hui and his colleagues decided to look at a type of rock called plagioclase, which is thought to have formed in a magma ocean inside the moon. Although the rocks later floated to the surface to form the crust, they contain a chemical time capsule from inside the young moon.

After examining the samples under a microscope equipped with a spectrometer, the researchers found that the rocks contained 6 parts per million of water. That's drier than an Earth desert, but far more than expected to survive in a rock from the moon's once-molten center.

The samples should have been bone-dry, Dr. Hui said, but "somehow we still detect this amount of water, so that makes things interesting."