Researchers have found the first evidence of past water on the moon: trace water molecules trapped in glassy, volcanic pebbles brought back to Earth by Apollo astronauts in the 1960s and early 1970s.

The pebbles, known as lunar volcanic glasses, were formed more than three billion years ago when molten underground rock (magma) erupted from the lunar surface in fiery sprays and solidified.

Using a highly sensitive type of mass spectrometry, which sorts particles by size and charge, researchers scanned the lunar samples for elements and molecules that boil at low temperatures, referred to as volatiles.

Along with water, they discovered chlorine, fluoride and sulfur in the 0.1- to 0.2-millimeter glasses (0.1 millimeter is four thousandths of an inch). The concentration of all the volatiles decreased toward the edges of the samples, indicating that they were not Earthly contaminants.

What the pattern does indicate, the researchers report in Nature, is that the gases were in the process of bubbling out of the magma droplets like soda fizz when they solidified into beads.

The concentration of water in the lunar glasses was 46 parts per million—hardly enough to sustain astronauts should they return to the moon by 2020 as planned. For that, NASA will have to hope for pockets of moon ice at the poles, hinted at in prior studies.

Researchers have tried for years to identify water and other volatiles in the lunar volcanic glasses, but were either unable to distinguish them from background gases or could not confirm that what they found was not a contaminant, says study author Alberto Saal, an assistant professor of geologic sciences at Brown University.

For the new study, researchers from Brown, the Carnegie Institution of Washington and Case Western Reserve University applied a technique called secondary ion mass spectrometry. They inserted the glasses in a sample holder made of the metal indium to minimize the presence of other gases.

They then applied a tightly focused beam of cesium ions, designed to dislodge volatile atoms or molecules with micron (thousandths of a millimeter) precision. They scanned the beam over the sample to map the concentration of volatiles.

Based on models of "degassing"—the soda fizz–like escape of gas from rock—they estimated that the moon once contained 750 parts per million of water in its mantle, which they note is similar to the water content of Earth's upper mantle.

"It's a little bit startling because you don't expect to find indigenous water in any lunar sample," says geologist Paul Spudis of the Lunar and Planetary Institute in Houston. Most researchers believe the moon formed some 4.5 billion years ago when a Mars-sized body smashed into Earth and knocked a piece of it loose, which became the moon.

Lacking evidence of lunar water, most scientists had assumed that the collision vaporized any hydrogen capable of forming water, Saal says. "People convinced themselves there was not water."

He says the apparent presence of water so long ago implies that it either managed to come along for the ride or that meteorites brought it there within 100 million years after the moon formed.

Prior scans of the moon have turned up possible evidence of frozen water in shadowy craters at the moon's poles—similar to Martian ice recently uncovered by NASA's Phoenix Lander—but follow-up studies proved inconclusive.

NASA plans to send its Lunar Reconnaissance Orbiter (LRO) to the moon later this year along with the Lunar Crater Observation and Sensing Satellite (LCROSS), which will impact the lunar south pole in 2009, to settle the case. If ice is found, Saal says, it would be possible to check whether it represented the same ancient water in glasses, preserved in polar cold spots.

But Spudis says that is unlikely, because in the course of three billion years the lunar cold spots would have changed positions if the moon's spin axis drifted, meaning that whatever is frozen there today wasn't necessarily frozen there in the past.

The bottom line, he says, is the discovery shows that "whatever the early history of the moon and it's early accretion [of water], it's not a simple process."