Past studies, however, have confirmed the presence of some light elements under the moon's surface, including sulfur, chlorine, fluorine, and carbon.
But evidence for water has eluded all efforts to find it for so long that Saal and his co-authors were denied funding for their study for three years.
"The scientific community thought it was a futile endeavor," he said. "They thought, We know the moon doesn't have any water."
The study authors now say that only 95 percent of the moon's water vapor was lost to space during the eruptive phase.
Evidence of the lingering 5 percent came from lunar volcanic glasses, pebble-like beads collected and returned to Earth by two of NASA's Apollo missions in the late 1960s and early 1970s.
Using a new, highly sensitive technique to determine the chemical makeup of the samples by measuring charged particles, the team was able to find minute amounts of water in the moon matter.
"We developed a way to detect as little as five parts per million of water," said Erik Hauri, a geologist at the Carnegie Institution and a co-author of the new study
"We were really surprised to find a whole lot more in these tiny glass beads, up to 46 parts per million," he said.
The research team calculated that moon magma might have contained water up to 750 parts per million before the "fire fountain" eruptions. That's similar to primitive magmas that erupted from mid-ocean ridges on Earth.
"This suggests the very intriguing possibility that the Moon's interior might have contained just as much water as the Earth's depleted upper mantle," Hauri said.
Lead author Saal said the research also may yield additional insight into how long water has been on Earth.
"It suggests that water was present within the Earth before the giant collision that formed the moon," he said.
"That points to two possibilities," he continued. "Water either was not completely vaporized in that collision or it was added a short time—less than 100 million years—afterward by volatiles introduced from the outside, such as with meteorites."
The meteoritic origin is difficult to reconcile, Saal said, with the fact that the water-containing magma came from deep within the moon's interior.
David Stevenson, a planetary scientist at the California Institute of Technology, called the new results "exciting."
"It is remarkable that people are going back to rocks that we've had lying around for decades and figure out things like what they've done," he said. "That's impressive."
Hauri pointed out that finding even trace amounts of water on the moon's surface is a "giant leap" from the former belief that the moon is dry.
"But even more intriguing: If the moon's volcanoes released 95 percent of their water, where did all that water go?" he asked.
The moon's gravity is too weak to retain an atmosphere, so the authors of the new study have speculated that the bulk of the water either drifted into space or migrated toward the poles, where it remains frozen as ice.
Several previous lunar missions have suggested the presence of ice at both poles.
Verifying the presence of this ice is one goal of the NASA Lunar Reconnaissance Orbiter mission, due to launch later this year. The task also is the primary objective of the Lunar Crater Observation and Sensing Satellite, which has a 2009 launch date.
This summer, meanwhile, Saal's team is looking for water in volcanic glasses gathered from other Apollo missions to determine how widespread and uniform the water might be.
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