Though the moon has many seas, scientists thought it was dry.
They were wrong.
In a study published today in Nature, researchers led by Brown University geologist Alberto Saal found evidence of water molecules in pebbles retrieved by NASA's Apollo missions.
The findings point to the existence of water deep beneath the moon's surface, transforming scientific understanding of our nearest neighbor's formation and, perhaps, our own. There may also be a more immediately practical application.
"Is there water there? That's important for lunar missions. People could get the water. They could use the hydrogen for energy," said Saal.
The pebbles were scattered by lunar volcanoes that erupted three billion years ago, when the moon was still a cooling hunk of magma cast into orbit by the collision of a Mars-sized asteroid with Earth.
That impact enveloped the Earth in temperatures reaching 7000 Kelvin -- more than enough, it was thought, to obliterate all traces of hydrogen and oxygen.
Though NASA's Lunar Prospector appeared to have struck ice in 1999, its findings proved inconclusive. Had they been supported, scientists predicted that any water would have come from gases emitted by meteorites striking the moon.
With so little reason to believe in native lunar water, said Saal, it took three years to secure the minimal funding necessary to take another look at the Apollo pebbles, gathered between 1969 and 1972.
But a high-powered imaging technique known as secondary ion mass spectrometry revealed a wealth of so-called volatile compounds, among them fluorine, chlorine, sulfur, carbon dioxide -- and water.
Critically, telltale hydrogen molecules were concentrated at the center of samples rather than their surfaces, assuring Saal's team that water was present in an infant moon rather than added by recent bombardment.
"That was not known," said William Feldman, a Los Alamos National Laboratory geophysicist who was not involved in the study.
If that water in fact came from the Earth, then planetary geologists can be certain that our planet contained water 4.5 billion years ago. That would change the dynamics of models of Earth's formations.
"Volatile elements play a fundamental role in planetary formation through their influence on melting," said Feldman. "Melting temperatures are lower, you get different kinds of volcanic flows and magma crystallization. It's important for a lot of the processes that determine surface mineralogy."
Alternatively, water could have been added after the moon was ejected into space but before it cooled, raising new questions about the water's origin.
"This opens up so many lines of study," said Saal.
More practically, the widespread presence of water beneath the moon's surface could prove a boon to future lunar colonies, who could harvest it for breathable oxygen and hydrogen fuel.
Whether that is possible depends on the water's extent and concentration. This is not now known.
Materials collected by the Lunar Reconaissance Orbiter, which will scour the moon's south pole later this year, and the Lunar Crater Observation and Sensing Satellite, scheduled for launch in 2009, should provide further insight.
"Could a colony use the water? That's like asking the final score of a football game in the first five minutes of the first quarter," said Saal. "But at least we know there's a game on."