Scientists have found water in some tiny beads of volcanic glass that Apollo astronauts collected on the moon decades ago, raising new questions about how the moon was born.
For a long time, scientists have thought "that the moon is completely dry," says Erik Hauri, a geochemist at the Carnegie Institution's Department of Terrestrial Magnetism in Washington, D.C.
They believed there couldn't be water on the moon because of how it was formed. According to the "giant impact" theory, the moon was born 4.5 billion years ago when an object the size of Mars came hurtling out of the void and smacked the young Earth.
This impact melted both objects and scattered a cloud of debris around the Earth. That debris became the moon — and the new moon was a hot ocean of magma.
Researchers thought there was no way a moon this hot could have retained any water.
But Hauri got together with some colleagues to look for water in a special kind of material collected on the moon by Apollo astronauts: tiny bits of glass created by volcanic eruptions on the early moon.
"They're very tiny, the size of a period on a typical printed page," says Hauri, who explains that as volcanoes on the moon erupted, tiny droplets cooled in the air, turning to glass before they even hit the ground.
The glass beads come in different colors. Some were almost invisible and had to be picked out of gray moon dust, but others could be seen by the astronauts as they walked around on the lunar surface.
Moonwalker and geologist Harrison Schmitt discovered some of them back in 1972, during the Apollo 17 mission, when he looked down and was shocked to discover orange soil at his feet.
A study about 10 years ago looked at the beads and saw hints of water. But the results also fell within the instrument's margin of error. So Hauri and his colleagues decided to look again using newer, more sensitive detection technology.
Some moon experts were skeptical, saying there was no way the team would find any water molecules. "We got a lot of comments like, 'Oh, well, we already know that the moon is dry, we've known this for 20 years,'" Hauri says.
But this time they did find water molecules, according to a report in the journal Nature.
It wasn't a lot of water. The beads had only up to 46 parts per million. But from this, the researchers estimate that the interior of the moon once probably contained an amount of water equal to that of the Caribbean Sea.
This is a problem for the giant impact theory, says Hauri. "It's hard to imagine a scenario in which a giant impact melts, completely, the moon, and at the same time allows it to hold onto its water," he says. "That's a really, really difficult knot to untie."
Others agree that the discovery raises questions. "It's obviously a very new result to say that the moon might have had a significant amount of water right after it formed," says Ben Bussey, with the Johns Hopkins University Applied Physics Lab in Laurel, Md.
Scientists have seen hints of ice at the moon's poles, in cold, shadowed craters. But because the moon was assumed to be dry, researchers thought this ice had to come from meteorite impacts.
Now there's a small but real possibility that some of that ice — if it exists — may have come from inside the moon. "There is a chance that there might be some very old water of the type that they discuss in the paper," Bussey says.
If the moon does have ice, Bussey says, scientists could figure out where it came from, but only if a rover or an astronaut goes to get a sample.