A new study of satellite data found that numerous volcanic deposits distributed across the surface of the Moon contain unusually high amounts of trapped water.
The finding of water in these ancient deposits, which are believed to consist of glass beads formed by the explosive eruption of magma coming from the deep lunar interior, bolsters the idea that the lunar mantle is surprisingly water-rich.
Scientists had assumed for years that the interior of the Moon had been largely depleted of water and other volatile compounds.
In 2008, researchers detected trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon.
In 2011, further study of tiny crystalline formations within those beads revealed that they actually contain similar amounts of water as some basalts on Earth, suggesting that the Moon's mantle contains as much water as Earth's.
"The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise 'dry' mantle," said Ralph Milliken, an associate professor at Brown University in the US.
"By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions," said Mr Milliken.
"The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet," he said.
By looking at which wavelengths of light are absorbed or reflected by the surface, scientists can get an idea of which minerals and other compounds are present.
Researchers looked at data from the Moon Mineralogy Mapper, an imaging spectrometer that flew aboard India's Chandrayaan-1 lunar orbiter.
They found evidence of water in nearly all of the large pyroclastic deposits that had been previously mapped across the Moon's surface, including deposits near the Apollo 15 and 17 landing sites where water-bearing glass bead samples were collected.
Water-rich deposits are spread across the lunar surface, which tells us that the water found in the Apollo samples is not a one-off, Mr Milliken said.
The idea that the interior of the Moon is water-rich could have implications for future lunar exploration.
The volcanic beads do not contain a lot of water - about 0.05 per cent by weight - but the deposits are large, and the water could potentially be extracted, researchers said.
"Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative," said Shuai Li, a postdoctoral researcher at the University of Hawaii in the US.