There's way more water locked inside the Moon than we previously thought, according to a new analysis of satellite data.
This unexpected finding about our planet's grey companion is giving scientists new insights into how the Moon formed and what its internal structure is like. And it has potentially huge implications for any of our future lunar missions.
For a long time we thought the Moon was totally bone dry. On the surface it's a super-dusty environment with no substantial atmosphere, temperature extremes, and not enough gravity to help retain water molecules.
But recent studies have uncovered several types of lunar water. In 2009, NASA deliberately crashed its LCROSS probe into the Moon's south pole, discovering loads of water ice in the debris that shot up from the impact.
These ice deposits were thought to be billions of years old, trapped in the permanently shadowy and extremely cold crevasses of the lunar poles. But this water wasn't always there.
Researchers thought it was likely produced through external forces, such as solar winds sweeping across the surface and providing the right chemical reactions. In fact, there's a background level of extremely minuscule amounts of this kind of trapped water across the Moon's surface.
Astronauts from several Apollo missions also brought back geological samples from various parts of the Moon's surface, and in 2008 these samples were re-analysed to reveal trace water locked up in tiny glass beads.
Those glass beads were found in pyroclastic deposits - rock deposits of volcanic origin from some 100 million years ago when the Moon was still a highly geologically active ball with a bubbling core and surface volcanoes.
Such water, locked up in the Moon's own geology, is considered to be of local origin or 'indigenous', meaning it could have stuck around ever since the Moon was still a chunk of matter violently torn off our young Earth.
But scientists couldn't tell whether these beads actually indicated a 'wet' layer right underneath the Moon's dusty crust, in the lunar mantle.
"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," says Ralph Milliken from Brown University, lead researcher of the latest study.
To answer that key question, Milliken and his team turned to orbital data from India's Chandrayaan-1 lunar orbiter, which carried aboard the handy Moon Mineralogy Mapper.
Using orbital data from previously mapped large pyroclastic deposits on the Moon's surface, laboratory analysis of Apollo mission samples, and a detailed model of lunar surface temperature data, the researchers found water-rich volcanic deposits all over the place.
"They're spread across the surface, which tells us that the water found in the Apollo samples isn't a one-off," says Milliken.
Some of these volcanic deposits stretch for thousands of square kilometres, and the team's data shows that there is four times more water in these than the measurable background level we mentioned above.
"[T]hese deposits are the result of magma that originally comes from deep within the lunar interior," Milliken told Samantha Mathewson at Space.com.
So did all that water once hitch a ride from Earth or was it dumped there by comets? Unfortunately, the new findings only tell us it's there - but it's a step towards finding out more about the Moon's history.
"Whether it is from the Earth or from impact delivery … we are not ready to answer that question," one of the team, Shuai Li from Brown University, told The Guardian.
But however it got there, scientists do think all that water could one day become a valuable resource.
"The nearly ubiquitous presence of water in large and small lunar pyroclastic deposits adds to the growing evidence that the lunar mantle is an important reservoir of water," the team writes in the study.
And if that's the case, our future Moon colonists might well be able to extract usable water from many of these large volcanic deposits, making their off-Earth home just a little bit more habitable.
The research was published in Nature Geoscience.