We've long known that water ice is holed up in the darkest, coldest craters on the moon, but now scientists think the story of lunar water runs much deeper, far below its surface. And this is good news if humanity wants to build that moon base we've been promising ourselves for so long.
Here's the backstory: In 2008, scientists analyzed small glass beads found in the samples of moon rock brought back to Earth by the Apollo 15 and 17 missions in 1971 and 1972. These volcanic beads originated from the moon's interior, and the crystalline structures contained a small, yet surprising amount of water, overturning the idea that the moon's subsurface was largely devoid of water.
More research found that the beads contained similar quantities of water as some basalts on Earth – another clue that the moon was formed from the debris of Earth after a massive ancient impact with another planetary body.
Now, in a new study published in the journal Nature Geoscience, spectroscopic data from the Moon Mineralogy Mapper onboard India's Chandrayaan-1 lunar orbiter was used to map the lunar surface; specifically it sought regions where there are elevated concentrations of water deposits in lunar rock that originated from volcanic activity. And guess what? It looks like the water-rich Apollo samples weren't an anomaly, which is welcome news.
"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. 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," said planetary geologist Ralph Milliken, in the press release. Milliken is the lead author of the new study and works at Brown University.
These spectrometer data were recorded by studying the reflected sunlight bouncing off the moon's surface. Some wavelengths of light are absorbed by certain chemicals, so when the reflected light is received, the Moon Mineralogy Mapper could identify which materials were present. But to tease out the signal of water in the pyroclastic flows, the researchers had to carefully remove the interference of thermal emissions caused by the sun's heating of the daytime surface.
"That thermally emitted radiation happens at the same wavelengths that we need to use to look for water," Milliken said in the statement. "So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component."
Using the Apollo samples in tandem with detailed thermal models of the moon's surface, the scientists corrected the map, revealing the distribution of water trapped in these ancient volcanic flows.
As these pyroclastic deposits contain a surprisingly high proportion of water, this likely means the moon's interior contains a lot more water than previously thought, a finding that has huge implications for our theories of how the moon formed. If the moon was formed from the molten debris of a massive Earth impact, how did the hydrogen required in water molecules survive the extreme heating?
"The growing evidence for water inside the moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the moon had completely solidified," said co-author and postdoctoral researcher Shuai Li, who works at the University of Hawaii. "The exact origin of water in the lunar interior is still a big question."
There are obvious implications for the future of possible human missions to the moon. If the moon was bone-dry, setting up any form of long-term human presence on the surface would be difficult, possibly unsustainably so. Launching supplies, materials and fuel from Earth to the lunar surface requires powerful and expensive rockets, but if we can source building materials and water from the lunar surface, we may eventually establish a self-sustaining, permanent moon base.
The researchers emphasize that the quantity of water is small – only 0.05 percent of the material by weight is water – but there's a lot of it on or close to the surface with, potentially, a huge supply in the moon's mantle. So, if we do set up a long-duration presence on the lunar surface, perhaps we can develop an efficient water extraction process that could make it sustainable – producing drinking water and fuel, and driving some form of space agriculture.
"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," Li concluded.