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Colored areas indicate elevated water content compared with surrounding terrains. Yellows and reds indicate the richest water content. Photo: Milliken Lab/Brown University

Researchers from Brown University have shared evidence suggesting the Moon’s interior mantle contains substantial amounts of indigenous water.

Up until about 2008, it was thought that the interior of the Moon was absent of water and other volatile compounds. But ancient volcanic deposits brought back to Earth from the Apollo 15 and 17 missions revealed traces of water. The volcanic deposits are believed to consist of glass “beads” formed by explosive eruption of magma that came from the deep lunar interior.

At the time, researchers were still unsure of whether the water-rich volcanic beads were an anomaly, or if they were representative of the rest of the Moon’s mantle.

Now, for the first time, researchers were able to detect widespread water within the ancient explosive volcanic deposits on the Moon’s surface, which suggests that the interior is rich with indigenous water.

To reach this conclusion, Brown University researchers reviewed satellite images taken by an imaging spectrometer that was aboard India’s Chandrayaan-1 lunar orbiter from 2008. The images displayed materials that were never examined by the Apollo missions, so the team used orbital spectrometers to measure the light bouncing off the surface. As a university release on the findings explained, scientists can get an idea of which minerals and other compounds are present by looking at which wavelengths of light are absorbed or reflected by the surface.

However, the Moon’s surface proposed a challenge for the team -- the lunar surface heats up throughout the course of the day, especially at latitudes where the volcanic deposits are located. Therefore, the spectrometer had to measure heat in addition to light.

Lead author Ralph Milliken and team used laboratory-based measurements of samples returned from the Apollo missions combined with a detailed temperature profile of the areas of interest on the Moon’s surface to accomplish the task.

“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water,” said Milliken, associate professor in the Department of Earth, Environmental and Planetary Sciences at Brown. “So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component.”

They found evidence of water in nearly all of the large deposits that had been previously mapped across the Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.

“The distribution of these water-rich deposits is the key thing,” Milliken said. “They’re spread across the surface, which tells us that the water found in the Apollo samples isn’t a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle.”

The discovery could prompt more long-term human visits to the Moon. Water from Earth wouldn’t need to be transported with astronauts if they could tap into water already on the Moon. While the beads only contain about .05 percent of water by weight, the deposits are large, and could potentially be extracted, according to the team.

 The findings also spark more questions about how the Moon formed.

“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 Shuai Li, postdoctoral researcher at the University of Hawaii, who co-authored the paper with Milliken. “The exact origin of water in the lunar interior is still a big question.”

The study is published in the journal Nature Geoscience.

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