Several volcanic deposits across the surface of the moon feature significant amounts of trapped water, according to new findings, the latest development in the quest for water on moon, which was first detected in volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions.
“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, lead author of the study and an associate professor at Brown University.
While scientists had assumed that the interior of the moon was largely depleted of water and other volatile compounds, analysis of the glass beads in 2008 revealed trace amounts of water. In 2011, further study of small crystalline formations in those beads showed that they contain similar amounts of water as some basalts – dark, fine-grained volcanic rock – on Earth.
The data shed light on layers of rock on the moon that likely formed from volcanic eruptions, called lunar pyroclastic deposits.
“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, Milliken said. “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.”
Scientists use orbital instruments to evaluate the water content of lunar volcanic deposits, measuring the light that bounces off a planetary surface. By observing which wavelengths are reflected or absorbed by the surface, researchers can get a sense of which minerals and other compounds are present.
It’s a challenging process, as the Moon’s surface heats up during the day, especially at the latitudes where the pyroclastic deposits are. This leads to spectrometers measuring heat as well as the light reflected from the surface.
“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water,” Milliken said. “So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component.”
To obtain accurate measurements, the team used laboratory-based measurements of samples from the Apollo missions, which the scientists combined with a detailed temperature profile of the areas of interest on the Moon. Using this approach, the researchers reviewed data from the Moon Mineralogy Mapper, an imaging spectrometer that was aboard India’s Chandrayaan-1 lunar orbiter.
The team found evidence of water in almost all of the large pyroclastic deposits that had been previously mapped, including near the Apollo 15 and 17 landing sites, where the water-bearing glass bead samples were found.
“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 water-rich interior of the Moon raises questions about the celestial body’s formation. Researchers believe the Moon formed from debris left behind after an object roughly the size of Mars slammed into Earth billions of years ago: This is known as the “great impact” theory. It led scientists to assume that the Moon’s interior is dry, as it seems unlikely that the hydrogen required to form water could have survived the heat of that impact.
“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 Shuai Li, a postdoctoral researcher at the University of Hawaii. “The exact origin of water in the lunar interior is still a big question.”
The new findings could aid missions from Earth.
“Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access,” Li 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.”