The Moon’s Interior Contains Lots of Water, New Evidence Suggests
Substantial amounts of water could be trapped in the rocky material that makes up the moon’s mantle — a potential benefit for future human exploration.
“The water that we detected in this study is OH/H2O (hydroxyl/water) that is trapped in volcanic glass beads,” lead author Ralph Milliken from Brown University said. “These glass beads are billions of years old and were formed by the explosive eruption of magma, similar to fire fountains that occur in some volcanic terrains on Earth, such as Hawaii.”
The beads are not ubiquitous, but embedded among numerous volcanic deposits scattered across the moon’s surface. Milliken and his co-author Shuai Li, a postdoctoral researcher at the University of Hawaii, used data from the Moon Mineralogy Mapper (M3, or “M-cubed”), an imaging spectrometer on India’s Chandrayaan-1 spacecraft that was in lunar orbit from 2008-2009. Since a spectrometer that can detect minerals on the moon’s surface is also sensitive to heat, they made temperature corrections to allow for the heating of the moon’s surface over the course of a lunar day.
The volcanic beads would have originally formed deep within the moon’s interior, and the spectrometer data showed unusually high amounts of trapped water in volcanic deposits compared with surrounding terrains.
“The water that we observe in the glass beads in these ancient fire fountain deposits came from the interior of the moon,” Milliken said. “This tells us that there is water in the moon’s mantle, and because the magma for these eruptions comes from very deep (several hundreds of kilometers down), there must be water in the deep interior of the moon.”
Milliken added it is important to note that the water is not in a liquid form, but rather it is mixed in with the ‘rocky’ materials that make up the mantle of the moon, similar to water that is within Earth’s mantle.
The volcanic glass beads are very tiny, sometimes orange in color and are intermixed with the dark lunar regolith. Some were in found in lunar samples from the Apollo 15 and 17 missions. You may recall astronaut/geologist Harrison Schmitt’s exclamation when he saw orange soil at the Taurus-Littrow landing site during the Apollo 17 mission. The beads are some of the finest particles ever brought back from the moon and range in size from 20 to 45 microns, about the same size as the particles that compose silt on Earth.
But during the days of Apollo, the prevailing theory was that the moon was bone dry. Even though trace amounts of water were found in some of the samples returned by the astronauts, it was thought the samples had been contaminated from Earth’s atmosphere, since the sample boxes in which the moon rocks were brought home weren’t airtight.
Although planetary scientists are still refining models of the moon’s formation, the accepted theory is that a Mars-sized body slammed into our early Earth, creating a big disk of debris that would ultimately form into the moon. But in this scenario, any water would have likely been vaporized by the high temperatures generated by the impact and cataclysm that followed, and vapor would have escaped into space.
“The giant impact event that formed the moon was very high energy and very hot,” Milliken said. “The traditional view is that highly volatile compounds like water would not have been able to survive this kind of process. However, multiple lines of evidence, including measurements of the returned Apollo samples as well as remote sensing studies like ours, indicate water is in fact present in the deep lunar interior. So, it either somehow survived the giant impact and moon-forming process or it was delivered later.”
The first confirmed detection of water inside the beads came in 2008, when a group of scientists used improved technology to re-look at the Apollo samples. While they determined there wasn’t a lot of water embedded in the beads – about 46 parts per million – they were able to estimate that the interior of the moon at one time contained an amount of water equal to that of the Caribbean Sea.
Then in 2010 another team released their findings of a surprisingly high abundance of water molecules — as high as several thousand parts per million — bound to phosphate minerals within volcanic lunar rocks, which would have formed well beneath the lunar surface and date back several billion years. Subsequent studies in 2011, 2013, and 2016 using data from M3 as well as NASA’s Lunar Reconnaissance Orbiter also showed concentrations of hydrogen atoms on various locations of the moon that strongly suggested the presence of water molecules.
Milliken and Li said their findings of a wide distribution of water-rich deposits across the surface of the moon is key, as it shows the water found in the Apollo samples isn’t a “one-off.” Since the lunar volcanic deposits seem to be universally water-rich, it suggests the same may be true of the mantle.
“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 in a press release. “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.”
All of these findings have been pushing lunar scientists to find possible alternative explanations for the moon’s formation to account for all the water.
“This is one of the big questions that has yet to be answered,” Milliken said in an email. “Either somehow the water survived the giant impact or moon-forming process or it was delivered later. We don’t know for sure which is correct, but I tend to like that latter option, where water-rich asteroids and comets could have delivered the water while the moon was still cooling and solidifying. The more samples and measurements that we can make of lunar materials, including returning new samples, and the better we understand how water-rich materials were distributed and mobilized throughout the early solar system, then the better we will be able to answer these key questions.”
While the volcanic beads don’t contain a lot of water — about 0.05 percent by weight, the researchers say — the deposits are large and widespread, and the water could potentially be extracted. This is exciting for possible future human exploration of the moon.
“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.”