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.”
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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.”