Scientists at Brown University have used satellite data to discover water within ancient explosive volcanic deposits on the Moon, indicating that its interior contains substantial amounts of indigenous water. They found that volcanic deposits distributed across the surface of the Moon contained a surprisingly large amount of trapped water in comparison with surrounding terrains. The ancient deposits, consisting of glass beads that were formed by the explosive eruption of magma originating from the depths of the lunar mantle (hundred of kilometers below the surface), reinforces the idea that the lunar mantle is rich in water.
The discovery was made by Ralph Milliken, assistant professor of earth, environmental and planetary sciences at Brown University, and Shuai Li, a postdoctoral researcher at the University of Hawaii and a recent Brown PhD graduate.
The study is published in Nature Geoscience.
Up until recently scientists believed that the interior of the Moon has been depleted of water and other volatile compounds. In 2008, however, a research team led Alberto Saal, professor of earth, environmental and planetary sciences at Brown University, found trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon. Three years later, the same team found similar amounts of water in crystalline deposits within those beads as some basalts on Earth. These findings suggest that at least parts of the Moon’s mantle (hundred of kilometers below the surface) contain as much water as Earth’s.
Milliken and Li used the previous findings as a springboard for their study. Milliken told The University Network (TUN) that the 2008 Saal study “was not clear” if the glass beads samples or the magmas “were truly representative of the bulk mantle of the Moon” or were merely “anomalies in an otherwise ‘dry’ mantle.”
“At the surface, there are a number of very large pyroclastic deposits that have long been recognized based on satellite data, but none of these were directly sampled by the Apollo missions,” he told TUN. “So is there evidence for water in these large pyroclastic deposits? This was the question we wished to answer, and by examining some of the newer orbital data (near-infrared reflected sunlight) we were able to show that yes, most of the large pyroclastic deposits do in fact show evidence for water. We interpret this water to be similar to what our colleague Alberto measured in the returned samples, that is, water trapped in the volcanic glass beads.”
In arriving at their conclusion, Milliken explained that they had to “remove a component in these data that was due to thermally emitted energy.” They had “to estimate and remove this component in order to isolate the part that is due solely to reflected sunlight and not due to energy being emitted by the surface because of its temperature.”
The researchers were able to exclude data from thermally emitted energy by using a combination of laboratory-based measurements of Apollos missions samples and a detailed temperature profile of the areas of interest on the Moon’s surface to study data from an orbital spectometer.
Milliken credits Li for leading the effort. “[B]ecause of his hard work we now have a much better understanding of the distribution and abundance of water on the Moon and what this means for lunar evolution,” he told TUN.
Milliken feels positive about the results of their research on future lunar exploration. “What excites me about this project is that it shows the importance of having global-scale satellite data that can be directly linked to small-scale measurements of samples in a lab setting,” he said. “Having data at these very different spatial scales really allows us to try and piece together a complete understanding of the evolution of a planetary body, in this case, the Moon.”
Does this mean that one day humans could live on the Moon? Milliken believes that it is in theory possible. While the water in pyroclastic deposits is small, possibly measuring about 0.05 weight percent at most and making it necessary to harvest 100-300 cubic feet of the pyroclastic material to get a liter of water, it may be easier to access these deposits than other “potential water ice deposits in permanently shadowed regions at the lunar poles.” More needs to be done, however, to determine the feasibility of people living on the Moon. “I think the key at this point in time is that we really need to sample these pyroclastic deposits directly in order to better understand their water content, how it might be extracted, and whether or not it is economically feasible to do so,” Milliken told TUN.