Much attention has been paid over the past two decades to the idea of water on the Moon—one of the most counter-intuitive concepts around. Most of this attention has focused on the presence of ice in the permanently dark polar regions of the Moon. This water ice is probably derived from sources external to the Moon—impacting meteorites and comets that contain ices and water-bearing minerals. Complementing this idea is the recent discovery of water within tiny glass beads derived from deep within the Moon, released during volcanic eruptions that occurred billions of years ago. This discovery, from lunar sample analyses, is bolstered in a recently published paper that describes remotely sensed detection of water in regional deposits of volcanic ash on the Moon, called pyroclastics (literally, “fire-broken”).
The significance of the new work is two-fold. First, it is additional evidence that the Moon’s mantle (i.e., the magnesium- and iron-rich zone below the crust) contained significant amounts of water at one time. The presence of this water is difficult to explain; it is not what one would expect from the extremely high temperature environment inferred from the collisional-ejection model for the origin of the Moon (the one currently favored by most lunar scientists). The fact that significant water exists (or existed) deep within the Moon indicates that water was part of the original lunar accretion disk from the beginning, which also implies that the Earth had significant water at that time as well. Secondly, because these tiny glass beads contain significantly more water (~several hundred parts per million) than the 10-50 ppm of typical lunar soils, thoughts naturally turn to mining the pyroclastics to extract water in order to support the future exploration of space.
Although that latter activity is possible, for several reasons it makes more sense to focus initially on the polar ice deposits to establish a lunar surface presence. Although the new observations suggest much larger than normal quantities of water, the concentrations in these deposits are still much lower than those of the polar cold traps—200 to 400 ppm is about 1,000 times lower in concentration than the typical polar ice deposits, which range from a few to several tens of weight percent. Most of the water in lunar pyroclastic glass is trapped within the glass beads, meaning that some process is needed to extract it. Such a process will likely involve either mechanical or thermal processing and will probably require large amounts of energy.
Virtually all of the regional pyroclastic deposits on the Moon are found within 25 degrees of the equator, meaning that these localities experience the normal lunar day/night cycle of 14 days of sunlight and 14 days of darkness. To undertake mining operations in this region requires a long-lived, constant power source. In the past, studies have almost always fallen back on the deployment of a megawatt-class nuclear reactor. Such a device does not now exist, and the development of that technology (halted in the 1990s) would likely require at least a decade of work and several tens of billions of dollars. I shudder at the idea of writing the environmental impact statement for such a project.
All of these concerns are eliminated or greatly alleviated by water harvesting at the poles of the Moon. The higher concentrations of polar ice deposits means that orders of magnitude less mass has to be moved and processed in these locations. The polar deposits consist of chemically unbound, physically discrete ice, probably mixed with varying quantities of lunar regolith. As such, it will require much less effort to separate the water from its host material at the poles than at the equator. But the most significant difference in operations is the presence of near-permanent sunlight at the poles. Because of the low obliquity of the Moon’s spin axis, the sun at the poles is always at or very near the horizon. In consequence, multiple locations at both poles experience sunlight for 80 to 95 percent of the lunar day, as the sun circles around the horizon versus rising and setting. This means that surface operations can be undertaken at the poles of the Moon at any time, and water harvesting can be conducted with continuous (rather than batch) processing. In terms of both the quality of the ore and ease of extraction, the poles offer a much better locality to collect lunar water, for all of the myriad uses it can offer for space operations—making the poles prime real estate (Location, location, location!), a fact that has not eluded China’s ongoing and active Moon architecture.
Beyond the immediate appeal and prospects of lots of ice at the poles, ash deposits may have accessible water—not the water derived from the deep lunar interior, but water derived from the Sun. The Moon has no atmosphere or magnetosphere, so particles of the solar wind (the effluence of material that streams out of the Sun) impinge directly on the lunar surface. In this process, solar wind protons implant themselves on the dust grains of the regolith. The quantities of solar wind hydrogen are correlated with composition and grain size, being enhanced in iron- and titanium-rich compositions and in the finer grain-sized fractions of the soil. A proton is simply a hydrogen ion, and in the presence of heat, these protons chemically reduce metal oxides in the lunar minerals and make a hydroxyl (OH) or water (H2O) molecule and native, metallic iron (Fe0). In fact, it may well be this process, and not water from the lunar interior, that is at least partly responsible for the enhanced amounts of water in the pyroclastics seen in the remote data.
Pyroclastic deposits are formed when silicate liquids (magmas) rich in dissolved volatiles (such as water) are erupted at high velocity through narrow conduits. The spray of liquid rock is jetted into space, cooling in flight into small (tens of microns) beads of liquid that quickly quench into glass spheres. The resulting eruption produces a deposit consisting of uniformly-sized glass beads that cover, drape and smooth the existing terrain. These properties make pyroclastic deposits potentially ideal material for mining—smooth landing sites, uniform and tiny (40 micron) grain sizes and iron- and titanium-rich compositions that offer solar wind protons many sites for attachment. These properties mean that dark pyroclastics on the Moon probably contain high levels of implanted solar wind gas. The significance of this property is that adsorbed solar wind hydrogen can be easily released by simple heating: 90 percent of such gas is released when regolith is heated to temperatures of 700 degrees C.
If the water features seen in the remote sensing data relate to water from the Sun rather than water from the deep interior, it will be much easier to extract this type of water than if it is contained only within the glass, making regional pyroclastics beneficial for water harvesting in the future. Although of low grade (compared to the polar ice), once we have established a permanent presence on the Moon, operations might expand out to include low-latitude sites. The possibility of mining lunar pyroclastics for solar wind gas makes them an attractive potential prospect.