The Value of the Moon

by Paul D. Spudis

  The Moon has undergone a complex and protracted geological history that we can study to understand early planetary evolution. From Apollo data, we found that the Moon is a differentiated object, with a metallic core, mantle, and crust. Its segregation into this tripartite condition was the result of global melting early in solar system history. If a body as small as the Moon could undergo global differentiation, it is likely that all the terrestrial planets did likewise. The study of early lunar geologic history is a guide to the interpretation of the history of all the rocky planets, and the Moon records events of an epoch for which evidence has been erased from the eroded, dynamic surface of the Earth. After this differentiation, the Moon underwent a protracted impact bombardment, hit by objects from the microscopic to the asteroidal; these collisions formed craters that span similar size ranges. While we understand the impact process in broad outline, details of the physical and compositional processes remain obscure, especially questions about how they scale with size. The Moon’s abundant craters (figure 6.2), on display for our study and enlightenment, offer innumerable examples of this process.

  Figure 6.2. Examples of fresh lunar craters. Rümker E (38.6°N, 302.9°E; 7 km diameter) is a simple crater, with a bowl shape and small, flat floor. Large blocks are visible near its rim crest. The complex crater Aristarchus (23.7°N, 312.5°E; 40 km diameter) shows wall terraces (from slumping after crater excavation), an extensive flat floor (impact melt sheet) and a central peak (brought up from the deep crust). (Credit 6.2)

  Billions of years ago, internal melting of the mantle of the Moon produced copious iron-rich magmas that rose upward to the surface and erupted as vast sheets of basaltic lava. These lavas make up the lunar maria, the dark smooth lowlands of the Moon. They are concentrated on the near side (for reasons that still elude us) and are made up of hundreds of individual flows with differing compositions, volumes and ages. By understanding the sequence of lavas over time, their source regions, and changes in composition, we can reconstruct the thermal and compositional evolution of the lunar deep interior. Again, because volcanism is ubiquitous on the terrestrial planets, knowledge of the lunar experience helps us to better understand this process across the solar system.

  The principal geological process on the Moon for the last three billion years is bombardment by a constant micrometeorite “rain” of tiny particles. The flux of debris acts as a giant “sandblaster,” grinding surface rocks into a fine powder. This layer of disaggregated rocky debris, the regolith, is exposed to space and thus, implanted with particles from sources external to the Moon. Because the Moon has no atmosphere or global magnetic field, plasmas and streams of energetic particles from the Sun, and the universe around us, impinge directly on its surface, becoming embedded onto these lunar dust grains. Thus, the Moon contains a unique, detailed record of the output of the Sun and galaxy through geological time.

  The solar wind is the most common source of particles, a stream consisting mostly of protons that collide with and stick to the lunar dust grains. As this process is constant, particles from the Sun emitted at varying times in history may be recovered from the ancient regolith and used to reconstruct the output of the Sun and galaxy as it was in the distant geological past. A special case occurs when an ancient regolith is buried by a lava flow. In this instance, the covered regolith becomes a closed-system, shut off from further particle implantation. The solar wind gases, preserved in such a closed-system, record a “snapshot” of the ancient Sun, dated by the ages of the bounding rock units above and below the ancient paleoregolith.

  Accessible regoliths on the Moon cover a time range of at least the last four billion years. The Sun is the principal driver of Earth’s climate, and by recovering solar output over time, a record unavailable anywhere on Earth, we can understand its cycles and singular events for the duration of the history of the solar system. Some initial results, from our study of the Apollo samples, suggest that the ancient Sun had a different composition of its nitrogen isotopes than it does now, a puzzling result not predicted by existing theories of stellar evolution. What other new and unexpected secrets of the Sun and stars lie embedded on the Moon, awaiting discovery?

  Because of the antiquity of the Moon, and its proximity to the Earth, the lunar surface retains a record of the impact bombardment history of both bodies. We know that the collision of large bodies has had drastic effects on the geological and biological evolution of the Earth and occur at quasi-regular intervals.4 Because our very survival depends on our understanding the nature and history of these collisions as a basis for the prediction of future events, the impact record on the lunar surface is critical to our understanding of this hazard. By dating a large population of individual craters on a surface of known age, we can establish whether the periodicity of the impact flux is real. Such periodic impacts may have driven the process of evolution on Earth. These studies could uncover fundamental, unknown aspects of the history of life on Earth and in the solar system.

  With no ionosphere, and a far side that is the only known area in the solar system permanently blocking the radio noise and static of Earth, a radio telescope on the far side of the Moon can examine low frequency wavelengths that are impossible to detect from Earth’s surface or in LEO. The seismically quiet lunar surface permits the construction of extremely sensitive and delicate instruments, such as interferometers at optical wavelengths. An array of such telescopes could achieve resolutions at the micro-arc second level, allowing the direct observation of phenomena such as star spots and the hemispheres of terrestrial planets in nearby systems. Such capabilities would revolutionize our understanding of the evolutionary paths of stellar and planetary systems.

  Finally, the environment of the Moon is itself a scientific asset of great value. The hard vacuum and extreme thermal regime permit unique material science experiments. The low gravity of the Moon allows us to quantify the effects of fractional gravity on physical and biological phenomena. The Moon is an isolated and sterilizing environment, permitting experimentation with hazardous materials and processes. Facilities on the lunar surface allow us to conduct dangerous or hazardous experiments that would be unwise to pursue on the Earth. These unique properties make the Moon an unparalleled asset for scientific experimentation and laboratory work.

  It’s Useful: The Utility of the Moon

  While the previous two attributes of the Moon are extremely important, its greatest value is its capacity to create new spacefaring capability through the exploitation of its material and energy resources. The idea of using the materials of other worlds to provision ourselves, and to supply and support spaceflight, is a very old one, but to date, it has not been attempted. Yet, development of this single activity could completely change the paradigm of spaceflight. Currently, anything that we need in space must be transported to Earth orbit at enormous cost, usually on the order of at least $1,000–10,000 per kilogram. This high cost applies to everything: It costs the same amount of money to launch a kilogram of high-technology electronics as it does a kilogram of water. If we could provide low-information density materials (like water, air, and rocket propellant) from local sources already present in space, we could accomplish much more for less money. In a nutshell, this is the driving motivation for the use of off-planet resources, or, in the term used in the business, in situ resource utilization (ISRU).5 This is a skill that we must master in order to become a truly spacefaring species.

  Although the physics and chemistry of extracting and using the resources of the Moon are simple and straightforward, there has been great resistance to incorporating ISRU into any spaceflight architecture. There are many reasons for this attitude, ranging from unfamiliarity with the processes involved to a natural and at least partly understandable conservatism in engineering design. For initial ISRU efforts, we would only undertake the simplest processes, such as bulldozing regolith to make blast berms around landing pads and to cover habitats for radiation shielding, along with heating polar regolith to extract
water ice. These are minimal, low-risk activities that provide useful products and pieces of outpost infrastructure. The techniques needed to begin ISRU are no more complex than everyday eighteenth-century industrial processes.

  The resources of the Moon are simple and require minimal processing. First, bulk regolith (soil) has many uses as thermal and radiation shielding and for construction. Although loose soil can be used as is, regolith can also be fused by microwave sintering or passive solar thermal heating (such as a concentrating mirror) into ceramics or aggregate for building material. Roads and landing pads can be manufactured by sintering the regolith in place using a microwave-heating element mounted on a rover.6 Microwaves fuse loose regolith into brick and ceramic because of the fine-scale, vapor-deposited free iron that coats the surfaces of dust grains. This coating permits RF energy to be efficiently coupled and transferred into heat, so that the grain boundaries fuse together to make glass. A microwave with a power level comparable to a kitchen oven can fuse the upper surface into a paved road or landing pad several centimeters deep. Fused regolith structures can be made as large or as long as needed. Structures and pieces can be produced with 3-D printer technology using fine regolith as feedstock.

  The Moon’s poles possess critical resources needed for long-term human presence on the Moon and in space. They have two key attributes that the rest of the Moon does not possess: water ice (and other volatile substances) and areas of near-permanent sunlight. We have verified the presence of water ice using several techniques of remote sensing, including hydrogen detection, near-infrared and ultraviolet reflectance, laser albedo, radar, and a physical impactor. In addition to water—the most cosmically abundant volatile substance in the solar system—other volatile species are present in the polar ice, including methane (CH4), carbon monoxide (CO), ammonia (NH3), hydrogen sulfide (H2S), and some simple organic molecules. All of these volatile substances can be chemically processed to help support a human presence on the Moon.

  Questions remain over how much water and other volatiles are present in total, on their distribution laterally and vertically, and over what physical form the different chemical ices take. These volatiles probably come from sources external to the Moon—the impact of water-bearing objects, such as cometary nuclei and volatile-rich meteorites. As such, they are deposited in extremely small amounts, in a vacuum and over a very long period. The likely nature of such a deposit would be a very porous mixture of dust grains and amorphous (noncrystalline) ice. In astrophysics, such a compositional fabric is called a fairy-castle structure and is a common state of materials in space.

  The dark areas where ice is stable are extremely cold, always less than —169°C (104 K), but in some cases as cold as —248°C (25 K) and widespread at both poles. These dark areas are typically found in crater interiors but in some cases as extended regions of shadow. The “cold traps” are all equally likely to contain ice, but current evidence suggests, for reasons we do not fully understand, that the ice is distributed heterogeneously (see figure 5.1). In addition, because lunar soil is an excellent thermal insulator, it is possible that extensive deposits of ice might be present in the shallow subsurface, in areas that receive partial solar illumination.

  We need to survey the potential mining areas to determine their content and grade. This is best accomplished by using a small robotic rover that traverses the polar areas and measures ice content and composition over many locations. The dark areas are close to the lit regions, as the grazing sunlight at the poles, both illuminates and shades. Although there are no areas of “permanent” sunlight, certain regions near both poles have been found to be in sunlight for more than 90 percent of the lunar year.7 Solar arrays mounted on a high mast could be in sunlight for longer periods; this possibility is a subject for current research. An outpost located in these areas would be able to generate electrical power on a nearly constant basis, with periods of darkness bridged by power storage, such as the use of a rechargeable fuel cell.

  Another advantage of these “quasi-permanent” sunlit areas is that they are thermally benign. At the equator of the Moon, the surface is heated during the daytime, which is fourteen Earth days long, reaching temperatures of up to 100°C. During the coldest part of the nighttime, also fourteen Earth days long, the surface may assume temperatures as low as —150°C, a 250° swing from the hottest part of the day. The high temperatures of lunar noon put stress on systems designed to keep machinery cool, while the cold night temperatures require moving parts to be heated. Within the sunlit areas near the poles, illumination is always at grazing incidence—that is, the Sun circles around near the horizon—and maintains the surface temperature at a near-constant —50°C. In such an environment, minimal power is required to maintain thermal equilibrium for complex machinery. Along with the pervasive presence of highly abrasive dust that can wear down parts and make machinery inoperative, the extreme thermal environment is one of our biggest technical challenges in developing the resources of the lunar poles. Mitigating strategies for each of these difficulties are currently the subject of intensive research.

  In addition to the constant solar power available at the poles, the Moon contains substances that, in the future, may be used to generate energy for use on the lunar surface and in space. Several regions of the western near side contain elevated amounts of the radioactive element thorium, which can be used to fuel nuclear reactors to generate electrical power. Via several nuclear reactions, thorium breeder reactors can produce their own fuel, making it possible for us to construct space reactors on the Moon. The use of nuclear power would allow us to survive the long lunar night and permit habitation of equatorial and mid-latitude regions of the Moon. The availability of abundant power also enables large-scale industrialization of the Moon.

  In the more distant future, some have proposed that the rare isotope helium-3, implanted in the lunar regolith by the solar wind, could be harvested to generate electrical power in a relatively “clean” nuclear reaction, one that does not generate excess neutrons and “dirty” reaction products.8 The fusion of deuterium (hydrogen-2) with helium-3 produces fewer neutrons and positively charged He ions, permitting the efficient conversion to electrical power over the standard deuterium-tritium (2H-3H) fusion. In fact, a variant of this process, whereby helium-3 fuses with itself (3He-3He), produces no harmful by-products at all. Potentially, helium-3 fusion could solve the world’s energy problems if a suitably large source of the isotope could be found—it is present on Earth as a component of natural gas, but in extremely small amounts.

  It has been proposed that we mine the lunar regolith for helium-3 and import the product back to Earth for commercial electrical power generation. The difficulty with this idea is twofold. First, we do not yet have reactors that can burn helium-3 nuclear fusion fuel. It takes a great deal of energy to start this reaction and then to contain and control it; no fusion reaction to date has achieved “breakeven,” the point at which the fusion reaction liberates more energy than it takes to start it. Research on this problem has been going on for decades; it is unlikely that we will see commercial applications of fusion power generation for many years. Second, although there is helium-3 in the lunar regolith, it is present in concentrations of less than about twenty parts per billion. This low concentration is for sampled sites in the lunar equatorial maria; we do not yet know the concentration of helium-3 in the polar volatiles. Extracting helium-3 from the mare regolith will require the mining and processing of hundreds of millions of tons of regolith, a scale of resource processing that may eventually occur, but certainly not in the early stages of lunar habitation. The mining of helium-3, often alluded to as the ultimate “pay dirt” on the Moon, is not likely near-term (~20 years) but may turn out to be significant in the multidecadal time scales of future lunar development.

  Water is the most useful material in space. In its native form, we can drink it and use it to reconstitute food, cool equipment, and jacket habitats for radiation protection, as well as for h
ygiene and sanitation needs. An electrical current can disassociate water into its component hydrogen and oxygen. These gases can be stored and used; oxygen can be used for breathing, and both gases can be recombined in a fuel cell to generate electricity. Used this way, water is a medium of energy storage. Finally, the hydrogen and oxygen can be cooled into cryogenic liquids and used as rocket fuel, the most powerful chemical propellant known. Because of its utilitarian value, water is truly the “currency” of spaceflight.

  The real lunar “El Dorado” consists of the water ice and the permanent sunlight near the poles. It is a location known to contain resources of material and energy that we can access and use. It is a place where we can learn the skills and technologies needed to become permanent residents of space.

  Why Not Mars?

  Virtually the entire space community, from those inside the agency to others working on spacecraft, missions or data analysis, presume that Mars is the “ultimate goal” for human spaceflight.9 In 1965, the imaginative pull that decades of science fiction and speculation about Mars as an Earthlike planet had dealt us were dashed when we found by direct investigation that the real Mars is a distant, cold, dry desert, with virtually no atmosphere. Subsequent missions over the years have shown that it may have been warmer and wetter in the past, which led to the idea that microbial life might have originated there. This single idea is largely responsible for the subsequent fixation on Mars as the “next destination” for humans in space. The obsession with “searching for life elsewhere” has hijacked our thinking about the future of people in space. It is virtually impossible to advance an idea or concept involving people at some space destination other than Mars, without proving that it “feeds forward” to our “ultimate destination.”