Lunar Material Resources: An Overview*
James L. Carter
Abstract
The analysis of returned lunar samples and a comparison of the physical and chemical processes operating on the Moon and on the Earth provide a basis for predicting both the possible types of material resources (especially minerals and rocks) and the physical characteristics of ore deposits potentially available on the Moon. The lack of free water on the Moon eliminates the classes of ore deposits that are most exploitable on Earth; namely, (a) hydrothermal, (b) secondary mobilization and enrichment, (c) precipitation from a body of water, and (d) placer.
The types of lunar materials available for exploitation are whole rocks and their contained minerals, regolith, fumarolic and vapor deposits, and non lunar materials, including solar wind implantations. Early exploitation of lunar material resources will be primarily the use of regolith materials for bulk shielding; the extraction from regolith fines of igneous minerals such as plagioclase feldspars and ilmenite for the produ.ction of oxygen, structural metals, and water; and possibly the separation from regolith fines of solar-wind-implanted volatiles. The only element, compound, or mineral that by itself has been identified as having the economic potential for mining, processing, and return to Earth is helium-3.
Introduction
To be economical, a lunar base operation requires the identification, characterization, development, and utilization of local resources (Flawn 1966, Dalton and Hohmann 1972, Criswell 1980, Haskin 1983, Carter 1985). Even though it is romantic to dream about exotic and fabulously rich mineral deposits, history has shown us repeatedly that in any area the natural resources that are exploited first are those that (a) are needed for basic survival, (b) are readily available, and (c) can be used with the least modification. Ore deposits that are remote, mineralogically complex, or low grade (and therefore must be dealt with in large volume) are not exploited until after the infrastructure necessary for their exploitation can be constructed. Meeting the criteria for early exploitation is material from the lunar regolith, the layer of debris that covers the surface of the lunar bedrock. Such material can be used as is for bulk shielding to reduce cosmic ray exposure. And from the lunar regolith such desirable elements as oxygen and iron can be extracted without extensive processing such as crushing. The ideal situation ultimately will be to use the lunar regolith material as a feedstock and to separate from it numerous elements and products (Lindstrom and Haskin 1979), but this requires an extensive infrastructure of sophisticated and elaborate processing equipment (Williams et ai, 1979).
In this paper I develop a general overview of what can be inferred from theoretical considerations of the physical and chemical processes operating on the Moon and what is known about possibly available types of lunar materials from analysis of samples returned by the Apollo and Luna missions. My overview will include an evaluation of lunar regolith fines (the less-than-1-mm fraction) as a source of volatile elements.
Physical and Chemical Processes
Comparison of physical and chemical processes operating on the Earth ana on the Moon provides a basis for predicting both the possible types of material resources, especially minerals, and the physical characteristics of ore deposits potentially available for exploitation on the Moon.
Active Surface Agents
One approach to evaluating possible lunar material resources is to compare the active surface agents that affect the Earth and the Moon. These are listed in table 1. The most striking feature described in table 1 is that the Moon has no atmosphere. Therefore, it has (a) no free water (and thus no freeze/thaw cycles and few, if any, water-bearing minerals such as clays), (b) no free oxygen (and thus no oxidation), and, most importantly, (c) no biological activity. The major physical (erosional) and chemical (weathering) agents-water and oxygen, respectively-and the resulting products we are familiar with on Earth play no role in shaping the surface of the Moon and thus they play no role in the formation of potential lunar ore deposits. The only indigenous lunar erosional agent is volcanic, especially basaltic, lava flows (Hulme 1973). The extremely low viscosities and thus the high extrusion rates of lunar basaltic lava flows (Moore and Schaber 1975) are conditions f~vorable for the formation of lava channels and tubes, which are very abundant on the lunar surface (Oberbeck et al. 1971). It may be possible to use the naturally sheltered environment of a large lava tube as housing for a lunar base (Harz 1985). Moreover, some lava tubes may contain accumulations of volatiles.
The lack of an atmosphere on the Moon allows meteorites, comets, micrometeorites, and the solar wind to bombard the lunar surface unimpeded. These are the most important agents shaping the lunar surface. They also contribute to its material resources (table 2; see also Williams and Jadwick 1980).
* This paper is based in part on research supported by NASA-JSC grant NAG-9-99.