Lunar Resources: a Review Reprints and Permission: Sagepub.Co.Uk/Journalspermissions.Nav DOI: 10.1177/0309133314567585 Ppg.Sagepub.Com Ian A

Article

Progress in Physical Geography 2015, Vol. 39(2) 137–167 ª The Author(s) 2015 Lunar resources: A review Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0309133314567585 ppg.sagepub.com Ian A. Crawford Birkbeck College, University of London, UK

Abstract There is growing interest in the possibility that theresourcebaseoftheSolarSystemmightinfuturebe used to supplement the economic resources of our own planet. As the Earth’s closest celestial neighbour, the Moon is sure to feature prominently in these developments. In this paper I review what is currently known about economically exploitable resources on the Moon, while also stressing the need for continued lunar exploration. I find that, although it is difficult to identify any single lunar resource that will be sufficiently valuable to drive a lunar resource extraction industry on its own (notwithstanding claims sometimes made for the 3He isotope, which are found to be exaggerated), the Moon nevertheless does possess abundant raw materials that are of potential economic interest. These are relevant to a hierarchy of future applications, beginning with the use of lunar materials to facilitate human activities on the Moon itself, and progressing to the use of lunar resources to underpin a future industrial capability within the Earth-Moon system. In this way, gradually increasing access to lunar resources may help ‘bootstrap’ a space-based economy from which the world economy, and possibly also the world’s environment, will ultimately benefit.

Keywords Moon, lunar resources, space economy, space exploration

I Introduction names Astrobotic Technology, Deep Space Industries, Golden Spike, Moon Express, Plane- To-date, all human economic activity has tary Resources and Shackleton Energy Com- depended on the material and energy resources pany; details may be found on their respective of a single planet, and it has long been recog- websites). As the Earth’s closest celestial neigh- nized that developments in space exploration bour, the Moon seems likely to play a major role could in principle open our closed planetary in these activities. economy to essentially unlimited external As a result of multiple remote-sensing mis- resources of energy and raw materials (e.g. sions conducted over the last two decades (see Hartmann, 1985; Lewis, 1996; Martin, 1985; Crawford et al. (2014) for a recent summary), Metzger et al., 2013; Schultz, 1988; Wingo, combined with the continued analysis of 2004; a thorough review of the literature as it stood at the end of the 20th century has been given by Hempsell, 1998). Recently, there has been renewed interest in these possibilities, with Corresponding author: Ian A. Crawford, Department of Earth and Planetary several private companies established with Sciences, Birkbeck College, University of London, Malet the stated aim of exploiting extraterrestrial Street, London, WC1E 7HX, UK. resources (these include companies with the Email: [email protected] 138 Progress in Physical Geography 39(2)

Figure 1. Lunar nearside (left) and farside (right) image mosaics, with the landing sites of the six Apollo and three Luna sample return missions indicated. Also marked are locations mentioned in the text: OP: Oceanus Procellarum; MI: Mare Imbrium; MT: Mare Tranquillitatis; SPA: South Pole-Aitken Basin; S: Schro¨dinger Basin (image courtesy, USGS/KH Joy). samples collected by the Apollo and Luna mis- In this paper I first summarise lunar geology sions 40 years ago (Heiken et al., 1991; Jolliff for readers who may be unfamiliar with it. I then et al., 2006), we are now able to make a first- review what is currently known about the order assessment of lunar resource potential. resource potential of the Moon, stressing the In doing so, it is useful to distinguish between limitations of present knowledge, and briefly three possible uses for lunar materials: (i) the discuss future exploration activities that will use of lunar materials to facilitate continued be required if we are to increase our knowledge exploration of the Moon itself (an application of lunar resource potential. I then discuss how usually referred to as In Situ Resource Utilisa- these resources might be economically utilized tion, or ISRU); (ii) the use of lunar resources within the three categories of potential applica- to facilitate scientific and economic activity tions identified above, and the international and in the vicinity of both Earth and Moon (so- legal context within which these developments called cis-lunar space, including operations will take place. My conclusions are presented in Earth orbit) as well as support for future in the final section. space activities elsewhere in the Solar Sys- tem; and (iii) the importation of lunar resources to the Earth’s surface where they II Lunar geology relevant to would contribute directly to the global econ- identifying possible resources omy. These three possible applications of The lunar surface is divided into two main geolo- lunar resources are not mutually exclusive, gical units: the ancient, light-coloured lunar although they do represent a hierarchy of highlands, and the darker, generally circular, increasing difficulty and, therefore, a tem- lunar mare (‘seas’) which fill the large impact poral order of likely implementation. basins, predominantly on the nearside (Figure. 1). Crawford 139

Figure 2. Distribution of regolith compositions on the lunar nearside (left) and the farside (right) based on Clementine multi-spectral imaging data. Blue: anorthositic highlands; yellow: low-Ti basalts; red: high-Ti basalts. The large yellow/greenish area in the southern hemisphere of the farside is the South Pole-Aitken Basin, where the colours mostly reflect the more Fe-rich nature of the lower crust exposed by the basin rather than basaltic material (Spudis et al., 2002; courtesy Dr Paul Spudis/LPI).

The compositions of these materials are now rea- orthopyroxene ((Mg, Fe)SiO3), clinopyroxene sonably well-characterised (see Heiken et al., (Ca(Fe, Mg)Si2O6), olivine ((Mg, Fe)2SiO4), 1991; Jaumann et al., 2012; Jolliff et al., 2006 for and ilmenite (FeTiO3)), although the propor- reviews), and this information is summarised in tions of these minerals differ in different lava Figures 2 and 3. The lunar highlands are thought flows. It follows that the mare basalts are rela- to represent the original crust of the Moon, and tively richer in Mg, Fe and Ti, and relatively are composed predominantly of anorthositic poorer in Ca and Al (although they are far from rocks (i.e. rocks containing more than 90% plagi- lacking in these elements owing to the ubiqui- oclase feldspar). Lunar plagioclase is invariably tous presence of plagioclase; see Figure 3). The Ca-rich, and to a first approximation the domi- principal classification of lunar basalts is based nant mineralogy of the highlands is anorthite on their Ti content: they are classed as ‘low Ti’ (CaAl2Si2O8), with iron and magnesium- if their TiO2 abundances are in the range of 1– bearing minerals (principally pyroxene and oli- 6% by weight (hereafter wt%), and ‘high-Ti’ vine) typically contributing only a few percent is TiO2 which is >6 wt% (Neal and Taylor, by volume. Thus, the lunar highlands are rich 1992). The main Ti-bearing phase in lunar rocks in Ca, Al, Si and O, but relatively poor in Mg and is the mineral ilmenite which, as discussed Fe (see Figure 3). below, is of particular importance for the reten- The lunar maria, on the other hand, are com- tion of some solar wind implanted volatiles and posed of basaltic lava flows. Their mineralogy for some proposed ISRU oxygen extraction pro- is dominated by a combination of five minerals cesses. In this context it is important to note that (plagioclase (mostly anorthite: CaAl2Si2O8), the ilmenite-bearing high-Ti basalts are mostly 140 Progress in Physical Geography 39(2)

Figure 3. Example chemical compositions of lunar soils: (a) lunar highland minerals (Apollo 16); (b) low-Ti basalts (Apollo 12); and (c) high-Ti basalts (Apollo 11). Based on data collated by Stoeser et al. (2010) and reprinted from Planetary and Space Science, Vol. 74, Schwandt C, Hamilton JA, Fray DJ et al., ‘The production of oxygen and metal from lunar regolith’, pp.49–56, Copyright (2012), with permission from Elsevier. confined to two geographically restricted near- et al., 1991), but also contains much larger rock side mare regions (i.e. Oceanus Procellarum in fragments (the sub-cm size fraction is often the west and Mare Tranquillitatis in the east; see referred to as the lunar ‘soil’). The mineralogi- Figure 2), and are therefore not a globally dis- cal and geochemical composition of the regolith tributed resource. at any given location largely reflects the compo- The entire lunar surface is covered in an sition of the underlying rock units, with a small unconsolidated layer of regolith which has been (generally 2%) additional meteoritic compo- produced by billions of years of meteorite and nent. Understanding the regolith is important micro-meteorite impacts (Lucey et al., 2006: for any consideration of lunar resources because McKay et al., 1991). The regolith is several it will form the basic feedstock for most pro- metres thick in mare regions, and perhaps 10 cesses which may be envisaged for extracting or more metres thick in the older highland areas, and refining lunar raw materials. and although the uppermost few cm have a pow- From a resource perspective, a particularly dery consistency (Figure 4) it becomes very interesting sub-set of lunar soils are the deposits compacted (relative density ~ 90%) by a depth of volcanic ash, otherwise known as pyroclastic of 30 cm (Carrier et al., 1991). The regolith has deposits, that appear to have been produced by an average grain size of about 60 mm (McKay fire-fountaining volcanic eruptions in some Crawford 141

a feedstock more straightforward (Hawke et al., 1990). With a few exceptions (discussed elsewhere in this paper), the extent to which the lunar crust may contain bodies of locally concentrated ‘ores’ of economically exploitable materials is still largely unknown. Given the Moon’s lack of atmosphere, and generally anhydrous nature (Vaniman et al., 1991), it seems certain that hydrothermal and biological processes, which are responsible for much ore formation on Earth, cannot have operated on the Moon. On the other hand, large quantities of silicate melts, from which economically valuable materials can be concentrated through fractional crystalli- sation and gravitational settling, have certainly existed in lunar history. Examples include: (i) Figure 4. Close up view of the lunar regolith with an early ‘magma ocean’ phase, when most or all Apollo 11 astronaut Edwin ‘Buzz’ Aldrin’s boot for scale, showing consistency of the uppermost surface of the Moon appears to have been molten (e.g. (NASA image AS11-40-5880). Elkins-Tanton et al., 2011); (ii) subsequent production of magnesium-rich magmas and their mare areas (e.g. Gaddis et al., 2003; Taylor intrusion into the lunar crust (e.g. Taylor et al., et al., 1991). Such deposits are identified from 1991); (iii) thick sequences of impact melt orbital remote-sensing observations as low deposits resulting from basin forming impacts albedo (‘dark mantle’) surficial material sur- (e.g. Vaughan et al., 2013); (iv) large-scale rounding known or presumed volcanic vents eruption of mare basalts of varying composi- (Figure 5). Over 100 possible lunar pyroclastic tions; and (v) more localised volcanic activity deposits have been identified (Gaddis et al., exhibiting an even wider range of magma com- 2003), with 20 having areas greater than 1000 positions (including some highly evolved, km2. Samples thought to be representative of silica-rich magmas which are otherwise rare these materials were collected at the Apollo 15 on the Moon; e.g. Glotch et al., 2010). These and 17 landing sites and found to consist of considerations led Papike et al. (1991) to con- small (tens of mm in diameter) glass, or partially clude that it is possible that ‘layered ore deposits crystallized, beads of essentially basaltic com- similar to or even larger than those of Earth may position. These deposits may have significant occur on the Moon’. resource implications for several reasons: (i) In addition, although true hydrothermal pro- they are relatively enhanced in volatiles com- cesses cannot have occurred on the lunar sur- pared to most lunar regoliths (e.g. Li and Milli- face, recent evidence that the lunar mantle, ken, 2014: Taylor et al., 1991); (ii) the glass and therefore partial melts derived from it, may component is more easily broken down for the not be as anhydrous as once thought (see extraction of oxygen than are crystalline sili- reviews by Anand, 2010; Anand et al., 2014) cates (Taylor and Carrier, 1993); and (iii) they means that we cannot be completely certain that are probably much more homogeneous in size hydrothermal concentration of economically and composition, and less compacted, than the useful materials has not occurred in localised, general regolith, which will make their use as and as yet undiscovered, magmatic settings. 142 Progress in Physical Geography 39(2)

Figure 5. Pyroclastic deposits on the Moon. (a) Outcrop of high-Ti orange glass at the Apollo 17 landing site; for scale, the gnomon’s feet are about 50 cm apart (NASA image AS17-137-20990). (b) Dark pyroclastic materials surrounding a presumed volcanic vent in the Schro¨dinger Basin on the lunar farside (see Fig. 1 for location; image obtained by NASA’s Lunar Reconnaissance Orbiter Camera, image courtesy NASA/ASU).

Indeed, there is evidence that volatile-mobilized of current knowledge concerning various differ- elements (e.g. F, S, Cl, Zn, Cd, Ag, Au, Pb) have ent categories of potential resources. become concentrated by the volcanic outgas- sing responsible for the formation of lunar pyroclastic deposits (e.g. Haskin and Warren, 1 Solar wind implanted volatiles 1991). Finally, we need to be aware that the Because the Moon has no atmosphere or mag- unique lunar environment may permit the con- netic field, the solar wind impinges directly onto centration of economically valuable materials its surface, and solar wind particles are thereby by processes that do not operate on the Earth. implanted directly into the lunar regolith. The This is especially true of the lunar poles, where solar wind dominantly consists of hydrogen and water and other volatiles may become trapped helium nuclei (with a He/H number ratio of in permanently shadowed craters (Feldman about 0.04), with heavier elements comprising et al., 1998) and, more speculatively, where pro- less than 0.1 percent by number (e.g. Wurz, cesses associated with the electrostatic charg- 2005). Analyses of samples returned by the ing, migration and trapping of metallic dust Apollo missions have shown that the lunar rego- particles may also occur (e.g. Platts et al., 2013). lith is efficient in retaining these solar wind implanted ions, which, depending on the age of the surface, have therefore been accumulat- III Potential lunar resources ing for hundreds of millions to billions of years. Haskin et al. (1993) gave a comprehensive geo- Studies of the Apollo samples have shown that chemical assessment of possible lunar resources most of these solar wind implanted volatiles can based on our knowledge in the early 1990s, and be degassed from the regolith by heating it to this work has been updated by, among others, temperatures of between 300 and 900C Duke et al. (2006), Schrunk et al. (2008) and (depending on the element outgassed; Fegley Anand et al. (2012). Here I summarize the state and Swindle, 1993), with 700C sufficient to Crawford 143

Table 1. Average concentrations of solar wind concentrations are approximately constant implanted volatiles in the lunar regolith (Fegley and within the uppermost 2–3 metres (Fegley and Swindle 1993), where the quoted errors reflect the Swindle 1993). With this in mind, Table 1 also range (+ one standard deviation) of values found gives the masses of these materials expected to at different sampling locations. The corresponding 3 3 be contained within a regolith volume of 1 m . average masses contained within 1 m of regolith (assuming a bulk density of 1660 kg m-3; Carrier In addition to the volatiles listed in Table 1, et al., 1991) are also given. lunar soils contain small quantities (typically 1 mg/g) of the solar wind-derived noble gasses 3 Concentration Average mass per m Ne and Ar (and much smaller quantities of Kr Volatile ppm (mg/g) of regolith (g) and Xe). Perhaps also interesting from a H46+ 16 76 resource perspective, lunar soils contain a not 3He 0.0042 + 0.0034 0.007 insignificant quantity of sulphur (715+216 4He 14.0 + 11.3 23 mg/g; Fegley and Swindle 1993) which has a C 124 + 45 206 range of industrial applications (e.g. Kutney, N81+ 37 135 2013); this is mostly derived from the break- F70+ 47 116 down of the mineral troilite (FeS), and would Cl 30 + 20 50 probably also be released by any process which extracts the other volatile elements. release most of the trapped H and He (Gibson There is an important caveat to note regard- and Johnson 1971; Haskin and Warren, 1991). ing the global distributions of solar wind Fegley and Swindle (1993) conducted a thor- implanted volatiles, and that is their unknown ough review of the concentrations of solar wind concentrations at high latitudes. All the num- implanted volatiles in the lunar regolith. The bers given in Table 1 are based on samples col- concentrations vary somewhat between the var- lected at low latitudes on the nearside (see ious Apollo and Luna sampling locations, in Figure 1). Although high lunar latitudes are part owing to the different regolith grain sizes exposed to a lower solar wind flux, volatiles and mineralogies (to which He is especially sen- may be retained more easily owing to the lower sitive). Allowing for this variation, Fegley and surface temperatures. There is remote-sensing Swindle find the average concentrations given evidence (Pieters et al., 2009) that this is the in Table 1. There is some evidence, based on the case. Indeed, as discussed in sub-section III(3), interpretation of orbital neutron spectroscopy high-latitude soils may contain several hundred (Sinitsyn, 2014), that H concentrations may ppm water derived ultimately from solar wind exceed 100 ppm (parts-per-million) in some interactions with the surface (e.g. Crider and highland areas not represented in the existing Vondrak, 2000; Ichimura et al., 2012), although sample collection. This is more than twice the quantifying its importance will require collect- average value listed in Table 1, and if solar wind ing samples, or making in situ measurements, implanted volatiles are to be utilised as a at much higher latitudes than has been attempted resource then further exploration of such areas to-date. may be called for. It should be noted that extracting these solar It is important to realise that, owing to the wind implanted materials will be quite energy continued overturning (‘gardening’) of the intensive. The ambient temperature of the rego- regolith by meteorite impacts, solar wind lith below the uppermost few centimetres is implanted volatiles are expected to be present about 20C (Vaniman et al., 1991), so extract- throughout the regolith layer, and studies of ing the bulk of the implanted volatiles will the Apollo drill cores indicate that their require raising this by about 720C. The specific 144 Progress in Physical Geography 39(2) heat capacity of lunar regolith is a function of where p represents a proton, and 18.4 MeV is temperature (see discussion by Rumpf et al., the energy released per reaction (1 mega- 2013), with an average value of about 900 J electron volt, MeV¼ 1.60210–13 Joules). 3He Kg-1 K-1. The regolith has a bulk density of is very rare on Earth whereas D is relatively approximately 1660 kg m-3 (Carrier et al., abundant in sea water (D/H¼1.6104), which 1991), so the energy required to raise the tem- has given rise to the suggestion that 3He might perature of a cubic metre of regolith by 720C be mined on the Moon and transported to Earth will be of the order of 109 J. This is approxi- where it could be used in future nuclear fusion mately the amount of energy which falls on a reactors employing the above reaction to pro- square metre of the equatorial lunar surface duce electrical power (Schmitt, 2006; Taylor (i.e. a surface perpendicular to the Sun) in nine and Kulcinski, 1999; Wittenberg et al., 1986). days. Application of the required energy to the Conceivably, it might also be useful for nuclear regolith might be achieved by direct concentra- fusion power in space. tion of sunlight (e.g. Nakamura and Senior, The first point to note is that, although it is 2008) or by microwave heating of the soil sometimes claimed that 3He is ‘abundant’ on the (Taylor and Meek, 2005). In any case, as many Moon (e.g. Tronchetti, 2009), this is actually far cubic metres of regolith will need to be pro- from being the case. As shown in Table 1, its cessed to yield useful quantities of these materi- average abundance in the regolith is about 4 als (see Table 1), it is clear that a significant parts-per-billion (ppb) by mass, with the highest processing infrastructure will be required. measured concentrations in lunar regolith sam- As will be discussed in more detail in ples being about 10 ppb (Fegley and Swindle, Section V, once extracted, some of these vola- 1993). Schmitt (2006) has argued that some of tile elements clearly have potential uses. For the 3He originally implanted in lunar samples example, solar wind implanted H could be may have been lost as a result of agitation dur- useful as a rocket fuel and as a reducing agent ing collection, transport to Earth, and storage for some of the schemes proposed for extract- and handling before the measurements were ing oxygen and metal from metal oxides (see made, and that therefore the measurements may sub-sections below); lunar 4He might concei- underestimate the actual concentrations (per- vably be a useful addition to terrestrial helium haps by as much as 40%). This suggestion is reserves; in the context of long-term human plausible, although proving it will require operations on the Moon, N may be required renewed sampling or in situ measurements on as a buffer gas breathable air, and local sources the lunar surface. In any case, it does not alter of C, N and S are likely to be required to support the basic fact that, by any objective standard, the lunar agriculture (see discussion by Kozyrovska 3He isotope is actually very rare in lunar soils, et al., 2006). However, most interest has focused having only ppb concentrations. on the light isotope of helium, 3He, to which we The second point to note is the non-uniform now turn. spatial distribution of lunar 3He. Studies of the Apollo samples have demonstrated that both isotopes of solar wind implanted He are prefer- 2 Helium-3 entially retained in the mineral ilmenite (Fegley In principle, and given a suitably designed reac- and Swindle, 1993), from which it follows that 3 tor, 3He could be fused with deuterium (D) to He will be most abundant in regoliths devel- yield energy through the nuclear reaction oped on high-Ti mare basalts. In addition, as would be expected, it has been found that the 3 4 D þ He ! He þ p þ 18:4 MeV He abundance correlates with measures of the Crawford 145

Figure 6. Estimated concentration of 3He (parts-per-billion by mass) in the lunar regolith: (a) nearside; (b) farside. The white contours in (a) enclose areas of enhanced 3He concentration in Oceanus Procellarum (left) and Mare Tranquillitatis (right; cf. Fig. 1). Reprinted from Icarus, Vol. 190, Fa W and Jin Y-Q, ‘Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer’, pp.15–23, Copyright (2007), with permission from Elsevier. ‘maturity’ of the lunar regolith (essentially a twice the highest concentration actually mea- measure of the length of time to which they suredinaregolithsample to date. Taking the have been exposed to the solar wind). As lunar 20 ppb concentrations estimated by Fa and Jin remote-sensing measurements can estimate (2007) for Mare Tranquillitatis and Oceanus both the Ti concentration and maturity of lunar Procellarum, which would clearly be the most soils, it is possible to map the estimated 3He economically exploitable deposits, and ass- concentrations in regions that have not yet been uming a regolith thickness of 3 m, would yield sampled (e.g. Fa and Jin, 2007; Fegley and a total mass of 3He for these two regions of Swindle, 1993; Johnson et al., 1999). about 2108 kg. Figure 6 shows the results of one such exer- Finally, it is important to reiterate the cise (Fa and Jin, 2007), which clearly shows point made above regarding the unknown dis- the expected non-uniform distribution of lunar tribution of 3He at high latitudes. Although 3He, with the high-Ti mare basalts of Mare the well-documented correspondence of 3He Tranquillitatis and Oceanus Procellarum dom- concentration with ilmenite abundance would inating. In these two areas, which collectively lead us to expect low polar concentrations, as cover about 2 million km2, the concentrations indicated in Figure 6, the extent to which the of 3He may exceed 20 ppb. This is about five lower temperatures of polar regoliths may times the global average value of 4.2 ppb found enhance retention of 3He is not currently by Fegley and Swindle (1993), and is about known. 146 Progress in Physical Geography 39(2)

3 Water (Colaprete et al., 2010). Supporting observa- The Moon is generally considered to be an anhy- tions have come from space and Earth-based drous body, and indeed until recently everything radar instruments (e.g. Nozette et al., 2001; Spu- we have learned from the analysis of lunar sam- dis et al., 2013), which may indicate the addi- ples and remote-sensing data indicates that it is tional presence of substantial ( 2 m thick) highly depleted in water and other volatiles deposits of relatively clean ice in some perma- compared to the Earth (e.g. Haskin and Warren, nently shadowed regions, although the interpre- 1991; Papike et al., 1991). However, in recent tation of these data has been questioned (e.g. years lunar exploration has shown that the Campbell et al., 2006; Starukhina, 2012). Con- Moon’s surface environment may contain a firmation of the presence of water ice in these number of possibly exploitable water reservoirs regions and the form the ice takes (e.g. blocks (reviewed by Anand, 2010; Anand et al., 2014). of relatively pure ice, or ice crystals mingled Clearly these will never be of any economic with the regolith) will almost certainly require value to planet Earth, but they may prove valuable additional in situ measurements. in the context of a future space-based economy McGovern et al. (2013) have determined that the cumulative area of permanently shadowed (e.g. Spudis and Lavoie, 2011; see discussion in 2 Section V). lunar surface is 13,361 km in the northern hemisphere and 17,698 km2 in the southern Of particular interest are craters near the 2 lunar poles, the floors of which are in permanent hemisphere, giving a total area of 31,059 km . As expected, in both hemispheres most areas of shadow owing to the low (1.5 ) obliquity of the Moon’s rotation axis. It was recognized by Wat- permanent shadow occur pole-ward of 80 north or south latitude, but small patches of permanent son et al. (1961) that water ice may be stable in such permanently shadowed ‘cold traps’, where shadow are identified at latitudes as low as 58 in surface temperatures have now been measured both hemispheres. The extent to which any or all to be below 40 K (Paige et al., 2010). Any water of these permanently shadowed areas contain ice found within such cold traps is likely ulti- water ice and other volatiles is not currently mately to be derived from the impacts of comets known. Clearly, if they all possessed ice at the and/or hydrated meteorites with the lunar sur- level implied by the LCROSS measurements in face, although it is also possible that H O and/ Cabeus then the total quantity of water could be 2 very high; assuming a concentration of 5.6% by or OH molecules produced by interactions –3 between the solar wind and the regolith at lower mass and a regolith density of 1660 kg m ,the latitudes (Ichimura et al., 2012) may migrate to total mass of water contained within the uppermost metre of permanently shadowed regolith the polar regions and become trapped there. 12 Indirect evidence for ice in permanently sha- would be 2.910 kg (or 2900 million tonnes). dowed polar craters was provided by the neu- In addition to possible ice in permanently tron spectrometer on the Lunar Prospector shadowed craters, infra-red remote-sensing Spacecraft (Feldman et al., 1998, 2001). This observations (Pieters et al., 2009) has found evi- interpretation was supported by the results of dence for hydrated minerals (and/or adsorbed NASA’s Lunar Crater Observation and Sensing water or hydroxyl molecules) covering high- Satellite (LCROSS) mission, which impacted latitude, but non-permanently shadowed, areas into a permanently shadowed region of the of the lunar surface (Figure 7). It is thought that southern polar crater Cabeus in 2009 and mea- this OH/H2O, which cannot exist as ice in sun-lit sured a water ice concentration of 5.6 + 2.9 areas, is produced by the reduction of iron oxi- wt% (1 sigma errors) in the target regolith des in the regolith by solar wind implanted hydrogen (Crider and Vondrak, 2000; Ichimura Crawford 147

Last, but not least, we note that there is evidence that lunar pyroclastic deposits have significant levels of hydration. Laboratory measurements of the Apollo 15 and 17 pyroclastic glasses have water contents mostly in the range 5–30 ppm (Saal et al., 2008). These concentrations are very modest from a resource perspective: at 10 ppm, 1 m3 of this material would yield only about 20 ml of water. However, the interpretation of recent infra-red remote- sensing observations implies that some pyroclastic deposits may contain as much as 1500 ppm (i.e. 0.15 wt%)ofH2O (Li and Milliken, 2014). If confirmed, such high concentrations may well be economically exploitable because, although an order of magnitude lower than those implied for permanently shadowed craters by Figure 7. False colour image of the lunar nearside the LCROSS results, they occur in much less based on data obtained by the Moon Mineralogy extreme environments where solar energy will 3 Mapper (M ) instrument on India’s Chandrayaan-1 be readily available. mission. Blue represents areas where a 2.8 to 3.0 mm absorption band attributed to bound H2O/OH was detected; red represents areas where the 1 mm 4 Oxygen band of the mineral pyroxene is strong, and picks out the basaltic maria; and green represents albedo. An indigenous source of lunar water, either in Note that evidence for hydration is restricted to the form of polar ice or in hydrated regoliths high latitudes, but is much more extensive than areas and/or pyroclastic deposits, would be the pre- of permanent shadow at the poles (image courtesy, ferred choice as a source of oxygen on the NASA/ISRO/Brown University/R. N. Clark, USGS/ Moon. Nevertheless, it has long been recog- AAAS). nized that, if necessary, oxygen could be extracted from anhydrous oxide and silicate et al., 2012), with OH/H2O being retained in minerals in the lunar regolith (see the extensive the relatively cold high-latitude regolith. reviews by Schrunk et al., 2008; Taylor and Pieters et al. (2009) estimated that the corre- Carrier, 1993). Indeed, Taylor and Carrier sponding water abundance might be as high (1993) discuss 20 different possible processes as 770 ppm in these areas, but this is for extracting oxygen from lunar regolith, and model-dependent and the actual concentra- produce a short-list of eight which they consider tions could be much lower. Moreover, the to be the most practical. It does have to be noted extent to which these hydrated materials are that all are quite energy intensive: Taylor and purely surficial (and thus probably of negligi- Carrier estimated that their eight shortlisted pro- ble value from a resource perspective) or cesses would require between 2–4 megawatt- may have been gardened into a substantial years of energy (i.e. 6–121013 J) to produce volume of the underlying regolith (in which 1000 tonnes of oxygen. The small-scale experi- case they may be volumetrically more impor- ments reported by Li et al. (2012) actually tant) is not currently known and will require required energy levels about two orders of mag- future exploratory missions to determine. nitude higher, although they noted that the 148 Progress in Physical Geography 39(2) efficiency can probably be increased signifi- proposed molten salt electrochemistry process cantly. In any case, these power levels would described by Schwandt et al. (2012), which, require a small nuclear reactor or several thou- although designed initially with ilmenite reduc- sand square metres of solar panels. tion in mind, would work equally well on Historically, one of the most studied of these anorthositic feedstock (where it would yield processes involves the reduction of the mineral an aluminium-silicon alloy as a by-product ilmenite, for example rather than titanium; C. Schwandt, personal communication, 2014). This would also have FeTiO þ 2H ! Fe þ TiO þ H O 3 2 2 the advantage of operating at a relatively low In common with all proposed oxygen extrac- temperature (~900C, compared to the 1300– tion schemes, high temperatures (700–1000C; 1700C required for processes based on the Li et al., 2012; Taylor and Carrier, 1993; Zhao electrolysis of silicate melts; Taylor and Carrier, and Shadman, 1993) are required for this reac- 1993). Another possible process would be tion, which in part accounts for the high ener- vapour phase reduction (reviewed by Senior gies required. Note that this particular route to (1993), and highly ranked in the comparison oxygen produces water in the first instance, and performed by Taylor and Carrier). In this pro- is therefore a possible route to producing water cess the high temperatures required (>2000C) on the Moon if indigenous sources (as described would be achieved by focusing sunlight in a above) are unavailable, provided that a source solar furnace, thereby avoiding problems asso- of hydrogen exists. ciated with electrical power conversion and Ilmenite is found mostly in high-Ti mare electrolysis, and some conceptual development basalts (where it can contribute more than work on such systems has already been per- 25% by volume; Papike et al., 1998), so it fol- formed (e.g. Nakamura and Senior, 2008; Naka- lows that this process would be most efficiently mura et al., 2008). employed in the same geographical regions dis- It is important to bear in mind that, even if 3 cussed above in the context of He extraction the existence of polar ice deposits are con- (i.e. Oceanus Procellarum, Mare Tranquillitatis firmed, processes able to extract oxygen from and additional small patches of high-Ti basalts lunar geological materials may still be impor- located elsewhere; see Figure 2). The reaction tant in the context of the future economic requires an initial source of hydrogen as a reduc- development of the Moon. For one thing, the ing agent (which might itself be extracted from difficulties of extracting and processing water the regolith, as discussed above), but as the last ice in permanently shadowed craters, where step of the process involves the electrolysis of the temperatures can be lower than 40 K (Paige water to produce oxygen and hydrogen the latter et al., 2010), may off-set the apparent advan- could be recycled. Note that this process also tages that ice has as a feedstock. In addition, produces Fe metal and rutile (TiO2), which there are many scientifically and economically might be further processed to produce Ti and interesting localities on the Moon that are additional oxygen that may also have additional located far from the polar regions, so it may economic value. be that a local, even if more energy intensive, Given that ilmenite reduction is only likely to source of oxygen will be preferable for sup- be viable in high-Ti mare regions, it is desirable porting operations in those localities. More- to identify processes which could extract oxy- over, as we have seen, oxygen extraction gen from the more ubiquitous anorthositic (i.e. from metal oxides also produces possibly use- plagioclase-rich) materials found in highland ful metals, which using water as a feedstock localities. One possibility might be the recently does not. Crawford 149

5 Metals PGMs and gold) are valuable owing to their cat- alytic and/or electrical properties; at concentra- With the possible exception of some high value 3 trace metals, such as the platinum group metals tions typical of meteoritic Fe, 1m of regolith (PGMs), one would not go into space for a could potentially yield 300 g of Ni and 0.5 g source of metals for use on Earth. Nevertheless, of PGMs. many metals will be essential for constructing Much higher localised concentrations of industrial and scientific infrastructures on the native Fe, and associated siderophile elements, Moon itself and in cis-lunar space, and we dis- could potentially be found in the vicinity of any cuss possible sources here. Fe-rich meteorites which partially survived col- lision with the lunar surface (Haskin et al., 1993; Wingo, 2004). Indeed, Wieczorek et al. (2012) a Iron and siderophile elements. Iron (Fe) is abun- have recently interpreted prominent lunar mag- dant in all mare basalts (~14-17 wt%; Papike netic anomalies as being due to surviving Fe- et al., 1998), but is mostly locked into silicate rich meteoritic debris. The strongest of these minerals (i.e. pyroxene and olivine) and the magnetic anomalies, on the northern rim of the oxide mineral ilmenite. Extracting iron from farside South Pole-Aitken Basin (Figure 8), these minerals will be energy intensive, cover an area of approximately 650,000 km2 although, as we have seen, it would be a natural and, given Wieczorek et al.’s preferred interpre- by-product of producing oxygen through ilme- tation that they are caused by surviving frag- nite reduction. ments of an originally ~110 km-sized metallic A more readily available source might be core of a differentiated asteroid, could poten- native Fe in the regolith, although the concen- tially indicate the presence of considerable trations are quite low (~0.5 wt%; Morris, quantities of near-surface metallic Fe (and asso- 1980). Native Fe in the regolith has at least three ciated Ni and PGMs). Smaller, scattered, mag- sources (e.g. Morris, 1980): meteoritic iron, iron netic anomalies elsewhere on the Moon (see released from disaggregated bedrock sources Figure 8) may similarly indicate the presence and iron produced by the reduction of iron oxi- of Fe-rich material at or near the lunar surface. des in the regolith by solar wind hydrogen. The Only further exploration will determine whether latter component occurs as nanometre sized or not this interpretation is correct, and how blebs (often referred to as ‘nano-phase Fe’) exploitable such meteoritic debris may be. within impact glass particles (‘agglutinates’) There is an argument (e.g. Elvis, 2012; Kar- and is likely to be difficult to extract. Morris gel, 1994; Lewis and Hutson, 1993) that the (1980) found the average concentration of the most easily exploitable extraterrestrial sources other two components to contribute of Fe, Ni and the PGMs will be near-Earth aster- 0.34+0.11 wt%. At least in principle this might oids rather than the Moon. Given the relative be extractable, perhaps through some form of ease of access of some of these objects, and their magnetic sieving, but given the very small sizes lack of gravity, this reasoning appears sound. of the individual Fe particles (generally < 1mm), Nevertheless, in the context of the future devel- the practicality of such a process is uncertain. If opment of the Moon, where an infrastructure the practical difficulties could be overcome, a may be developed to support human operations native Fe concentration of ~0.3 wt% is not an for multiple purposes (see Section V), access to entirely negligible amount, corresponding to ‘crashed’ metallic asteroids (and/or their melted about 5 kg for 1 m3 of regolith. Moreover, and re-crystallised remnants) on the lunar sur- meteoritic iron will be associated with sidero- face may nevertheless prove to be economically phile elements, some of which (e.g. nickel, the valuable. 150 Progress in Physical Geography 39(2)

Figure 8. Lunar magnetic anomalies found in the Moon’s farside southern hemisphere (left) and nearside northern hemisphere (right). The north and south lunar poles are marked ‘N’ and ‘S’, respectively; the two white ovals at left denote the inner basin floor and outer structural rim of the South Pole–Aitken Basin (SPA) and the outer ellipse has a major axis of 2400 km. Note the occurrence of magnetic anomalies along the northern rim of the basin, and scattered anomalies elsewhere (Reiner-g and Descartes are two prominent nearside examples). These may represent remains of iron and/or chondritic meteoritic debris in the near sub- surface. From Wieczorek MA, Weiss BP and Stewart ST ‘An impactor origin for lunar magnetic anomalies’, Science, Vol. 335, pp.1212–1215 (2012), reprinted with permission from the AAAS. b Titanium. Titanium (Ti) is a potentially useful context of a future space economy, the Moon metal for aerospace applications which has a may have a significant advantage over asteroids significant concentration (typically in the range as a source of Ti. The fact that oxygen is also 5–8 wt%) in the high-Ti mare basalts of the produced as a result of Ti production from ilme- lunar nearside (see Figure 2). As we have seen, nite could make combined Ti/O2 production one it occurs almost entirely in the mineral ilmenite of the more economically attractive future (FeTiO3), from which it may be extracted industries on the Moon. by electrochemical processes such as that described by Schwandt et al. (2012). Moreover, c Aluminium. Aluminium (Al) is another poten- unlike the case for Fe discussed above, Ti is a tially useful metal, with a concentration in lunar lithophile element and has a very low concen- highland regoliths (typically10–18 wt%) that is tration in metallic asteroids/meteorites (there orders of magnitude higher than occurs in likely are only a handful of experimentally measured asteroidal sources (i.e. ~1wt% in carbonaceous values in the literature, but most iron meteorites and ordinary chondtites, and <0.01 wt% in iron have ‘whole rock’ Ti concentrations <0.07 wt%, meteorites; Lodders and Fegley, 1998; Meyer and this figure may reflect the presence of sili- et al., 2010). It follows that, as for Ti, the Moon cate inclusions rather than the concentration in may become the preferred source for Al in cis- the meteoritic metal itself; Nittler et al., 2004). lunar space. Extraction of Al will require break- Ti concentration in chondritic asteroids/meteor- ing down anorthitic plagioclase (CaAl2Si2O8), ites is comparably low (typically <0.09 wt%; which is ubiquitous in the lunar highlands, but Lodders and Fegley, 1998). Therefore, in the this will be energy intensive (e.g. via magma Crawford 151 electrolysis or carbothermal reduction; Duke important optical, electrical, magnetic and cata- et al., 2006; Taylor and Carrier 1993). Alterna- lytic properties (e.g. British Geological Survey, tive, possibly less energy intensive, processes 2011; Chakhmouradian and Wall, 2012; Hatch, include the fluoridation process proposed by 2012). It has been known since the return of the Landis (2007), acid digestion of regolith to pro- Apollo samples that some lunar rocks contain duce pure oxides followed by reduction of relatively enhanced levels of REE, which, Al2O3 (Duke et al., 2006), or a variant of the because of corresponding enhancements in molten salt electrochemical process described phosphorus (P) and potassium (K), have by Schwandt et al. (2012). become known in lunar geology as KREEP- rich (Warren and Wasson, 1979). Because ura- 6 Silicon nium and thorium are also concentrated in KREEP-rich materials (see below), their distri- Silicon (Si) is abundant in all rocks (about 20 bution can be mapped by orbiting instruments wt% in lunar materials; see Figure 3), and one able to detect gamma-rays emitted by these would not normally think about going into space radioactive elements (e.g. Prettyman et al., to obtain it. However, in the context of the 2006). Figure 9 shows a map of lunar surficial future industrialisation of space, it is of poten- Th concentrations as mapped by the Gamma tially enormous importance for the production Ray Spectrometer on the Lunar Prospector of arrays of solar cells for the conversion of spacecraft, from which it is evident that sunlight into electricity. Some proposed appli- KREEP-rich lithologies are mostly associated cations (discussed in Section V) will require with Oceanus Procellarum and the Imbrium such large quantities of Si that a space-based Basin on the north-western nearside (a region source would be desirable. Proposed extraction of the Moon that has become known as the Pro- strategies are similar to those that would be cellarum KREEP Terrain, PKT; Jolliff et al., used for extracting Al from lunar materials 2000). In the context of lunar geological (i.e. magma electrolysis, carbothermal reduc- evolution, KREEP is thought to have become tion, fluorination and/or molten salt electro- concentrated in the final stages of mantle crys- chemistry; e.g. Duke et al., 2006; Landis, tallisation and then exhumed by the impact 2007; Schwandt et al., 2012; Taylor and which excavated the Imbrium Basin. Carrier 1993). The purity requirements for use Warren and Wasson (1979) studied the com- in semi-conductor devices are very stringent, position of this mantle reservoir (which they and may be challenging to achieve in the lunar named urKREEP, adopting the German prefix environment. However, routes to the produc- ‘ur-’ for ‘primitive’), and obtained an estimated tion of solar cell grade silicon by molten salt total REE concentration of about 1200 ppm electrolysis on Earth are currently under study (0.12 wt%). This total hides considerable varia- (e.g. Oishi et al., 2011), and a conceptual out- tion among individual REE, with yttrium and line for solar cell production on the lunar sur- cerium having concentrations of about 300 ppm face has been proposed (e.g. Duke et al., each, but europium and lutetium having concen- 2001; Ignatiev and Freundlich, 2012). trations of only 3-5 ppm. These REE concentrations are very much at the lower end of the scale 7 Rare earth elements for economically exploited REE deposits on The rare earth elements (REE; usually defined Earth, which generally contain somewhere in as the 15 lanthanides, spanning lanthanum to the range of 0.1 to 10 wt% REE (BGS, 2011). lutetium in the periodic table, plus scandium Moreover, because most lunar crustal rocks, and yttrium) exhibit a wide range of industrially even in the PKT, only contain a component of 152 Progress in Physical Geography 39(2)

Figure 9. Map of lunar surficial Th concentrations on the nearside (left) and farside (right) measured using the Gamma Ray Spectrometer on NASA’s Lunar Prospector Spacecraft (Prettyman et al., 2006; image courtesy of NASA). urKREEP, in most cases the REE concentra- exploitable. However, and in spite of their tions will be lower. name, Earth itself has abundant REE deposits At first sight, this would seem to indicate that (e.g. Hatch, 2012), so it seems unlikely that the Moon is unlikely to be a useful source of exporting lunar REE to Earth will be economi- REE. However, it is worth pointing out that cally viable (unless the environmental costs of some samples from the Apollo 14 and 15 land- REE extraction on Earth become prohibitive, a ing sites within the PKT actually have REE point to which I return in Section V), and lunar abundances several (typically 2 to 4) times REE mining may have to wait until off-Earth higher than the urKREEP values (Haskin and markets are developed. Warren, 1991; Papike et al., 1998). Although still low by the standards of terrestrial ores, it is important to stress that the very localised 8 Thorium and uranium areas where remote-sensing indicates the high- In principle, uranium (U), either on its own or as est likely concentrations of REE (essentially the a feedstock for the production plutonium (Pu), white dots in Figure 9, where surficial Th has its could prove to be an important element for the maximum concentration) have not yet been development of space-based nuclear power and sampled directly. Moreover, the current spatial nuclear propulsion concepts (e.g. Dewar, 2007; resolution of the remote-sensing data is quite Turner, 2005). Conceivably, Th may one day coarse (typically several tens of km; Prettyman have similar applications, for example in the et al., 2006), and smaller scale outcrops with production of fissile 233U (Ashley et al., potentially higher abundances will not be 2012), although such applications have not yet resolved. Future prospecting in these areas may been developed. On the Moon, we expect U and yet identify lunar REE deposits which might be Th to be concentrated in KREEP-rich terrains, Crawford 153 and therefore to be spatially correlated with elements that have become locally concen- enhancements in the REE discussed above. In trated. Whether or not these will be economi- lunar materials the abundance of uranium has cally exploitable will, as always, depend on been found to scale linearly with Th, with a U/ the value placed on them and on the energy Th ratio of approximately 0.27 (Korotev, and infrastructure available to support their 1998), so the lunar Th map shown in Figure 9 extraction. also indicates the locations where U enhance- Moreover, although not normally considered ments may be expected. This expectation was as economic resources as such, the natural lunar confirmed when Yamashita et al. (2010) pub- environment has some properties which might lished the first global lunar map of U, obtained be economically exploitable, at least in the con- with the gamma ray spectrometer onboard the text of lunar development. For example, essen- Japanese lunar orbiter, Kaguya. This work tially unprocessed lunar regolith may be revealed a maximum surficial U abundance of required for radiation shielding of human habi- about 2 ppm in the highest Th-bearing areas of tats (e.g. Eckart, 1999), and perhaps also for the PKT south of the Imbrium Basin. habitat construction if it can be sintered (e.g. The Kaguya U map has a spatial resolution of Desai et al., 1993; Taylor and Meek, 2005), con- 130 km (Yamashita et al., 2010), and so higher verted into concrete (e.g. Lin, 1985), 3D-printed concentrations are likely to exist that are cur- (e.g. Lim and Anand, 2014) or otherwise turned rently unresolved in existing remote-sensing into useable structural components. data. For comparison, the urKREEP concentra- Furthermore, the hard vacuum, and night- tion of U is about 5 ppm (Warren and Wasson, time/polar cryogenic temperatures of the lunar 1979), and some small individual Apollo sam- environment may positively facilitate some ples of highly evolved lithologies (i.e. quartz industrial processes, such as vacuum vapour monzodiorites and felsites; Papike et al., 1998) deposition, element concentration via ion sput- are known with U concentrations in the range tering (essentially industrial-scale mass-spec- 10–20 ppm. However, even the latter concentra- troscopy), and the manufacture of ultra-pure tions are well below what would currently be materials (Schrunk et al., 2008). Finally, the considered a low grade U ore on Earth (>100 unadulterated sunlight (available for >90% of ppm; World Nuclear Association, 2012), and a year at some polar localities; e.g. Speyerer and only further exploration of high U/Th-bearing Robinson, 2013) will provide essentially unlim- terrains on the Moon will determine if higher ited supplies of solar energy. Indeed, solar concentrations exist which might be exploitable energy has the potential to be a significant lunar in the future. export, as it could be collected by solar panels on the lunar surface and beamed to Earth and other locations in cis-lunar space (e.g. Criswell, 9 Other potential lunar resources 1998; Criswell and Harris, 2009). In this section I have summarised some of the more obvious natural resources that the Moon may be able to contribute to human civilisation IV The need for continued in the future, but other possibilities may remain exploration to be discovered. Indeed, it is important to rea- Any discussion of lunar resource potential lise that every element in the periodic table will needs to recognize that our knowledge of the exist on the Moon at some level, just as they do Moon’s geology is still very incomplete. All on Earth, and as our understanding of lunar geo- of the discussion to-date has been based on the logical processes improves we may find other study of samples returned from just nine 154 Progress in Physical Geography 39(2) localities (all at low latitudes and on the near- separation of solar wind implanted volatiles; side; see Figure 1), and on the interpretation (ii) demonstration of in situ oxide reduction of orbital remote-sensing data which only and/or electrochemical processes for the extrac- probe the uppermost few metres (and often tion of oxygen, and production of metal, from only the uppermost few microns) of the lunar the regolith; (iii) magnetic sieving of regolith surface. It is entirely possible that further to assess the extent to which metallic iron parti- exploration will reveal additional concentra- cles might be extracted; and (iv) assuming that tions of economically useful materials on the polar ice is confirmed, demonstration of in situ Moon, and indeed the history of geological extraction of water and subsequent electrolysis exploration on our own planet would lead us to yield hydrogen and oxygen. to expect this. Such an extensive programme of investiga- It follows that, in order to properly assess the tions will doubtless take many decades, extent to which the Moon may host economi- although a start can be made by new robotic cally useful materials, an extensive future pro- lunar missions that are already in the planning gramme of exploration will be required. Key stages (summarized by Crawford and Joy, objectives of such a programme would include: 2014). Within the next decade these space (i) confirmation of the presence of water ice and agency-led activities are likely to be sup- other volatiles in permanently shadowed polar plemented by an increasing number of private regions, and assessment of the total polar water ventures, such as those stimulated by the Goo- inventory; (ii) assessment of the extent to which gle Lunar X-Prize (Bowler, 2014; Hall et al., high-latitude, but non-permanently shadowed, 2013), many of which have an explicit focus regoliths contain hydrated materials, and espe- on resource identification. In the longer term, cially the depth to which such hydration extends it seems clear that exploration on this scale below the surface; (iii) assessment of the vola- would be greatly facilitated by renewed human tile inventory of lunar pyroclastic deposits; operations on the lunar surface (e.g. Crawford, (iv) in situ investigations of impact melt sheets, 2003; Spudis, 1996, 2001). Of course, even if lava lakes and magmatic intrusions to assess the such exploratory activities and associated pilot extent to which fractional crystallization may extraction schemes are successful in identifying have enhanced concentrations of economically exploitable resources, significant technical and valuable minerals; (v) in situ investigations of infrastructural development will still be required lunar magnetic anomalies to determine whether to bring them to an industrial scale of operation or not they indicate the locations of exploitable (some discussion of these issues is given by Duke concentrations of meteoritic iron and associated et al., 2006). siderophile elements; and (vi) in situ investigations of high concentrations of KREEP-rich V Applications for lunar resources areas to determine whether or not they contain exploitable concentrations of REE, uranium and 1 In Situ Resource Utilisation (ISRU) on the thorium. Moon In addition to further exploration, pilot stud- There are considerable scientific reasons for ies will be required to determine the practicality wanting to establish a scientific infrastructure and efficiency of extracting potential resources on the lunar surface, at least comparable to that we already know about. Such pilot studies, which currently exists in Antarctica (e.g. Craw- designed to operate on several m3 of regolith, ford, 2003; Crawford and Joy, 2014; Crawford could include: (i) in situ heating of regolith to et al., 2012; Eckart, 1999; Ehrenfreund et al., ~700C to demonstrate the extraction and 2012; McKay, 2013; Spudis, 1996; Taylor, Crawford 155

Figure 10. Orbital and potential energies in the Earth–Moon system. The vertical axis shows the ‘well depth’ expressed in units of km (i.e. reaching a stable orbit at a given distance from the Earth would require the same energy as would be expended by climbing against a constant one Earth gravity (9.8 ms–2) for the corresponding vertical distance; to convert to energy units (i.e. Joules per kg of lifted mass) multiply by 9800). For example, escaping the effective gravitational influence of the Earth (the zero level in the diagram) requires the same energy as climbing a 6400 km high mountain; the corresponding value for the Moon is only 290 km (i.e. it requires 22 times less energy to escape from the Moon as from the Earth). Note that all locations in cis-lunar space, including low Earth orbit (LEO) and geostationary orbit (GEO), require far less energy to access from the surface of the Moon than from the surface of the Earth.

1985). In addition, as discussed below, a lunar Probably the most important, at least in the surface infrastructure may become necessary early stages of operations, would be water for to support economic activities in cis-lunar drinking (given suitable purification), personal space. Moreover, far-fetched as it may seem at hygiene and, in the fullness of time, agriculture. present, in the context of growing interest in the Water would also be an important local source possibilities of space tourism, the coming of oxygen for life-support, and water-derived decades may witness an emerging market for oxygen and hydrogen would also help reduce the transport of fare-paying passengers to the transport costs if used as rocket oxidiser/fuel. lunar surface (e.g. Benaroya, 2010; Moskowitz, Other volatiles, such as C and N, whether 2013; The Economist, 2012). Whatever the rea- obtained from polar cold traps or released from sons for establishing a human infrastructure on the regolith by heating, will also be important, the lunar surface, there is no doubt that it would especially if lunar agriculture is contemplated. benefit from ISRU, simply because any Local sources of building materials will also resources obtained from the Moon itself will not be desirable at an early stage. Initially small- need to be hauled up from the Earth (Figure 10). scale scientific outposts (and/or hotels) are 156 Progress in Physical Geography 39(2) unlikely to require the local refinement of lunar infrastructure could include the supply metals (although, as noted above, some oxide- of rocket fuel/oxidiser (such as hydrogen and reduction schemes will produce these in any oxygen; especially oxygen, which dominates case as a by-product of oxygen production). In the mass budget for liquid hydrogen/oxygen the longer term, if a substantial lunar infrastruc- propulsion), and simple structural components. ture is developed to support wider economic Later, as the lunar industrial infrastructure activities in cis-lunar space, then local sources becomes more mature, the Moon may be able of metals, semi-conductors (for solar arrays) to provide more sophisticated products to and even uranium (for nuclear power) would Earth-orbiting facilities, such as Ti and Al become desirable in addition to indigenous alloys, silicon-based solar cells and uranium volatiles and building materials. (or plutonium derived from it) for nuclear power/propulsion systems. One particular area where significant future 2 Use of lunar resources in cis-lunar space expansion of economic activity in cis-lunar It has long been recognized (e.g. Eckart, 1999; space may occur would be the development of Ehricke, 1985; Koelle, 1992; Metzger et al., solar power satellites (SPS; Glaser, 1977) in 2013; Spudis, 1996; Spudis and Lavoie, 2011) geostationary orbit (GEO). By efficiently cap- that lunar resources will be valuable, and per- turing sunlight, converting it into microwave haps essential, in the future economic develop- energy and transmitting it to ground stations ment of near-Earth space. This is because, on Earth, SPS have the potential to become an while establishing resource extraction indus- important component in Earth’s future energy tries on the Moon will be a significant invest- mix (Flournoy, 2012; Glaser et al., 1998). How- ment, the energy required to transport ever, as noted by Maryniak and O’Neill (1998), materials from the lunar surface to any location the cost of transporting all the materials in Earth or lunar orbit, or indeed anywhere else required to establish a global network of SPS in the inner Solar System, will be much less than from Earth’s surface to GEO may be prohibi- the cost of lifting these materials out of Earth’s tive, and a SPS programme would be exactly the gravity (see Figure 10). kind of large-scale cis-lunar industrial activity Our global civilisation is already highly that would benefit from the use of lunar dependent on Earth-orbiting satellites for com- resources (again, note the small energy differen- munications, navigation, weather forecasting tial which exists between the surface of the and resource management, and this reliance is Moon and GEO compared to that between the only likely to increase. The currently high costs surface of the Earth and GEO; see Figure 10). of these activities are largely dictated by high launch costs (~ US$ 5000/kg even for the most economic launch vehicles currently available; 3 Use of lunar resources on Earth see discussion by Spudis and Lavoie, 2011), and Although a developing cis-lunar economy will by the fact that failed satellites, or satellites that necessarily yield wider economic benefits to the have exhausted station-keeping propellants or Earth as well, the opportunities for lunar cryogens, cannot be repaired or replenished in resources to make a direct contribution to the orbit. The availability of resources obtained world economy in the foreseeable future are from the much shallower potential well of the quite limited. This is because our planet con- Moon (see Figure 10) would help mitigate these tains the same basic mix of elements as the obstacles to further economic development in Moon and the rest of the Solar System, many Earth orbit. Near-term lunar exports to a cis- of them in higher localised concentrations (i.e. Crawford 157 ores) and, unlike the Moon, Earth already has a Earth (e.g. Wingo, 2004), where the high cost of well-developed infrastructure for extracting and these materials is currently a limiting factor refining raw materials. There appear to be three (Debe, 2012). Ultimately, whether or not lunar possible exceptions to this general observation, (or indeed any extraterrestrial) sources of which I discuss below: (i) rare elements for PGMs, or other rare and expensive materials, which the market value may render lunar will prove economic in the long-run will depend sources economic; (ii) lunar-derived energy on whether the prices that these materials can sources; and (iii) materials for which the envi- fetch on Earth will remain sufficiently high in ronmental cost of terrestrial mining may make the face of a significant increase in supply. lunar sources more attractive. Addressing that topic would require an economic analysis beyond the scope of the present a Lunar sources of rare elements. Much thought paper. has been given to identifying materials for which their present high market value may ren- b Lunar sources of energy. Most discussion of der the exploitation of extraterrestrial sources lunar energy sources for planet Earth have economic (e.g. Lewis, 1996). The PGMs usu- focused on the possible use of solar wind- ally feature prominently in these discussions, derived 3He in terrestrial nuclear fusion reactors with near-Earth asteroids (NEAs) identified as (Kulcinski et al., 1988; Schmitt, 2006; Taylor possible sources (Elvis, 2012; Kargel 1994; and Kulcinski, 1999; Wittenberg et al., 1986; see Lewis and Hutson, 1993). However, the possi- sub-section III(2) above). However, there are a bility of finding ‘crashed’ iron meteorites on the number of serious problems with this scenario. Moon (e.g. Wieczorek et al., 2012; Wingo, First, nuclear fusion has not yet been demon- 2004; see the discussion in sub-section III(5) stratedtobeaviablesourceofenergyonEarth, above) means that lunar sources should not and current large-scale experiments (such as be excluded from consideration. Moreover, the international ITER facility due to become although the Moon has a much higher gravity operational in 2028; Butler, 2013) are based well than do NEAs, it has at least two advantages on deuterium-tritium (D – 3H) fusion rather which may off-set this relative inconvenience. than D 3He. An oft-stated advantage of First, it is much closer than any NEA (many 3He fusion is the lack of neutrons produced NEAs, while they may transiently pass close to by the primary reaction (which would induce Earth, spend most of their orbits at much greater radioactivity in the reactor structure), but as distances and have long synodic periods). Sec- some level of D-D fusion cannot be prevented ond, as discussed above, in the coming decades in a D – 3He plasma (Close, 2007) this claimed a local infrastructure may develop on the Moon advantage is often exaggerated (it would apply to support a mix of scientific, industrial and tour- to pure 3He – 3He fusion, but that reaction ism activities, and this could provide local sup- requires much higher temperatures to initiate). port for mining operations of a kind unlikely to Even assuming that D 3He fusion is found be available at NEAs. Indeed, the presence of to be technically practical as a power source, the such a local infrastructure may reduce the cost low concentration of 3He in the lunar regolith and risks of extracting these materials on the (see sub-section III(2) above) implies that min- Moon to the point that the Moon becomes eco- ing very large areas would be required. In order nomically more attractive than NEAs. to estimate the quantity of regolith that would One potentially important future application need to be processed we need to estimate the of lunar PGMs may prove to be as catalysts for end-to-end efficiency of converting lunar 3He a future fuel-cell-enabled hydrogen economy on rest-mass energy into electricity on Earth. 158 Progress in Physical Geography 39(2)

Conventional power stations generally have year, which may be impractical, and then the efficiencies of about 30%, but advocates of accessible reserves would hold out for only 3He fusion estimate efficiencies of 60–70% for about 200 years. this process based on the (hypothetical) direct It therefore seems that, at best, lunar 3He electromagnetic conversion of the fusion prod- could only ever make a relatively small contri- uct energy into electricity (Kulcinski et al., bution to Earth’s total long-term energy needs. 1988; Schmitt, 2006). However, this figure Given the uncertainty as to whether the world ignores the efficiency of extracting 3He from economy will ever come to rely on nuclear the regolith; as discussed in sub-section III(1) fusion as a power source, and the practicality above, this will require heating large volumes of 3He fusion in particular, not to mention the to about 700C (which would itself consume scale of the investment that will be required to about 5% of the energy obtained from the fusion extract lunar 3He and its non-renewable nature, of the released 3He unless efficient ways of it appears at least premature to identify lunar recycling heat from one regolith batch to 3He as a solution to the world’s energy needs. another can be implemented), and complete That said, assuming the question of practicality efficiency in the release, collection and purifica- can be resolved, the high energy density of 3He tion of 3He can hardly be expected (~80% may as a fuel (~5.81014 Jkg–1 for D – 3He fusion), be reasonable based on the figures given by Kul- and its corresponding potential value (several cinski et al., 1988). Furthermore, transport to million US dollars per kg at current wholesale Earth will also require energy, as will the extrac- energy prices, with the exact value depending tion, processing and transport of D obtained on the energy sector with which the comparison from sea water to act as a reaction partner is made; US Energy Information Administra- (which has been entirely neglected here). Bear- tion, 2014), may nevertheless result in the iden- ing all this in mind, it is hard to see how the end- tification of economically viable, if relatively to-end efficiency could exceed 50%, and may small-scale, future applications. Possibilities be much less. might include space-based fusion power sys- Annual world electricity consumption is pre- tems, and mobile low-radioactive fusion reac- dicted to rise to 1.41020 J (40,000 TW-hours) tors for use on Earth (e.g. Schmitt, 2006). by 2040 (US Energy Information Administra- Moreover, there is currently a shortage of 3He tion, 2014), and if lunar 3He were required to on Earth for non-fusion applications (e.g. Kra- produce, say, 10% of this then, for an assumed mer, 2010), and lunar 3He could possibly help overall efficiency of 50%, about 500 km2 of satisfy this demand. As 3He will automatically high-concentration (i.e. 20 ppb 3He) regolith be released as a by-product of extracting other, would need to be processed every year down more abundant, solar wind implanted volatiles to a depth of 3 metres. Note that, owing to the from the lunar regolith (see sub-section III(1) ubiquitous presence of craters and other obsta- above), it is worth keeping an open mind to such cles on the lunar surface, only a fraction of a possibilities. given surface area (estimated at 50% by Schmitt As far as the long-term energy requirements (2006)) will actually be amenable to processing, of planet Earth are concerned, it would appear and allowing for this the highest concentration more logical instead to invest in developing deposits of this non-renewable material (see genuinely inexhaustible energy sources on Figure 6) would last for about 2000 years. Pro- Earth itself. Possibilities include some combina- ducing all of Earth’s anticipated mid-21st cen- tion of D-T and D-D fusion, and/or solar, tidal or tury electrical energy requirements from lunar geothermal power, supplemented if necessary 3He would require processing 5000 km2 per by space-based solar power systems. As noted Crawford 159 above, lunar resources could contribute to the Moreover, preservation of Earth’s rich natural latter either by supporting the construction of habitats is an important social and ethical geostationary SPS (Maryniak and O’Neill, imperative irrespective of merely economic con- 1998), or through the construction of solar siderations. Although there are certainly good arrays on the lunar surface and beaming the ethical reasons for not unnecessarily despoiling energy to Earth (Criswell, 1998; Criswell and extraterrestrial environments (e.g. Rolston, Harris, 2009). Just to put this latter point in per- 1986), if the gradual development of a space- spective, covering a given area of the near- based infrastructure eventually enables us to equatorial lunar surface with solar panels would choose between inflicting further environmental yield as much electrical energy in seven years damage on an inhabited planet or obtaining raw (31010 Jm–2, assuming a conservative 10% materials from uninhabited ones, then the latter conversion efficiency) as could be extracted choice may come to be seen as ethically prefer- from all the 3He contained in the 3 m of regolith able (e.g. Hartmann, 1986). Indeed, as stressed below it (assuming 20 ppb 3He and an optimistic by Bernasconi and Bernasconi (2004), in the long 50% overall efficiency). If the conversion effi- term it is only by accessing space resources that ciencies were the same (say 20% each, which the human burden on Earth’s biosphere can be in practice might not be unreasonable) then lightened and eventually reduced. Possible lunar solar energy would out produce 3He in just 1.4 resources to which this argument may come to years. But whereas, once extracted, the 3He apply might include the REE (see sub-section would be gone forever, the solar power would III(7)), for which the world economy is certain be continuous. Looked at from this perspective, to demand increasing supply over the coming it would be far more efficient to beam lunar decades owing to their many technological appli- solar energy to Earth than it would be to extract, cations (e.g. Hatch, 2012), but which can be and physically transport to Earth, 3He for environmentally damaging to extract (British hypothetical fusion reactors which have not yet Geological Survey, 2011; Chakhmouradian and been shown to be practical. Wall, 2012). However, regardless of the possible environmental benefits, the option of shifting c Environmental benefits of extraterrestrial resources. resource extraction from Earth to extraterrestrial The environmental costs of resource extraction sources will only be possible in the context of a on Earth are becoming of increasing concern, developing space-based economy, which (as dis- and these pressures will only increase as world cussed above) is itself likely to depend on lunar population rises towards an estimated 10 billion resources. people by mid-century (e.g. Tollefson, 2011), all of whom will aspire to increased standards of liv- ing. Indeed, if the economic value (to say nothing VI The international and legal of the wider societal value) of damaged ecosys- context tems were included in the extraction costs then In addition to the technical challenges, the many sources of raw materials on which the development of lunar (and other extraterrestrial) world economy depends would be much more resources will require the establishment of an expensive than they are, and some might not be international legal regime which encourages economic at all (e.g. Costanza et al., 1997). It fol- large-scale investment in prospecting and lows that, when the true value of Earth’s natural extraction activities, while at the same time environment is properly taken into account, the ensuring that space does not become a possible relative cost of exploiting extraterrestrial raw flashpoint for international conflict. On the lat- materials may not be as high as first appears. ter point, Hartmann (1985) has wisely cautioned 160 Progress in Physical Geography 39(2) that ‘space exploration and development should (Article I) that ‘the exploration and use of outer be done in such a way so as to reduce, not aggra- space ... shall be carried out for the benefit and vate, tensions on Earth’, and this will require the in the interests of all countries’, which implies development of an appropriate, internationally some kind of international regulatory frame- supported, legal framework. work. Currently there is ambiguity in the inter- Currently space activities are governed by a pretation of the OST in these respects (Pop, small set of international treaties negotiated 2012; Simberg, 2012; Tronchetti, 2009; Viikari, under United Nations auspices (UN, 1984). The 2012), and this ambiguity is unhelpful as it acts most important of these, and the foundation on to inhibit investment. Simberg (2012) explains which the international legal regime for outer the problem well when he writes ‘[w]hile the space is built, is the 1967 ‘Treaty on Principles technology is a challenge, one of the biggest Governing the Activities of States in the Explo- business uncertainties these new [commercial ration and Use of Outer Space, including the space] companies face is the legal status of any Moon and Other Celestial Bodies’, more com- output from their off-world mining operations, monly known as the Outer Space Treaty (OST). and the corresponding ability to raise the funds The OST now has 102 States Parties, including needed to make them successful’. all nation-states having a spacefaring capability. The one serious attempt to set up a legal As its name suggests, the OST was intended to regime which went beyond the OST in this establish a number of important ‘principles’ for respect (namely the 1979 ‘Agreement Govern- the activities of nation-states in space. These ing the Activities of States on the Moon and include the concept that space should be consid- Other Celestial Bodies’, otherwise known as the ered ‘the province of all mankind’ (Article I), Moon Treaty) has failed to gather the support of that outer space is free for the ‘exploration and any spacefaring nation because it was perceived use’ by all states (Article I), that the Moon (and as being overly restrictive on commercial activ- other celestial bodies) cannot be appropriated ities (Delgado-Lo´pez, 2014; Simberg, 2012; (by claim of sovereignty or otherwise) by Tronchetti, 2009; Viikari, 2012). Although nation-states (Article II) and that international there are reasons to believe that some of the law, including the UN Charter, applies to outer opposition to the Moon Treaty was based on space (Article III). This has proved to be an an exaggerated view of these alleged restric- excellent foundation for international space tions (e.g. Goldman, 1985), it is clear that a new law, but it is clearly a product of its time in that approach is now required. This is not the place it makes no explicit reference to the exploitation to review all the suggestions that may be found of space resources, or to other commercial space in the space policy literature, but there is a spec- activities that were not envisaged in 1967. trum of possibilities from extreme laissez-faire Most importantly in the current context, positions that would repudiate not only the although it is a clear principle of the OST that Moon Treaty but possibly even the OST itself, nation-states cannot ‘appropriate’ the Moon as to more internationalist approaches that would a celestial body, it seems certain that private retain the spirit of the Moon Treaty while companies will not invest in the exploitation attempting to make its basic approach more of space resources unless they can be guaran- compatible with commercial activities in space. teed a return on their investment. This in turn An example of the former position is given implies that they must have legal entitlement by Simberg (2012), who advocates that the US to at least the resources they extract (e.g. Pop, (and, by inference, other nation-states) unilater- 2012). This, however, then has to be squared ally establish legislation recognising extrater- with the principle established by the OST restrial land claims (albeit under carefully Crawford 161 controlled conditions, including actual occupa- claims for lunar 3He in this respect, through its tion of a site on the Moon and/or asteroid by the alleged ability to make a major contribution to claimant) as a means of encouraging private Earth’s future energy needs, have been exagger- investment. However, it is difficult to see how ated. Rather, I have identified a hierarchy of such unilateral action would be acceptable to applications for lunar resources, beginning with the international community (or consistent with the use of lunar materials to facilitate human the OST; see, for example, Tronchetti, 2014), activities on the Moon itself (including scien- and it would run the risk of violating Hart- tific exploration, resource prospecting and, per- mann’s injunction (1985) that space develop- haps, tourism), progressing to the use of lunar ment should not increase political tensions on resources to underpin a growing industrial capa- Earth. An intermediate position, recognizing the bility in cis-lunar space. In this way, gradually need for international cooperation, but also the increasing access to lunar resources may help difficulty of negotiating a new treaty, calls for ‘bootstrap’ a space-based economy from mod- the development of international ‘norms of est beginnings on the lunar surface until much behaviour’ in space, which would recognize the of the inner Solar System lies within the sphere interests of commercial companies while of human economic activity (e.g. Metzger et al., remaining within the spirit of current interna- 2013; Spudis and Lavoie 2011; Wingo, 2004). tional law (e.g. Delgado-Lo´pez, 2014; Tronch- In the fullness of time, this has the potential to etti, 2014). Finally, at the other end of the yield significant economic benefits to human spectrum, Tronchetti (2009) has put forward a society on Earth by opening the world economy detailed suggestion for the creation of an ‘Inter- to external sources of energy and raw materials. national Space Authority’ to manage the exploi- Of course, the Moon is not the only source of tation of lunar (and other extraterrestrial) extraterrestrial raw materials which could con- resources, while also incentivising private tribute to the development of such a space- investment and protecting sites of special scien- based economy, and NEAs are often discussed tific importance. in this context. However, while resources from Whatever legal framework is eventually NEAs will doubtless come to play an important adopted, if humanity is to reap the potential role, the Moon is likely to remain of central rewards of utilising the material and energy importance because of its constant proximity, resources of the Solar System, it is imperative its probable concentrations of accessible vola- that these outstanding legal and political issues tiles (especially water ice at the poles) and the be addressed in a timely manner. fact that diverse lunar geological processes have concentrated many economically important materials (e.g. Ti, Al, the REEs and U) in crustal VII Conclusions reservoirs to levels which far exceed those From the discussion above, it is clear that the found in most known asteroid classes. The Moon does possess abundant raw materials that Moon also has abundant energy, in the form of are of potential economic value for future sunlight, to extract and process these materials human activities, and especially for future activ- in situ. Moreover, although it has long been con- ities in space. It is true that, on the basis of cur- sidered that NEAs would be the preferred extra- rent knowledge, it is difficult to identify any terrestrial sources for iron, nickel and the PGMs single lunar resource that is likely to be suffi- (e.g. Elvis, 2012; Kargel, 1994), the possibility ciently valuable to drive a lunar resource extrac- that the Moon may retain partially intact iron tion industry which has near-term profit as an and/or chondritic meteorites (Wieczorek et al., objective. In particular, I have argued that 2012; Wingo, 2004) means that it may become 162 Progress in Physical Geography 39(2) a useful source for these materials as well. This (acknowledged in the figure captions). Finally, I is likely to be especially true if a local infra- wish to acknowledge my decades-old debt to two structure is developed to support the utilisation former teachers at North Cestrian Grammar School of other lunar materials. Indeed, it is the fact (Altrincham, Cheshire): Brian Thirlwell, who taught that the lunar surface has the potential to sup- me physics and encouraged my interest in astron- port a growing scientific and industrial infra- omy, and Trevor Laverock, who allowed me to write my first ever geography essay on the topic of ‘The structure, in a way that asteroidal surfaces do Solar System’ and who later introduced me to geol- not, which is likely to make the Moon the ogy – truly has it been observed that teachers rarely linchpin of humanity’s future utilisation of the get to know the limits of their influence. Solar System. Finally, it is necessary to reiterate that our Funding knowledge of lunar geological evolution, and This research received no specific grant from any therefore of the extent to which the Moon may funding agency in the public, commercial, or not- offer economically useful resources in addition for-profit sectors. to those identified above, is still very incomplete. Gaining a more complete picture of the References Moon’s economic potential will require a much Anand M (2010) Lunar water: a brief review. Earth Moon more ambitious programme of lunar exploration and Planets 107: 65–73. than has been conducted to-date, and this will Anand M, Crawford IA, Balat-Pichelin M, et al. (2012) A probably require the return of human explorers brief review of chemical and mineralogical resources to the lunar surface. Fortunately, such a pro- on the Moon and likely initial In Situ Resource gramme is under active international discussion. Utilization (ISRU) applications. 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