WHITE PAPER
Ice Prospecting: Your Guide to Getting Rich on the Moon Version 1.0 // May 2019 // This work is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License. Kevin M. Cannon ([email protected])
Introduction Water ice has been detected indirectly and directly within permanently shadowed regions (PSRs) at both poles of the Moon. This ice is stable against sublimation on billion-year timescales, and represents an attractive target for mining to produce oxygen and hydrogen for propellant, and water and oxygen for human life support. However, the mere presence of ice at the poles does not provide much information: Where is it exactly? How much is there? Is it thick layers of pure ice, or small amounts mixed in the soil? How hard is it to excavate? This white paper attempts to offer answers to these questions based on interpretations of the best data currently available. New prospecting missions in the future–particularly landers and Figure 1. Ice accumulation mechanisms. rovers–will continue to change and improve our understanding of ice on the Moon. This guide will be updated on an ongoing expected to be old. (2) Solar wind. The solar wind is a stream of basis to incorporate new findings. electrons, protons and other particles that are constantly colliding with the unprotected surface of the Moon. This Why is there ice on the Moon? process can create individual OH and H2O molecules that are Two factors create conditions that allow ice to accumulate able to ballistically hop across the surface, eventually migrating and persist at the lunar poles: (1) the Moon has a very small axial to the PSRs. This process could lead to the PSRs at lower tilt of 1.54° compared to 23.5° for the Earth, and (2) the Moon latitudes accumulating more ice because hopping molecules get has rough topography, mostly due to large craters formed by trapped before traveling further poleward. (3) Volcanic asteroid and comet impacts. Combined, these factors lead to outgassing. Volcanoes erupt lava on the surface, but also spew topographic lows mostly in crater floors near the poles that out significant amounts of gasses including water vapor. never receive direct sunlight. As one can imagine, regions of Volcanic activity has more or less stopped on the Moon, but was permanent darkness will be cold on a planetary body with no much greater in the past with ancient eruptions outgassing large atmosphere, and the PSRs on the Moon reach temperatures as amounts of H2O, some of which could have ended up in the low as 40 K or colder. At average temperatures below about 110 PSRs if eruptions were spaced closely together. K, water ice in a hard vacuum is stable against sublimation on Multiple processes operate to modify the concentration billion-year timescales, allowing any ice that accumulated in the and distribution of ice once it has accumulated at the surface. past to remain there today. Ice is even more stable when covered Impact cratering churns up the soil and mixes ice both vertically with a layer of lunar regolith (soil). and horizontally. Given enough time this will break up any How did ice accumulate in the first place? There are three continuous ice layers at the surface and incorporate this ice into main ideas that probably all contributed to ice accumulation in the surrounding soil. The result will be patchy and diffuse ice in the past (Figure 1): (1) Impacts of large comets and volatile-rich the regolith that varies in concentration on scales of meters or asteroids. These high-energy collisions delivered water and more. Under certain conditions ice at the surface can be other volatiles to the Moon, forming a transient atmosphere in “pumped” deeper into the underlying regolith by diffusion, but the hours to days after the impact. Some of the H2O molecules this process may be inefficient on the Moon. Ice is lost due to in this atmosphere were cold trapped in the PSRs, possibly sublimation and space weathering at the surface, both of which building up thick deposits. Accumulation within different PSRs reduce the ice concentration over time without continual was likely nonuniform and depended on the random locations resupply. A complicating factor is that the current spin axis of of sequential impacts. Cratering rates were orders of magnitude the Moon may not have always been the same. If the spin axis higher in the past, so most ices deposited by this process are changed in the past (as some have argued) certain areas where ice accumulated will have become warmer, causing ice to migrate up towards the surface and partially or fully sublimate away. Overall there has been a complex history of ice accumulating, being diluted into surrounding soils, and being lost over time. Current thinking suggests most major accumulation at the poles took place more than 2-3 billion years ago, and mixing and loss processes have dominated since then. If there ever were meters-thick sheets of nearly pure ice in the PSRs, they have been battered away.
Ice locations and concentrations Figure 2 shows locations of the PSRs in the north and south polar regions. There are more PSRs at the north pole but they tend to be smaller than those at the south pole. Every PSR on the Moon has the potential to host ice but the available data do not necessarily indicate all of them do. It’s also possible for ice to exist outside the PSRs if it is buried at depth under a layer of dry regolith. Ice concentrations have been estimated by different instruments on orbiting satellites, and by the LCROSS experiment that impacted into Cabeus crater near the south pole and created an ejected plume of material in which water ice and other volatile molecules were detected. Orbital remote sensing instruments have widely different spatial resolutions and sensing depths: this makes interpreting ice locations and concentrations challenging. Instruments that rely on reflected UV, visible, and near-infrared light only sense to microns or millimeters below the surface, while microwave, radar and neutron spectroscopy penetrate centimeters to meters. So, two different instruments can give different values for ice abundance at the same location and this is not necessarily a contradiction. Table 1 lists the main instruments used to infer or directly detect polar ice on the Moon. The table describes limitations of these measurements, depths below the surface they sense to, and presents ice concentrations as pessimistic, realistic, and optimistic cases, attempting to capture different interpretations by the many people who have studied these data. Overall the data are consistent with the presence of two broad categories of ice deposits: surface ice (including frost), and deeper diffuse ice. Figure 2. Locations of Permanently Shadowed Regions at the Surface ice and frost north and south poles from Mazarico et al. 2011. In this paper surface ice is defined as ice within the upper microns to millimeters, including frosts that are thin coatings of the case then the higher concentrations indicated by the data (as ice on the surface of regolith grains. If true frosts are present, much as 20 weight percent ice) could not extend very deep as a they probably formed more recently than deeper ice but could uniform layer, otherwise they would not be consistent with still be billions of years old. Figure 3 shows locations of likely neutron spectroscopy data. surface ice based on orbital data that use reflected UV, visible Frosts or surface ice can be readily mined by mechanical and near infrared light to detect the spectrum of ice or enhanced excavation or thermal methods (see below), but yields will likely brightness interpreted to be caused by ice. Ice is detected where be low if ice is only present as discontinuous frost (Table 2). For current average temperatures are below 110 K such that ice is example, if the entirety of the 233.7 km2 Shackleton crater PSR stable at the upper surface. The data indicate any frost layers was covered in a patchy micron-thick frost layer this would add present are patchy, perhaps with 7-20% areal coverage at 100- up to only about 40 mT (metric tonnes) of ice. Because of this, meter scales. However, the data could also be interpreted as extreme caution should be taken when using ice concentrations diffuse ice in the upper soil layers rather than true frost. If this is Table 1. Reported ice concentrations from orbital datasets. Spatial Sensing Instrument Limitations Ice: pessimistic Ice: realistic Ice: optimistic Resolutiona Depth Lunar Cannot distinguish Very low ice Heterogeneous ice Ice highly clustered with local Prospector uniform low abundances concentrations distributions with richest 500 m 0-70 cm regions and buried wet layers Neutron from buried wet layer, (<1%) spread over PSRs at 1-4% ice, buried at >10% ice Spectrometer senses all H not just H2O wide regions wet layers with >1% ice Lunar Crater Represents single point Observation 2.7% ice at the 5.6-6.3% ice at the 8.5% ice at the 20-25 mb 0-3 m on the surface; may not and Sensing impact site impact site impact site be representative Satellite Enhanced circular Enhanced CPRs in Upper limit of 5-10% ice polarization ratios polar craters are Meters-thick layers of nearly Mini-RF 15 m 0-10 mc in the subsurface, but ice indicative of subsurface entirely due to rocks, pure ice in the subsurfaced not uniquely identified ice are non-unique no ice indicated Relies on ratios/slopes Frosts covering up to Lyman-Alpha Signal can be instead of ice spectrum; 10% of the surface, or Even more extensive ice not Mapping 76.4 m μm-mm attributed to OH, cannot distinguish H2O diffuse surface ice with detected due to low SNR Project very little H2O from OH 0.1-2% concentration Single wavelength, High albedos caused Even more extensive ice not Lunar Orbiter Thin frosts or diffuse ice 240 m μm-mm enhanced albedo not by other factors, little detected due to strict criteria Laser Altimeter with 7% concentration unique to ice to no ice used to report detections Lunar Instrument may not Much lower spatial Ice highly clustered with local Exploration operate as reported (see resolution than Up to 0.5% ice over large 10 km 0-70 cm regions or buried wet layers Neutron discussion in Teodoro et reported, no spatially areas covering the PSRs at >10% ice Detector al. 2014) resolved ice detected Up to 20 wt.% ice if Moon Uses multiply bounced Ice present as thin present as intimate ice- Even more extensive ice not Mineralogy 280 m μm-mm photons for PSR frosts with low regolith mixture, or 20 detected due to low SNR Mapper investigations: low SNR overall yields vol.% if pure ice patches Chang'E Unclear how to link the Dataset not fully Areas of subsurface ice in More extensive ice than microwave 280 m cm-m band ratios to physically understood, ice not some large PSRs reported in initial analysis radiometer meaningful properties uniquely detected aRepresents maximum resolution used in studies reporting ice detections; these are often binned pixels and the maximum resolution of the instrument can be higher. bEstimated diameter of the crater created by LCROSS. cFor S-band. dThis interpretation is widely disputed.
Table 2. Ice concentrations for given frost thicknesses. Frost thickness Mass of ice (kg/m2) assuming 20% areal frost coverage
1 nm 1.8x10-7
1 μm 1.8x10-4
1 mm 1.8x10-1
Deeper diffuse ice Evidence for ice deeper than the optical surface comes mostly from orbital neutron spectroscopy and from the LCROSS mission. Neutron spectroscopy is sensitive to small amounts of ice in the subsurface but it has very poor spatial resolution and cannot distinguish between higher-grade wet layers buried under dry soil and lower-grade uniform layers. Most reported abundances are 1% ice by weight or less for uniform layers (Tables 1, 4), but there are reasons to be more optimistic: (1) These values are averaged over large areas, and ice ores could be concentrated in smaller clusters such that local or regional concentrations are much higher. (2) Most PSRs are located in craters, and the roughness of craters can affect neutron measurements leading to underestimates of ice contents. The LCROSS experiment detected 6% ice in an area where neutron data indicated only 1%, demonstrating richer ores are in fact present at local scales. For reference, Table 3 shows expected yields in a cubic meter of regolith for different grades of diffuse water ice.
Table 3. Expected ice yields for given concentrations of diffuse ice mixed with regolith. Ore grade Mass of ice in 1 cubic meter (weight percent ice) of regolith (kg)
1% 14.9
5% 73.5
10% 144.2 Figure 3. Locations of likely frosts/surface ice detected by the LOLA, LAMP and M3 instruments. Modified Table 4 lists estimates of ice concentrations in some of the larger from Li et al. 2018, their Fig. 4. PSRs based on the Lunar Prospector Neutron Spectrometer dataset, but because of low spatial resolution and non- reported from orbital datasets at shorter wavelengths (Table 1). uniqueness in layering (Table 1), these modeled values should These data should be coupled with neutron spectroscopy or be used only as a guide for crude tonnage estimates and for other products, and ground investigations will likely be needed targeting further prospecting efforts. Promising ore locations to drill or otherwise penetrate beneath any surface ice to can also be estimated based on temperature data and thermal properly assess ore grade at depth. models of where ice was likely stable in the past and where it is currently stable. Figure 4 shows an example of a map where I have used ranking criteria to identify preferred ice deposits.
Table 4. Modeled water equivalent hydrogen* (WEH) missions to fully address. These are some of the more pressing concentrations for major PSRs. From Teodoro et al. questions relevant to mining: 2010, their Table 1. Crater Location Area (km2) WEH • How do ice concentrations vary laterally within the PSRs at (wt.%) scales of meters to tens of meters (i.e., how strong is the nugget effect in lunar ice deposits)? Hermite 86.0°N, 89.9°W 225 3.7 • If surface frosts are present, how thick are they and what is Peary B 89.2°N, 128.0°E 100 3.2 their areal extent? • Unamed 87.0°N, 19.44°E 250 3.1 For diffuse ice, how do concentrations vary as a function of depth? Is the ore richer or poorer at depth? If diffuse ice is Unamed 86.2°N, 37.9°E 150 1.9 present at the upper surface of PSRs, how can this be Cabeus A 82.4°S, 53.0°W 50 1.6 reconciled with interpretations of neutron spectroscopy Cabeus 84.9°S, 35.5°W 275 1.0 data that argue for a dry layer overlying a wet layer? Unamed 85.7°N, 52.7°E 75 1.0 • Are significant amounts of buried ice present in the non- Shackleton 89.7°S, 110.0°E 200 0.6 shadowed regions surrounding the PSRs? Faustini 87.2°S, 89.0°E 725 0.3
Nansen F 84.5°N, 62.2°E 250 0.3 Physical nature of lunar ice deposits Shoemaker 87.6°S, 38.0°E 1150 0.2 As currently interpreted, orbital data do not support the Haworth 87.4°S, 5.0°W 1050 0.2 presence of pure ice sheets or permafrost-like materials within * Water equivalent hydrogen is the mass of H2O assuming that all the PSRs. These types of deposits could in theory be present at detected hydrogen is in the form of ice. depths greater than a meter, but there is no reason to think this is the case. Some studies have used radar data to claim thick pure deposits exist and are buried under the surface, but this is widely disputed. Lunar ice is likely to be in the form of frosts, and diffuse ice grains mixed in the soil at low to moderate concentrations as discussed above. The composition and physical properties of the soil are probably similar to highlands soils elsewhere on the Moon: these soils are mostly made of the rock type anorthosite and are loose with high porosity near the surface, and are more compact at depth. Bulk densities vary between about 1500-1900 kg/m3. Soils are poorly sorted (well graded), and follow a power law size distribution with average grain diameters of 90-110 μm if highlands soils at the Apollo 16 site are representative (basaltic soils at other landing sites had 40-60 μm average grain diameters). Some people stress almost nothing is known about the physical properties of material within PSRs, and treat the PSRs themselves as a kind of mysterious twilight zone. In fact, the Apollo astronauts encountered mini PSRs beneath large boulders, and noted nothing strange or unique about the soil within them. Recent studies found locations where boulders rolled from non- shadowed regions into PSRs, and found no difference in the
Figure 4. Example of a map where criteria based on bearing capacity of the soils. It is true that no rovers or landers temperature and ice stability have been used to rank have investigated the polar PSRs, but a reasonable geologic locations for likely ore grades. Dark blues = highest inference is that their materials are more or less the same as other materials on the Moon, just colder with enhanced potential; pinks = lowest potential. concentrations of water ice and other volatiles. In-Situ Resource Utilization (ISRU) studies have created simulated ice-bearing lunar materials for experiments involving Outstanding questions drilling and water extraction, but unfortunately these efforts Reasonable interpretations about polar ice deposits can be give incorrect impressions about PSR material properties. These made when orbital datasets are combined and their limitations experiments mix dry lunar regolith simulant with liquid water are understood (Table 1). That said, there are major then freeze the mixtures. As expected, this leads to hard outstanding questions requiring future orbital and landed cemented materials similar to permafrost. Some studies quote compressive strengths over 100 MPa, and these results are used solves issues of transferring power over kilometer-scale to argue it is prohibitive to excavate ice ore in the PSRs with horizontal distances and opens up more areas for mining that mechanical diggers. These experiments do not reflect the are not located close to peaks of near-perpetual sunlight. Tower- geologic processes that have formed and then modified polar supported outposts may also allow for more efficient logistics ices (Figure 1): the same impact cratering that has turned solid because all aspects of mining, processing and mission control basalt into a meters-thick powder on the Moon has been can be located in close proximity. However, towers will have to pummeling the PSRs for billions of years, and hard cemented be fairly tall (hundreds of meters or more) to reach heights materials are unlikely to have survived intact. Mechanical where solar power is viable, and there might only be a small excavation is probably just as viable in the PSRs as it is elsewhere subset of craters or other depressions at very high latitudes that on the Moon. meet the narrow set of constraints required to make this Ice mining on the Moon has been discussed and architecture work. Ultimately, nuclear power will offer the best planned under two broad architectures: (1) Mechanically access to PSRs that are not located near sunlit peaks and are too excavating soil then hauling it to a fixed processing plant. (2) deep for reasonable tower heights. Nuclear will be critical to Heating or irradiating soils in situ then capturing released open up the massive PSRs in Haworth, Shoemaker, Faustini volatiles in a mobile tent or dome. There are pros and cons to and Sverdrup craters at the south pole that may host especially each approach, but a full discounted cash flow analysis rich deposits. An added benefit is that waste heat from nuclear comparing the two options is beyond the scope of this guide. reactors can be put to use as part of the mining effort to liberate water from the icy ores. Operability 3. High-bandwidth communication to Earth will be Mining activities in extreme locations on Earth are proof important for mining operations, particularly in early phases that ore body characteristics generally take precedence over before large lunar bases are established. Existing mapping operability. This may not be true on the Moon at first, and products are available that show the fraction of time Earth is lower grade ores could be targeted initially because they are visible from different locations at the lunar poles. Locations on located in more favorable areas than richer ores. Operability in the near side of the Moon are more favorable, but ground relays the challenging lunar environment can be divided into three or satellite relays will probably still be needed to provide data categories for simplicity: (1) terrain, (2) power, and (3) connectivity to mine sites within the PSRs themselves. This is communications. doubly true for far-side locations. 1. Terrain includes topographic slopes, roughness, and rockiness. These are important because most lunar mining Contaminants activities will be carried out by semi-autonomous to fully Ore deposits in the PSRs are not likely to be pure water ice. autonomous mobile vehicles, and more benign terrains will The LCROSS experiment found evidence for a variety of increase the efficiency of mining operations. Planetary rovers contaminants including sulfur-bearing phases, carbon-bearing generally cannot operate on slopes steeper than 20-30 degrees, phases, and mercury. Many of these compounds are toxic, and and must traverse between large boulders and small craters processing methods will be needed to filter out a variety of where they are present. Areas of major terrain hazards can metallic and organic materials to purify the ice. Prospecting already be identified using existing orbital datasets, and efforts could target regions where average temperatures are supplemented with future orbital platforms and on the ground above the sublimation temperatures of most carbon- and sulfur- using rover-based cameras and LiDAR systems. bearing compounds (~54 K) but still below the sublimation 2. Access to power is likely to be a major limiting factor for temperature of ice (110 K). Elemental mercury and elemental initial mining operations. Near the poles there are peaks and sulfur will remain an issue between 54-110 K. crater rims with nearly perpetual sunlight, often in proximity to PSRs with detected ice deposits. Conventional thinking is to Conclusions land and establish a solar-powered base in the sunlit regions Exploring lunar ices at the poles will never be complete while conducting mining operations nearby in shadowed crater from a scientific point of view. But from a mining perspective, floors. However, distances between sunny areas and ore deposits the amount of knowledge about the ore body necessary to start are at least 5-10 km, and often involve untraversable descents a lunar mine can be quantified based on the risk tolerance of an down steeply sloped crater walls. It is not clear how power, organization. Have we reached the point where such decisions equipment and materials will be transferred between the two. can be made? The following information is known to fairly high There are only a small handful of large PSRs located near peaks confidence based on multiple orbital datasets, the LCROSS of high illumination (but there may be many more “micro cold mission, and Apollo experience: traps”), and available data suggest they are not the most • Surface ice has been directly detected within multiple PSRs favorable for ice accumulation and persistence. Another option and has been mapped at 240 m resolution (Figure 3). is to use towers that extend from the floors of shadowed craters However, it cannot be determined whether this represents a to heights where illumination is much more favorable. This very thin frost or diffuse ice that extends deeper. • Ice that likely extends deeper than the shallow surface has Lamelin, M. et al. (2014), High-priority lunar landing sites for been directly detected by the LCROSS experiment at Cabeus in situ and sample return studies of polar volatiles. Planetary and crater (6% ice) and is inferred at many other PSRs (1-4% ice) Space Science 101, 149. from neutron spectroscopy data (Table 4). • There is no robust evidence that permafrost or thick ice sheets Lawrence, D. J. (2017), A tale of two poles: Toward are present anywhere in the polar regions. understanding the presence, distribution, and origin of volatiles • There is no robust evidence that ice deposits have at the polar regions of the Moon and Mercury. JGR Planets 122, fundamentally different physical properties than highlands 21. soils elsewhere on the Moon. Ice deposits are likely to be in the form of unconsolidated anorthositic soil with low to Li, S. et al. (2018), Direct evidence of surface exposed water ice moderate concentrations of ice and other volatile in the lunar polar regions. PNAS 115, 8907. contaminants mixed in. This information is surely enough to narrow down sites for Mazarico, E. et al. (2011), Illumination conditions of the lunar future prospecting with higher-resolution orbital instruments polar regions using LOLA topography. Icarus 211, 1066. and with landed spacecraft. For the highly risk tolerant, it may be on the verge of sufficient to choose the location of the first Miller, R. S. et al. (2014), Identification of surface hydrogen mine on the Moon. enhancements within the Moon’s Shackleton crater. Icarus 233, 229. Acknowledgements Much thanks to Ariel Deutsch for fruitful discussions and for Neish, C. D. et al. (2011), The nature of lunar volatiles as reviewing the content in this guide. revealed by Mini-RF observations of the LCROSS impact site. JGR 116, E01005. References & further reading Qiao, L. et al. (2019), Analyses of Lunar Orbiter Laster Arnold, J. R. (1979), Ice in the lunar polar regions. 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