Resources for prospective Lunar miners
This page hosts a growing list of resources and tools related to ice deposits at the Lunar poles, including a freely available white paper aimed at non-experts. If you have any questions about the material, want to suggest updates, or would like to work together please don't hesitate to contact me: Kevin Cannon (firstname.lastname@example.org; 401-340-6608).
- PSR Rankings
- Polar Maps
- Ice Prospecting Guide
Below is my top 15 ranking of Permanently Shadowed Regions (PSRs) at both Lunar poles, specifically focused on predicted (not detected) total recoverable water ice. The scores are derived from proprietary metrics and will be updated in the future based on new datasets/models. Data indicating surface frosts are not considered, nor are operability constraints such as proximity to well-lit crater rims. Scores break well into three tiers as indicated.
The collection of maps below were created using publicly available datasets (as indicated) and are free to use or modify in any way. Additional mapping products and ArcGIS files will be added in the future. Customized maps are available upon request.
Blank maps: 75° to 90°
Click to view full size (~55 MB). Based on LRO_LOLA_DEM_NPole75_30m and LRO_LOLA_DEM_SPole75_30m datasets.
Annotated maps with PSRs: 75° to 90°
Click to view full size (~55 MB). Based on LRO_LOLA_DEM_NPole75_30m and LRO_LOLA_DEM_SPole75_30m datasets for topography, and the LPSR_75N_120M_201608.JP2 and LPSR_75S_120M_201608.JP2 datasets for PSR locations.
Ice Prospecting: Your Guide to Getting Rich on the Moon
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 propellants, 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 with 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 rovers–will continue to change and improve our understanding of ice on the Moon. This guide will be updated on an ongoing basis to incorporate new findings and new thinking.
Why is there ice on the Moon?
Two factors create conditions that allow ice to accumulate and persist at the Lunar poles: (1) the Moon has a very small axial tilt of 1.54° compared to 23.5° for the Earth, and (2) the Moon has rough topography, mostly due to large craters formed by asteroid and comet impacts. Combined, these factors lead to topographic lows mostly in crater floors near the poles that never receive direct sunlight. As one can imagine, regions of permanent darkness will be cold on a planetary body with no atmosphere, and the PSRs on the Moon reach temperatures as low as 40 K or colder. At average temperatures below about 110 K, water ice in a hard vacuum is stable against sublimation on billion-year timescales, allowing any ice that accumulated in the past to remain there today. Ice is even more stable when covered with a layer of Lunar regolith (soil).
How did ice accumulate in the first place? There are three main ideas that probably all contributed to ice accumulation in the past (Figure 1): (1) Impacts of large comets and volatile-rich asteroids. These high-energy collisions delivered water and other volatiles to the Moon, forming a transient atmosphere in the hours to days after the impact. Some of the H2O molecules in this atmosphere were cold trapped in the PSRs, possibly building up thick deposits. Accumulation within different PSRs was likely nonuniform and depended on the random locations of sequential impacts. Cratering rates were orders of magnitude higher in the past, so most ices deposited by this process are expected to be old. (2) Volcanic outgassing. Volcanoes erupt lava on the surface, but also spew out significant amounts of gasses including water vapor. Volcanic activity has more or less stopped on the Moon, but was much greater in the past with ancient eruptions outgassing large amounts of H2O, some of which ended up in the PSRs. (3) Solar wind. The solar wind is a stream of electrons, protons and other particles that are constantly colliding with the unprotected surface of the Moon. This process can create individual OH and H2O molecules that are able to ballistically hop across the surface, eventually migrating to the PSRs. This process could lead to the PSRs at lower latitudes accumulating more ice because hopping molecules get trapped before traveling further poleward.
Figure 1. Ice accumulation mechanisms.
Multiple processes operate to modify the concentration and distribution of ice once it has accumulated at the surface. Impact cratering churns up the soil and mixes ice both vertically and horizontally. Given enough time this will break up any continuous ice layers at the surface and incorporate this ice into the surrounding soil. The result will be patchy and diffuse ice in the regolith that varies in concentration on scales of meters or larger. Under certain conditions ice at the surface can be “pumped” deeper into the underlying regolith, but this process may be inefficient on the Moon. Ice is lost due to sublimation and space weathering at the surface, both of which reduce the ice concentration over time without continual resupply. A complicating factor is that the current spin axis of the Moon may not have always been the same. If the spin axis 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 amounts
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 abundances have been estimated by different instruments on orbiting satellites, and by the LCROSS experiment that impacted into the Cabeus crater PSR near the south pole and created an ejected plume of material in which water ice and other volatile molecules were detected.
Figure 2. Locations of Permanently Shadowed Regions at the north and south poles from Mazarico et al. 2011.
Orbital remote sensing instruments have widely different spatial resolutions and sensing depths: this makes interpreting ice locations and abundances 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 in 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 two broad categories of ice deposits present: surface ice (including frost), and deeper diffuse ice.
Table 1. Reported ice concentrations from orbital datasets.
|Lunar Prospector Neutron Spectrometer
||Cannot distinguish uniform low abundances from buried wet layer, senses all H not just H2O
||Very low ice concentrations (<1%) spread over wide regions
||Heterogeneous ice distributions with richest PSRs at 1-4% ice, buried wet layers with >1% ice
||Ice highly clustered with local regions and buried wet layers at >10% ice
|Lunar Crater Observation and Sensing Satellite
||Represents single point on the surface; may not be representative
||2.7% ice at the impact site
||5.6-6.3% ice at the impact site
||8.5% ice at the impact site
||Enhanced circular polarization ratios indicative of subsurface ice are non-unique
||Enhanced CPRs in polar craters are entirely due to rocks, no ice indicated
||Upper limit of 5-10% ice in the subsurface, but ice not uniquely identified
||Meters-thick layers of nearly pure ice in the subsurfaced
|Lyman-Alpha Mapping Project
||Relies on ratios/slopes instead of ice spectrum; cannot distinguish H2O from OH
||Signal can be attributed to OH, very little H2O
||Frosts covering up to 10% of the surface, or diffuse surface ice with 0.1-2% concentration
||Even more extensive ice not detected due to low SNR
|Lunar Orbiter Laser Altimeter
||Single wavelength, enhanced albedo not unique to ice
||High albedos caused by other factors, little to no ice
||Thin frosts or diffuse ice with 7% concentration
||Even more extensive ice not detected due to strict criteria used to report detections
|Lunar Exploration Neutron Detector
||Instrument may not operate as reported (see discussion in Teodoro et al. 2014)
||Much lower spatial resolution than reported, no spatially resolved ice detected
||Up to 0.5% ice over large areas covering the PSRs
||Ice highly clustered with local regions or buried wet layers at >10% ice
|Moon Mineralogy Mapper
||Uses multiply bounced photons for PSR investigations: low SNR
||Ice present as thin frosts with low overall yields
||Up to 20 wt.% ice if present as intimate ice-regolith mixture, or 20 vol.% if pure ice patches
||Even more extensive ice not detected due to low SNR
|Chang'E microwave radiometer
||Unclear how to link the band ratios to physically meaningful properties
||Dataset not fully understood, ice not uniquely detected
||Areas of subsurface ice in some large PSRs
||More extensive ice than reported in initial analysis
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.
Surface ice and frost
In this paper surface ice is defined as ice within the upper microns to millimeters, including frosts that are thin coatings of ice on the surface of regolith grains. If true frosts are present, they probably formed more recently than deeper ice but could still be billions of years old. Figure 3 shows locations of likely surface ice based on orbital data that use reflected UV, visible and near infrared light to detect the spectrum of ice or enhanced brightness interpreted to be caused by ice. Ice is detected where current average temperatures are below 110 K such that ice is stable at the upper surface. The data indicate any frost layers present are patchy, perhaps with 7-20% areal coverage at 100-meter scales. However, the data could also be interpreted as diffuse ice in the upper soil layers rather than true frost. If this is the case then the higher concentrations indicated by the data (as much as 20 weight percent ice) could not extend very deep as a uniform layer, otherwise they would not be consistent with neutron spectroscopy data.
Figure 3. Locations of likely frosts/surface ice detected by the LOLA, LAMP and M3 instruments. Modified from Li et al. 2018, their Fig. 4.
Frosts or surface ice can be readily mined by mechanical excavation or thermal methods (see below), but yields will likely be low if ice is only present as discontinuous frost (Table 2). For example, if the entirety of the Shackleton crater PSR was covered in a patchy micron-thick frost layer this would add up to only about 40 mT (metric tonnes) of ice, and would require a mining architecture that can cover huge areas efficiently. Because of this, extreme caution should be taken when using ice abundances reported from orbital datasets at optical wavelengths (Table 1). These data should be coupled with neutron spectroscopy or other products, and ground investigations will likely be needed to drill or otherwise penetrate beneath any surface ice to properly assess ore grade at depth.
Table 2. Ice concentrations for given frost thicknesses.
||Mass of ice (kg/m2) assuming 20% areal frost coverage
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 bowl shape 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 (weight percent ice)
||Mass of ice in 1 cubic meter of regolith (kg)
Table 4 lists estimates of ice concentrations in some of the larger PSRs based on the Lunar Prospector Neutron Spectrometer dataset, but because of low spatial resolution and non-uniqueness in layering (Table 1), these modeled values should be used only as a guide for crude tonnage estimates and for targeting further prospecting efforts. Promising ore locations can also be estimated based on temperature data and thermal 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.
Figure 4. Example of a map where criteria based on temperature and ice stability have been used to rank locations for likely ore grades. Dark blues = highest potential; pinks = lowest potential.
Table 4. Modeled water equivalent hydrogen* (WEH) concentrations for major PSRs. From Teodoro et al. 2010, their Table 1.
|Peary B||89.2°N, 128.0°E||100||3.2
|Cabeus A||82.4°S, 53.0°W||50||1.6
|Nansen F||84.5°N, 62.2°E||250||0.3
* Water equivalent hydrogen is the mass of H2O assuming that all detected hydrogen is in the form of ice.
Reasonable interpretations about polar ice deposits can be made when orbital datasets are combined and their limitations are understood (Table 1). That said, there are major outstanding questions requiring future orbital and landed missions to fully address. These are some of the more pressing questions relevant to mining:
- How do ice concentrations vary laterally within the PSRs at scales of meters to tens of meters?
- If surface frosts are present, how thick are they and what is their areal extent?
- For diffuse ice, how do concentrations vary as a function of depth? Is the ore richer or poorer at depth? If diffuse ice is present at the upper surface of PSRs, how can this be reconciled with interpretations of neutron spectroscopy data that argue for a dry layer overlying a wet layer?
- How strong is the nugget effect in Lunar ice deposits?
- Are significant amounts of buried ice present in the non-shadowed regions surrounding the PSRs?
Physical nature of Lunar ice deposits
As currently interpreted, orbital data do not support the presence of pure ice sheets or permafrost-like materials within the PSRs. These types of deposits could in theory be present at 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. Particle sizes follow a power law distribution and are about 90-110 μm in diameter on average if highlands soils at the Apollo 16 site are representative (basaltic soils at other landing sites had 40-60 μm average particle diameters). Some people stress that 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 bearing capacity of the soils. It is true that no rovers or landers have investigated the polar PSRs, but a reasonable geologic inference is that their materials are more or less the same as other materials on the Moon, just colder with enhanced concentrations of water ice and other volatiles.
Many In-Situ Resource Utilization (ISRU) studies created simulated ice-bearing Lunar materials, but unfortunately these efforts give incorrect impressions about PSR material properties. These experiments often mix dry Lunar regolith simulant with liquid water then freeze the mixtures. As expected, this leads to hard cemented materials similar to permafrost. Some studies quote compressive strengths over 100 MPa, and these results are used to argue it is prohibitive to excavate ice ore in the PSRs with mechanical diggers. These experiments do not reflect the geologic processes that have formed and then modified polar ices (Figure 1): the same impact cratering that has turned solid basalt into a meters-thick powder on the Moon has been pummeling the PSRs for billions of years, and hard cemented materials are unlikely to have survived intact. Mechanical excavation is probably just as viable in the PSRs as it is elsewhere on the Moon.
Mining ice on the Moon has been discussed and planned under two broad architectures: (1) Mechanically excavating soil then hauling it to a fixed processing plant. (2) Heating or irradiating soils in situ then capturing released volatiles in a mobile tent or dome. There are pros and cons to each approach, but a full discounted cash flow analysis comparing the two options is beyond the scope of this guide.
Mining activities in extreme locations on Earth are proof that ore body characteristics generally take precedence over operability. This may not be true on the Moon at first, and lower grade ores could be targeted initially because they are located in more favorable areas than richer ores. Operability in the challenging Lunar environment can be divided into three categories for simplicity: (1) terrain, (2) power, and (3) communications.
1. Terrain includes topographic slopes, roughness, and rockiness. These are important because most Lunar mining activities will be carried out by semi-autonomous to fully autonomous mobile vehicles, and more benign terrains will increase the efficiency of mining operations. Planetary rovers generally cannot operate on slopes steeper than 20-30 degrees, and must traverse between large boulders and small craters where they are present. Areas of major terrain hazards can already be identified using existing orbital datasets, and supplemented with future orbital platforms and on the ground using rover-based cameras and LiDAR systems.
2. Access to power is likely to be a major limiting factor for initial mining operations. Near the poles there are peaks and crater rims with nearly perpetual sunlight, often in proximity to PSRs with detected ice deposits. Conventional thinking is to land and establish a solar-powered base in the sunlit regions while conducting mining operations nearby in shadowed crater floors. However, distances between sunny areas and ore deposits are at least 5-10 km, and often involve untraversable descents down steeply sloped crater walls. It is not clear how power, equipment and materials will be transferred between the two. As well, there are only a small handful of PSRs located near peaks of high illumination, and available data suggest they are not the most favorable for ice accumulation and persistence. Another option is to use towers that extend from the floors of shadowed craters to heights where illumination is much more favorable. This solves issues of transferring power over kilometer-scale horizontal distances and opens up more areas for mining that are not located close to peaks of near-perpetual sunlight. Tower-supported outposts may also allow for more efficient logistics because all aspects of mining, processing and mission control can be located in close proximity. However, towers will have to be fairly tall (hundreds of meters or more) to reach heights where solar power is viable, and there might only be a small subset of craters or other depressions at very high latitudes that meet the narrow set of constraints required to make this architecture work. Ultimately, nuclear power will offer the best access to PSRs that are not located near sunlit peaks and are too deep for reasonable tower heights. Nuclear will be critical to open up the massive PSRs in Haworth, Shoemaker, Faustini and Sverdrup craters at the south pole that may host especially rich deposits. An added benefit is that waste heat from nuclear reactors can be put to use as part of the mining effort to liberate water from the icy ores.
3. High-bandwidth communication to Earth will be important for mining operations, particularly in early phases before large Lunar bases are established. Existing mapping products are available that show the fraction of time Earth is visible from different locations at the Lunar poles. Locations on the near side of the Moon are more favorable, but ground relays or satellite relays will still probably be needed to provide data connectivity to mine sites within the PSRs themselves. This is doubly true for far side locations.
Ore deposits in the PSRs are not likely to be pure water ice. The LCROSS experiment found evidence for a variety of contaminants including sulfur-bearing phases, carbon-bearing phases, and mercury. Many of these compounds are toxic, and processing methods will need to be able to filter out a variety of metallic and organic materials to purify the ice. Prospecting efforts could target regions where average temperatures are above the sublimation temperatures of most carbon- and sulfur-bearing compounds (~54 K) but still below the sublimation temperature of ice (110 K). Elemental mercury and elemental sulfur will remain an issue between 54-110 K.
Exploring Lunar ices at the poles will never be complete from a scientific point of view. But from a mining perspective, the amount of knowledge of the ore body necessary to start a Lunar mine can be quantified based on the risk tolerance of an organization. Have we reached that point where decisions can be made? The following information is known to fairly high confidence based on multiple orbital datasets, the LCROSS mission, and Apollo experience:
- Surface ice has been directly detected within multiple PSRs and has been mapped at a 240 m resolution (Figure 3). However, it cannot be determined whether this represents a very thin frost or diffuse ice that extends deeper.
- Ice has been directly detected by the LCROSS experiment at Cabeus crater (6% ice) and is inferred at many other PSRs (1-4% ice) from neutron spectroscopy data (Table 4).
- There is no robust evidence that permafrost or thick ice sheets are present anywhere in the polar regions.
- There is no robust evidence that ice deposits have fundamentally different physical properties than highlands soils elsewhere on the Moon. Ice deposits are likely to be in the form of unconsolidated anorthositic soil with low to moderate concentrations of ice and other volatile contaminants mixed in.
Much thanks to Ariel Deutsch for fruitful discussions and for reviewing the content in this guide.
References & further reading
Arnold, J. R. (1979), Ice in the Lunar polar regions. JGR 84, 5659.
Colaprete, A. et al. (2010), Detection of Water in the LCROSS Ejecta Plume. Science 330, 463.
Eke, V. R. et al. (2009), The spatial distribution of polar hydrogen deposits on the Moon. Icarus 200, 12.
Elphic, R. C. et al. (2007), Models of the distribution and abundance of hydrogen at the Lunar south pole. GRL 34, L13204.
Fa, W., and V. R. Eke (2018), Unravelling the Mystery of Lunar Anomalous Craters Using Radar and Infrared Observations. JGR Planets 123, 2119.
Feldman, W. C. et al. (2000), Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles. Science 281, 1496.
Fisher, E. A. et al. (2017), Evidence for surface water ice in the Lunar polar regions using reflectance measurements from the Lunar Orbiter Laser Altimeter and temperature measurements from the Diviner Lunar Radiometer Experiment. Icarus 292, 74.
Hayne, P. O. et al. (2015), Evidence for exposed water ice in the Moon’s south polar regions from Lunar Reconnaissance Orbiter ultraviolet albedo and temperature measurements. Icarus 255, 58.
Lamelin, M. et al. (2014), High-priority Lunar landing sites for in situ and sample return studies of polar volatiles. Planetary and Space Science 101, 149.
Lawrence, D. J. (2017), A tale of two poles: Toward understanding the presence, distribution, and origin of volatiles at the polar regions of the Moon and Mercury. JGR Planets 122, 21.
Li, S. et al. (2018), Direct evidence of surface exposed water ice in the Lunar polar regions. PNAS 115, 8907.
Mazarico, E. et al. (2011), Illumination conditions of the Lunar polar regions using LOLA topography. Icarus 211, 1066.
Miller, R. S. et al. (2014), Identification of surface hydrogen enhancements within the Moon’s Shackleton crater. Icarus 233, 229.
Neish, C. D. et al. (2011), The nature of Lunar volatiles as revealed by Mini-RF observations of the LCROSS impact site. JGR 116, E01005.
Qiao, L. et al. (2019), Analyses of Lunar Orbiter Laster Altimeter 1,064-nm Albedo in Permanently Shadowed Regions of Polar Crater Flat Floors: Implications for Surface Water Ice Occurrence and Future In Situ Exploration. Earth and Space Science 6, 467.
Schorghofer, N., and O. Aharonson (2014), The Lunar thermal ice pump. ApJ 788, 169.
Siegler, M. A. et al. (2016), Lunar true polar wander inferred from polar hydrogen. Nature 531, 480.
Teodoro, L. F. A. et al. (2010), Spatial distribution of Lunar polar hydrogen deposits after KAGUYA (SELENE). GRL 37, L12201.
Teodoro, L. F. A. et al. (2014), How well do we know the polar hydrogen distribution on the Moon? JGR Planets 119, 574.
Thomson, B. J. et al. (2012), An upper limit for ice in Shackleton crater as revealed by LRO Mini-RF orbital radar. GRL 39, L14201.
Watson, K. et al. (1961), On the possible presence of ice on the Moon. JGR 66, 1598.
Yang, F. et al. (2019), Study of Chang’E-2 Microwave Radiometer Data in the Lunar Polar Region. Hindawi Advances in Astronomy 2019, 3940837.