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