Too often as casual observers, but sometimes as geoscientists, we look at the moon and just see an orb of various shades of gray, maybe reflect on its surface features, maybe think about its impact on the formation of the Earth or maybe its daily and seasonal impact on our climate, our near-surface depositional processes, biological impacts, and maybe even think about it in terms of our literature, history and even our love life.
This is a lot to ascribe to the moon, but it’s true. As geologists, there is so much more to consider about the moon and our critical understanding is needed for detailed exploration and development of lunar resources.
Plans are afoot for the moon to serve as our future intermediate base for trans-lunar spaceflights, as well as a low-gravity laboratory system and to be utilized for long-term Earth observations, and become an inhabited, multifunctional, diverse, and high-technology population center with resource utilization, engineering, mining and manufacturing.
In the interest of these future activities there are two new, robotic boots-on-the-ground exploration geology missions that are part of a new space race to discover, develop and utilize lunar resources. Besides minerals and base materials as a historic primary goal, the new primary goal is the exploration, mining and exploitation of volatiles – primarily solar wind gases and water ice.
Not As Easy As It Looks
The news and other media make it look like going to and exploring the moon is easy. Yet, after almost 65 years of lunar space flight there are still mission failures. Of the last five years of lunar landing missions, Beresheet, Luna 25, Hakuto-R, Omotenashi and Rashid failed to successfully land. Chandrayaan-3, Chang’e 4 and Chang’e 5 were successful landers, although the Vikram Lander and Pragyan Rover had only short-term success. This is a recent international success rate of less than 50 percent. Of the 140 lunar missions launched to date, 64 failed or partially failed. To add a little perspective, the U.S. success rate is 74 percent, the Russian (former USSR) success rate is 31 percent and the Chinese success rate is, according to their reports, 100 percent.
It should be noted that the United States and Russia have launched 82 percent of all lunar missions. There have been six human-crewed lunar landing missions, several successful commercial uncrewed lunar flybys and five uncrewed lunar landers/rovers. There are more than 30 lunar missions, 24 of which are lunar lander missions, planned in the next five years, including six crewed lander missions. Of these, two major efforts are the geological-based landers, PRIME-1 and NASA VIPER Rover prospector missions to the lunar south pole.
Composition of Lunar Regolith
To understand the target geology, think of the lunar surface as an igneous provenance desert regolith ranging from very fine-grained silt to scattered rocky outcrops. For this discussion, the term “regolith” is used to represent the full range of lunar surface materials as described in the U.S. Geological Survey lunar map. There are no significant organic compounds on the solar exposed surface of the moon due to temperature (about 110 to minus-150 degrees Celsius, but minus-223 degrees in shaded zones), solar wind flux, cosmic radiation, etc. In the non-sun-exposed surfaces, the constant dark, continuous cold traps, there are volatiles that have been detected from orbit. The average lunar surface composition is SiO2, AL2O3, FeO, CaO, MgO, TiO (about 97 percent), plus localized KREEP (K-potassium, rare Earth elements, and P-phosphorous). Lunar regolith contains abundant oxygen (about 40 percent), trace metals in variable concentrations and mineable He-3 (5-60 parts per billion). Representative lunar simulants, used in laboratory studies, are often dominantly basaltic with plagioclase group anorthites and a mix of mafic and oxide minerals.
Lunar regolith analysis mainly comes from the samples returned by the six Apollo missions, the 1976 Luna 24 mission, and more recently from the Chang’e-5 probe. These earlier data are recorded in the Lunar Sourcebook (Carrier et al. 1991). The following geotechnical data exist: particle size distribution, particle shapes, specific gravity, bulk density and porosity, relative density, compressibility, shear strength, permeability and diffusivity, bearing capacity, slope stability, and trafficability (the capacity of soils to successfully support vehicles or other weight without being damaged). Lunar regolith, weathered by a spectrum of impacts from large-scale and to micro-meteors, solar winds, internal alpha particle radiation and extreme temperature change, is broadly graded as medium-grained sand to fine silt and is commonly described as a silty sand or a sandy silt and is generally described as well-graded. Lunar soil has a bulk density, ρ, of 1.30 grams per centimeters-cubed at the surface, 1.52 at 10 centimeters, 1.83 at 100 centimeters, asymptotic at 1.92 grams per centimeters-cubed. Lunar soil porosity, φ, varies between about 30 to 50 percent. The most recent data is from Chang’e-4 and the Chandrayaan 3 Vikram Lander with its Pragyan Rover. The Pragyan Rover, even though having a very limited lifetime, validated the presence of Fe, Ti, Ca and Al at the lunar south pole and confirmed the presence of sulpur in lunar materials adding to the available elements for lunar use. The Chang’e-4 Yutu Rover found olivine and orthopyroxene on the far side of the moon as expected, and the more recent Change-5 sample return probe, landing in Oceanus Procellarum on the near side, reported the presence of water known to be present at the poles and possible in the form of OH elsewhere. It returned 1.73 kilograms of regolith with additional and new geotechnical parameters and chemical/mineral analyses for that landing site.
The Race for Lunar Volatiles
The ongoing surface and subsurface exploration of the moon is heating up. Most recent lunar orbiting missions are for surface mapping geology and geophysics. Many of these instruments were developed, designed and planned before the discovery of water ice in cold traps (but predicted since the early 1960s) and do contain critical sensors for geoanalyses. Minerals plus water opens opportunities for a wide variety of in-situ resource utilization strategies which, in turn, opens the door to habitability, commercial space utilization and human deep space missions beyond the moon.
All lunar data are important and critical but in the new race, lunar volatiles – especially water-bearing ice and minerals – are the most sought after. The existence of lunar water ice was proposed in the early 1960s by physicist Kenneth Watson and his colleagues, but data were sparse and discussed in the literature as a concept, a possibility, through the 1980s. Its discovery at the poles of Mercury increased speculation that it would be found at the moon’s poles as well. Apollo landing sites were not at the poles but in equatorial areas, so the lunar sediments interrogated from those missions were primarily dry. The Clementine orbital geology mission was flown in 1994 – the first since 1972 – and the onboard neutron spectrometer detected a water ice signature over the poles in dark craters. In the Nov. 29, 1996 issue of “Science,” it was announced that interpretation of data from Clementine suggested the possibility of water ice on the surface of the moon. The Lunar Prospector mission (1998) included a neutron spectrometer experiment designed to detect water ice at a level of less than 0.01 percent by looking for “slow” (thermal) and “intermediate” (epithermal) neutrons. The data showed a distinctive 4.6-percent water ice signature over the north polar region and a 3.0-percent signature over the south down to a depth of about half a meter.
This was a game changer.
What had been just a hypothetical now had instrumental support from two different sources, both detecting potentially significant volumes of water ice. The nature of the poles became a key topic of study and the cold trap concepts were developed. Subsequently, the impact plumes of the Lunar Crater Observation and Sensing Satellite (LCROSS) and its Centaur rocket stage in the Cabeus crater near the south pole of the moon on Oct. 9, 2009 showed the spectral signature of hydroxyl, a key indicator that water ice is present in the floor of the crater at concentrations of roughly 6-percent water in the impact area including potentially some nearly pure water ice crystals. The Indian Chandrayaan-1 Moon Mineralogy Mapper (2009) experiment showed low-concentration hydroxyl signatures over much of the lunar surface, and possible large deposits of water ice in the northern lunar craters.
Water became a primary driver for resource discovery and utilization for the moon. To transport water to the lunar surface is tremendously expensive – water for human use, potential agricultural use, for oxygen, hydrogen and for propellants. Water for use in space might be the most valuable commodity of all. The search for water ice now drives most lunar government-funded or encouraged expeditionary science and there is a competitive race between nations.
Two U.S. missions that will accelerate this process are the PRIME-1 (Polar Resources Ice Mining Experiment – 1) and VIPER (Volatiles Investigation Polar Exploration Rover) missions, currently scheduled to launch in 2024. These missions utilize four primary exploration instruments, a drill system TRIDENT (The Regolith and Ice Drill for Exploration of New Terrains), the NIRVSS (Near-Infrared Volatiles Spectrometer System), the MSolo (Mass Spectrometer Observing Lunar Operations) and the NSS (Neutron Spectrometer System).
Scheduled to launch first, as currently planned in early 2024, PRIME-1 is a lander-hopper mission just outside of the Shackleton Crater. Following on the initial results of the Apollo sample analyses, this will be the first in-situ resource utilization demonstration on the moon and will test the TRIDENT and MSolo instruments while generating useful multisite subsurface data. VIPER is capable of wide ranging and sampling and will be launched in November 2024 to land near the Nobile Crater.
The TRIDENT will be on both missions and is a rotary percussive drill capable of depths to 1,020 millimeters with a bit diameter of 24.5 millimeters. It can sample at 10 centimeters and shorter intervals and measure cuttings size, bearing capacity and regolith heat flow temperature. The MSolo will also be on both missions and interrogate the composition of gases around the lunar lander, including those emanating from the lander itself, and any present in the lunar exosphere. This provides a baseline, lander-induced background measurement for future and more complex science missions.
Additionally, as it travels, the MSolo will monitor how the local environment’s composition changes during the transition from lunar day to lunar night. This allows for a baseline study of the lifetime and transport properties of identified compounds, including CO2, CO, H2, H2S, NH3, SO2, CH4 and C2H4, released from lunar surface and subsurface regolith. Also measured will be sublimating water ice at concentrations as low as 0.5 percent (by weight), the distribution of hydrogen-bearing volatiles, the isotopic ratios for deuterium/hydrogen and the isotopic ratios for O18/O19. On the VIPER, the MSOLO will be able to scan a five-meter swath with a horizontal sample measurement resolution of 10 centimeters.
The NIRVSS will be on the VIPER and be capable of detecting and analyzing minerals, water-ices, carbon dioxide, methane and ammonia and generally the nature of any hydrogen present. The VIPER is designed to explore the extreme environment area near the southern lunar pole with a planned 100-day-plus mission. Also on the VIPER is the neutron spectrometer system, a hydrogen-detection instrument that will scan the terrain ahead of the VIPER to look for potential water ice and other hydrogen-bearing regolith.
The VIPER is approximately 2.5 meters tall and 1.5 meters in length and width and weighs 430 kilograms. It will travel at a rate just under half a mile per hour. VIPER can explore dark areas using its headlights and onboard camera and avionics systems and has the ability to traverse various incline levels and multiple regolith types while using its NSS and cameras to scan the terrain in a five-meter swath with 10 centimeter sample resolution. The VIPER utilizes batteries while in shadowed dark areas and recharges when in areas of sunlight.
Much of the lunar in-situ resource utilization research over the last two decades has concentrated on oxygen and helium by heating dry regolith, as well as materials for use in 3-D printing of habitats and processing facilities, and water ice by heating via excavation or in-situ processing. In general, the current research concentrates on producing oxygen by melt processing, oxygen and hydrogen from ice disassociation and nitrogen potentially from regolith. Assuming we can bring the carbon with us to the moon, derive from regolith solar wind volatiles, or derive from lunar regolith a la carbonaceous asteroids, we now have C, H, O, N (about 99 percent of animal/plant mass) and can produce: water, alcohol, methane, NH3, NO3, etc., fertilizer (with metals from regoliths). Sulfur and phosphorous are all that’s missing for life and fuels, plus other metals that are on the moon can lead to energy and manufacturing. However, Pragyan found sulfur. If we can extract helium (there is much more helium-4 available than the rare and valuable helium-3, although helium-3 estimates are probably minimum estimates) from lunar regoliths, as proposed by Harrison H. Schmitt, then we have one of the primary industrial/aeronautical working fluids (especially for cryogenic operations).
Launching Lunar Geoscience into the Future
PRIME-1 and VIPER, standing on the shoulders of all the missions that came before, will ensure a strong future of geological and exploration development on the moon, and beyond.
What does this mean for expanded lunar geoscience? It means lot:
• Stratigraphy: If various volatiles are located, then both a vertical and lateral geometry can be measured, or at least estimated, then a stratigraphic relationship/concept can be made.
• Geochemistry, Geotechnical, Geophysics: If the relationships between volatiles found and volumetrics can be made depending on regolith type and stratigraphy, with temperature and nature of occurrence, that will lead to a decision-making process for next generation lunar mining excavators and surface mapping methodologies.
• Exploration: Once stratigraphy and in-situ relationships are known, a lunar ice geomorphology and depositional system can be developed.
• Mining, Economics: Volatile chemical forms, once quantified, will allow for a detailed estimate of lunar manufacturing capability (water, O2, H2, etc.) necessary for human sustainability.
• Geochemistry, Systemic Planetology: Once lunar volatiles are quantified, isotopic ratios determined and physical chemistry is established, then a comparison can be made to Terran water to understand age, source and history.
• Deposition and Diagenetics: Knowing the variation in the volatile chemistry and the regolith chemistry will allow for predictive mineralogy and maybe hint at some form of lunar diagenesis.
• Space Biology and Agriculture: Following these, soil sciences can be evoked for plant growth potential.
• Geotechnical and Civil Engineering and Construction: Once the geotechnical parameters are further developed and various geotechnical geospatial changes known, basic lunar materials for building can be utilized.
• Geophysics: If ice/volatile/metallic ionic variations exist geospatially then, in the presence of solar disturbances, regolith based electrical conductance/generation may be possible.
• Geochronology, Seismology, Tectonics: A comparison of indigenous lunar waters and deep Earth crustal and/or mantle waters could greatly aide in understanding the Earth system.
• Development, Exploration: If VIPER, including the MSolo, TRIDENT. NSS and NIRVSS instruments work as planned, numerous rovers could be deployed giving a rapid understanding of lunar resources.
Lunar geological science is developing at an increasing rate and is certain to grow. The year 2024 might be the year of lunar geology and provide a quantum leap in our understanding of the future potential of the moon for exploration of space and for the benefit of Earth.