Lunar Polar Regions Research Articles

Terahertz Emission Modeling of Lunar Regolith

We investigate the terahertz (THz) scattering and emission properties of lunar regolith by modeling it as a random medium with rough top and bottom boundaries and a host medium situated beneath. The total scattering and emission arise from three sources: the rough boundaries, the volume, and the interactions between the boundaries and the volume. To account for these sources, we model their respective phase matrices and apply the matrix doubling approach to couple these phase matrices to compute the total emission. The model is then used to explore insights into lunar regolith scattering and emission processes. The simulations reveal that surface roughness is the primary contributor to total scattering, while dielectric contrasts between the volume and the boundaries dominate total emission. The THz emissivity is highly sensitive to the regolith dielectric constant, particularly its imaginary part, making it a promising alternative for identifying previously undetected water ice in the lunar polar regions. The THz emissivity model developed in this study can be readily applied to invert the surface parameters of lunar regolith using THz observations.

Effect of Hydrogen on Albedo Neutrons in the Lunar Surface

The remote sensing observations of the lunar soil by using neutron spectroscopy as well as infrared spectroscopy in various studies suggest the presence of hydrogen in the lunar surface, particularly around the lunar polar regions. Additionally, the cosmic ray protons incident on the lunar surface induce the generation of secondary neutrons depending on the composition of the surface soil and the associated density, a certain number of which, called albedo neutrons, leak from the lunar regolith. Motivated by the interaction of the cosmic ray protons with the lunar surface in the absence and presence of hydrogen, a series of GEANT4 simulations are employed in the current study to unveil the influence of hydrogen on the energy spectrum of the albedo neutrons that escape from the lunar surface. Initially, a discrete 69-bin proton energy spectrum between 0.4 and 115 GeV based on the PAMELA spectrometer is implemented into GEANT4. Subsequently, a single-volume lunar surface of 2-m thickness is constructed where it is assumed that a single layer of either 1.6 or 1.93 g/cm^3 constitutes the lunar regolith. The material composition in the present study includes 43.7 wt.% oxygen, 0.3 wt.% sodium, 5.6 wt.% magnesium, 9 wt.% aluminium, 21.1 wt.% silicon, 8.5 wt.% calcium, 1.5 wt.% titanium, 0.1 wt.% manganese, and 10.2 wt.% iron in accordance with another study. Then, 0.1 wt.% hydrogen is introduced by replacing oxygen in order to assess the impact of hydrogen on the secondary neutrons. In the wake of irradiating a lunar surface of 3*2*3 m^3 with a planar vertical PAMELA proton beam of 20*20 cm^2, the depth profile of the generated neutrons as well as the corresponding initial energy spectrum is primarily obtained in the absence and presence of 0.1 wt.% hydrogen by voxelizing the entire geometry with 100 cells of 2-cm thickness. Next, a surface detector is placed at the top of the lunar surface in order to collect the albedo neutrons in both cases, and the energy spectra of the albedo neutrons are acquired in terms of thermal, epithermal, and fast neutrons. From the GEANT4 simulations in this study, it is shown that the presence of 0.1 wt.% hydrogen is observable in each energy regime of the albedo neutrons at the lunar surface, thereby providing an indication about the elemental variation of the lunar soil.

PneumoPlanet: An Inflatable Moonbase Concept for the European Space Agency 1. Design and Location

One of the greatest objectives of space exploration today is to establish a permanent outpost on the Moon that would allow exploration of the lunar polar regions and the exploitation of local water resources. We envision this lunar outpost as a structure made up of inflatable elements, which would utilize solar energy to produce biomass, and thus, both oxygen and food for the astronauts. Due to the extremely low weight of the inflatable elements, we could build a very large infrastructure on the Moon with the help of only a few rocket launches. The low cost and the sustainable nature of such infrastructure can lay the foundation for a permanent human presence on the Moon. Keywords: Inflatable, Lunar Habitat, Moonbase, Illumination, Sustainability, Greenhouse, Cultivation

The ephemeral state of ice in micro cold traps on the Moon

Micro cold traps stand out as promising targets for future investigation missions to the lunar polar regions, due to their greater accessibility compared to macro cold traps. However, this advantage naturally comes at the cost of their size, which limits the amount of ice they can potentially harbor. Here, we investigate the permanently shadowed volume (PSV) of micro cold traps — an upper limit for their potential ice capacity. We find that, as expected, the PSV depth increases with latitude and surface roughness, but is on average much shallower (∼0.5−1%) compared to the topographic baseline. By comparing the expected destruction and accumulation rates of ice to the potential maximum capacity of micro cold traps, we predict their infill as a function of lateral size, and the lifetime of the ice they harbor. Our results could be used by future investigation missions to the lunar polar regions to constrain the delivery rate and delivery mechanism of ice to the Moon.

Lunar regolith water ice simulation method and characterization

The exploration of lunar water resources has always been a cutting-edge topic in human space exploration. This paper presents a method for gas deposition and ice formation to simulate the mixture of ice and regolith in lunar polar shadowed regions. Following the principles of “physical structure equivalence and environmental parameter approximation,” based on the principle of water molecule deposition and adsorption in a cold trap environment, water ice is converted into water vapor by raising the temperature in a vacuum environment. Then, water molecules deposit and adsorb on the surface of dry, low-temperature mineral particles, forming an ice film under the action of the cold trap capture principle. The morphology and occurrence status of the particle surface ice film has been obtained through microscopic characterization of the samples in a cold environment. Vacuum pressure monitoring equipment is used to verify the pressure changes of water during the sublimation and deposition, and the phase diagram of water is analyzed to understand its state changes within the simulated setup. Samples are retrieved from a high-purity nitrogen glove box, and their water content is verified using thermogravimetric analysis (TGA). The effectiveness of the proposed lunar regolith simulation method is validated through macroscopic and microscopic approaches. This method provides high-fidelity samples for lunar water resource utilization, scientific exploration, and payload development on the Moon.

On the reachability and genesis of water ice on the Moon

Understanding the reachability of water ice by future in-situ experiments near the lunar poles is crucial for supporting growing exploration plans and constraining the uncertainties on its genesis and distribution. To achieve this objective, we perform a thorough three-dimensional mapping of the distribution of water ice in the lunar polar regions (70° onward), integrating radar, optical, and neutron detector observations from the Lunar Reconnaissance Orbiter mission (LRO). Our analysis reveals ∼5-to-8-fold larger expanse of subsurface water ice (∼1-3 m depth) compared to surface water ice (up to 1 m depth) for the north and south poles, respectively. Our investigation cannot rule out the possibility of deep-seated water ice deposits in the lunar poles that remains beyond the detection capabilities of existing instruments on LRO. Moreover, we find that the extent of water ice in the northern polar region (∼1100 ± 74 km2) is twice that in the southern polar region (∼562 ± 54 km2). Our mapping also suggests that the dichotomous latitudinal distribution and the antipodal longitudinal distribution of water ice are likely driven by Mare volcanism and preferential cratering. We provide additional evidence that outgassing during Imbrian volcanism was probably the primary source of subsurface water ice in the lunar poles, which favors larger expanse over meteoritic sources.

Enhanced Interactive Rendering for Rovers of Lunar Polar Region and Martian Surface

Appropriate environmental sensing methods and visualization representations are crucial foundations for the in situ exploration of planets. In this paper, we developed specialized visualization methods to facilitate the rover’s interaction and decision-making processes, as well as to address the path-planning and obstacle-avoidance requirements for lunar polar region exploration and Mars exploration. To achieve this goal, we utilize simulated lunar polar regions and Martian environments. Among them, the lunar rover operating in the permanently shadowed region (PSR) of the simulated crater primarily utilizes light detection and ranging (LiDAR) for environmental sensing; then, we reconstruct a mesh using the Poisson surface reconstruction method. After that, the lunar rover’s traveling environment is represented as a red-green-blue (RGB) image, a slope coloration image, and a theoretical water content coloration image, based on different interaction needs and scientific objectives. For the rocky environment where the Mars rover is traveling, this paper enhances the display of the rocks on the Martian surface. It does so by utilizing depth information of the rock instances to highlight their significance for the rover’s path-planning and obstacle-avoidance decisions. Such an environmental sensing and enhanced visualization approach facilitates rover path-planning and remote–interactive operations, thereby enabling further exploration activities in the lunar PSR and Mars, in addition to facilitating the study and communication of specific planetary science objectives, and the production and display of basemaps and thematic maps.

Mineralogical Characterization of the Lunar South Polar Region: 1. The Artemis Exploration Zone

AbstractThe lunar south pole is a region of focused scientific and exploration interest, with several crewed and robotic missions to this region planned within the next decade. Understanding the mineralogy of the region is essential to inform landing site characterization and selection and provides the key context for interpreting samples and in situ observations. At high latitudes, extreme illumination conditions (high phase angles) can negatively impact the data quality of orbital instruments. This is especially true for passive near‐infrared spectrometers such as the Moon Mineralogy Mapper (M3) and the Kaguya Spectral Profiler, which measure the spectral properties of the surface using reflected sunlight. Using Moon Mineralogy Mapper data, we observed that the south polar region is associated with a detectable mafic signature consistent with the presence of pyroxenes. The strongest mafic signatures are associated with the South Pole—Aitken Basin, suggesting that impact melt and basin ejecta from the lower crust and upper mantle are present within this region. This observation is validated in several ways: (a) comparisons between M3 data acquired during different mission phases, (b) comparisons between multiple spectral parameters sensitive to the presence of mafic minerals, (c) comparisons between the north and south lunar polar regions, and (d) comparisons with publicly available Kaguya polar mineralogy maps and Lunar Prospector elemental abundances. We also investigate the nature of an anomalous high‐albedo region within 2–3° of the south pole observed in Lunar Orbiter Laser Altimeter reflectance data exhibiting a spatially conflicting apparent FeO abundance pattern between several data sets.

The Long‐Term Flux of the Solar Wind Suprathermal Ions That Precipitate on the Lunar Surface

AbstractSolar wind ions weather the optical, physical, and chemical properties of the lunar surface. While the solar wind ion number density is dominated by the thermal population, the study of suprathermal ions is important to understand weathering effects that may develop at depth larger than the penetration depth of the bulk solar wind. Possible manifestations of suprathermal ion weathering include the creation of thick amorphous rims, contamination of isotopic ratios at depth, and alteration of the optical reflectance at infrared wavelengths. Furthermore, while the thermal population forms a beam parallel to the ecliptic plane, suprathermal ions may have a large northward or southward velocity component and can therefore directly access and weather the lunar polar regions, including the interiors of the Permanently Shadowed Regions (PSRs). In this article, we constrain the long‐term properties of the solar wind suprathermal ions using 17 years of observations by the SupraThermal Ion Composition Spectrometer onboard the Wind spacecraft (Wind‐STICS). We find that the 17‐year omnidirectional energy spectra of protons, helium ions, and oxygen ions observed by Wind‐STICS are in excellent agreement with the 11‐month estimates published by Mewaldt et al. (2001, https://doi.org/10.1063/1.1433995) and Mewaldt et al. (2007, https://doi.org/10.1007/978-0-387-74184-0_32). This agreement validates the conclusions of Nénon and Poppe (2020, https://doi.org/10.3847/PSJ/abbe0c) who proposed that suprathermal ions heavier than helium may increase the rate of amorphous rim formation in lunar samples by 50%. Furthermore, the Wind‐STICS observations of out‐of‐ecliptic ions reveal that the lunar PSRs are weathered by the same flux of >15 keV/nucleon as anywhere else on the lunar surface.

Origin of the Moon and Lunar Water

Three principal concepts regarding lunar formation have been examined: the accretion hypothesis, the mega-impact theory, and the multi-impact model. The multi-impact model amalgamates the salient facets of the mega-impact theory and the accretion hypothesis. As per this model, fragments of the terrestrial crust are ejected into space during collisions with numerous planetesimals (proto-asteroids) with diameters around 10-100 kilometers. In the vicinity of Earth’s orbit, this ejecta interacts with the accretion disk, augmenting its mass. Numerical computations demonstrate that particles within the prograde-rotating accretion disk effectively capture prograde-rotating ejecta while shedding retrograde-rotating ejecta onto the planetary body. The multi-impact theory provides an explanation for the creation of not just the Moon and Charon but also the satellites of asteroids. Several predictions regarding the parameters of asteroid satellites posited by this theory have already been validated through statistical analysis of binary asteroids. Different models of lunar formation yield varied conclusions regarding the quantity of lunar water, its subsurface distribution, and isotopic composition. The mega-impact theory postulates the genesis of a largely desiccated Moon. Consequently, the modest lunar water content may arise from comets, solar wind, or the transport of water from Earth’s atmosphere. These mechanisms for lunar water formation imply its superficial occurrence in polar regions and substantial deviation of the deuterium-to-hydrogen (D/H) ratio from terrestrial norms. Conversely, the multi-impact theory posits lunar water’s origin from planetesimals, akin to terrestrial water. Thus, a significant quantity of lunar water is inferred, expected to be pervasive across the lunar surface at varying depths, and possessing isotopic composition analogous to terrestrial water. Geomorphological structures in the lunar polar regions (smoothed craters, landslides, regular patterns) suggest the presence of a substantial permafrost layer with an approximate thickness of a kilometer.

Analysis of the relation of hydrogen distribution and topographic roughness in the lunar south polar region

Appreciable amounts of hydrogen-bearing compounds have been detected within the lunar polar regions. Estimating the effect of the presence of water ice on surface topographic roughness is important for future exploration and activities in the vicinity of the lunar pole. To investigate this issue, we analyzed the correlations between water equivalent hydrogen in the top 1-m surface layer and topographic roughness of lunar south polar regions. The results show that water ice probably plays an important role in the surface roughness at the hectometer scale, and might has a suppressive effect on surface roughness. In the detailed analysis, most the surface roughness at the floor of Shoemaker, Faustini, Slater and Sverdrup shows a decreasing trend with increasing water equivalent hydrogen at different decreasing slopes; Haworth shows slightly increasing trend, contrary to the trend of other studied craters. These observations may be related to small-scale topographic features at the surface and/or subtle changes in surface and subsurface WEH, which in turn affect the roughness characteristics in detail.

Pilot Study of Low-Light Enhanced Terrain Mapping for Robotic Exploration in Lunar PSRs

The recent discovery of water ice in the lunar polar shadowed regions (PSRs) has driven interest in robotic exploration, due to its potential utilization to generate water, oxygen, and hydrogen that would enable sustainable human exploration in the future. However, the absence of direct sunlight in the PSRs poses a significant challenge for the robotic operation to obtain clear images, consequently impacting crucial tasks such as obstacle avoidance, pathfinding, and scientific investigation. In this regard, this study proposes a visual simultaneous localization and mapping (SLAM)-based robotic mapping approach that combines dense mapping and low-light image enhancement (LLIE) methods. The proposed approach was experimentally examined and validated in an environment that simulated the lighting conditions of the PSRs. The mapping results show that the LLIE method leverages scattered low light to enhance the quality and clarity of terrain images, resulting in an overall improvement of the rover’s perception and mapping capabilities in low-light environments.

Numerical Simulation of the Lunar Polar Environment: Implications for Rover Exploration Challenge

The lunar polar regions are key areas for future exploration due to the long-term continuous illumination and persistently shadowed regions that can cold trap abundant water and other volatiles. However, the complex terrain, dynamic lighting, and solar wind-induced electric-field environment present multiple challenges for polar investigation and sampling missions. China’s Chang’E-7 (CE-7) will explore the Moon’s south polar region in 2026. One of the scientific goals is to drill samples in a wide area with a rover for in situ analysis. This study analyzes the engineering constraints of the polar illumination condition, slopes, and electric field for landing and sampling-site selection. Then, we create a 3D model of CE-7’s lunar rover in three operating environments by employing the Spacecraft Plasma Interaction Software, with the rover sampling (i) on a flat surface, (ii) in a shadow, and (iii) near a meter-scale crater under different solar altitude angles. The results show that the rover can be charged to different potentials under the combined effects of solar wind incident angles and surrounding terrains. We find that a favorable traversing and/or sampling site of the rover for future polar exploration is in the upwind direction of a bulge (positively elevated terrains, such as the lander or boulders) or crater, which will cause a minimum charging effect on the rover. Our results have important implications for minimizing the risk of charging effects and guiding the lunar polar region exploration.

Multi-orbit lunar GNSS constellation design with distant retrograde orbit and Halo orbit combination

The Moon is the closest natural satellite to mankind, with valuable resources on it, and is an important base station for mankind to enter deep space. How to establish a reasonable lunar Global Navigation Satellite System (GNSS) to provide real-time positioning, navigation, and timing (PNT) services for Moon exploration and development has become a hot topic for many international scholars. Based on the special spatial configuration characteristics of Libration point orbits (LPOs), the coverage capability of Halo orbits and Distant Retrograde Orbit (DRO) in LPOs is discussed and analyzed in detail. It is concluded that the Halo orbit with a period of 8 days has a better coverage effect on the lunar polar regions and the DRO has a more stable coverage effect on the lunar equatorial regions, and the multi-orbital lunar GNSS constellation with the optimized combination of DRO and Halo orbits is proposed by combining the advantages of both. This multi-orbital constellation can make up for the fact that a single type of orbit requires a larger number of satellites to fully cover the Moon, using a smaller number of satellites for the purpose of providing PNT services to the entire lunar surface. We designed simulation experiments to test whether the multi-orbital constellations meet the full lunar surface positioning requirements, and compare the coverage, positioning, and occultation effects of the four constellation designs that pass the test, and finally obtain a set of well-performing lunar GNSS constellations. The results indicate that the multi-orbital lunar GNSS constellation combining DRO and Halo orbits can cover 100% of the Moon surface, provides there are more than 4 visible satellites at any time on the Moon surface, which meets the navigation and positioning requirements, and the Position Dilution of Precision (PDOP) value is stable within 2.0, which can meet the demand for higher precision Moon surface navigation and positioning.

Sampling of lunar regolith with a deep fluted auger: Experimental study on sampling performance

Currently, sampling and water detection of lunar regolith in the permanently shadowed regions (PSRs) of lunar polar regions is a hotspot in lunar exploration. Using deep fluted augers (DFAs) for lunar regolith sampling was proposed long ago. However, a comprehensive study on how to effectively capture regolith using a DFA has not been reported so far. In this study, based on the principles of regolith capturing with a DFA (RCDFA), we analyzed the factors possibly affecting the mass of the captured sample in terms of lunar soil conditions, drilling operating parameters, and structural design. Using the CUG-1A lunar regolith simulant (LRS), an experimental study was conducted on the influence of drilling operating parameters and the structure of the auger flutes on the mass of the captured sample during sampling operation. The experimental results revealed that the regolith sample might not be captured at all with unreasonable operating parameters; however, when using operating parameters within the feasible range, the mass of the captured sample was positively correlated with the cut per revolution (CPR) at the sampling and retracting stages and with the length and depth at the sampling stage. Among all factors, the CPR at the sampling stage had the most significant impact on the mass of the captured sample. Moreover, the width-to-depth ratio (WDR) of the auger flutes should be neither too small nor too large to obtain a good trade-off between the mass of the captured sample and the stability of the brushing efficiency. This study lays the foundation for further research on the mechanism of how DFAs capture lunar regolith and the optimal design of drilling tools.

Possible Formation Mechanism of Lunar Hematite

Hematite, a ferric mineral with diagnostic features in the visible and infrared spectral range, has recently been discovered in the polar regions of the Moon by the Chandrayaan-1 Moon Mineralogy Mapper (M3). The oxygen involving the oxidization process producing lunar hematite is supposed to originate from the Earth’s upper atmosphere, and hematite with different ages may have preserved information on the oxygen evolution of the Earth’s atmosphere in the past billions of years. The discovery of lunar hematite may provide insight into the understanding of the oxidation products on the Moon and other airless bodies. In this work, we analyze hematite abundance distribution in the lunar polar regions, showing that the content of hematite on the lunar surface increases with latitude, and is positively correlated with surface water abundance. We suggest that the latitude dependence of hematite is derived from the latitude dependence of water, which indicates that water may play an essential role in the formation of hematite. The correlation between hematite and the optical maturity parameter (OMAT) was analyzed and a significant positive correlation was observed, which suggests that the hematite in the polar regions is the result of gradual and persistent oxidation reactions. In addition, based on the analysis of oxygen particles in the Earth wind, it was found that O+ and O2+ are much more abundant, suggesting that low-energy O+ or O2+ ions escaping from the upper atmosphere of the Earth may play a crucial role in the formation of hematite in the lunar polar regions.

Research of Lunar Water-Ice and Exploration for China’s Future Lunar Water-Ice Exploration

Since the 1990s, the existence of water-ice in the permanent shadow areas of the lunar polar regions and the problem of water in the early lunar period have become the hot spot of international lunar exploration. This paper analyzes the research progress and existing problems of lunar water-ice detection in recent years. Based on the analysis of expected foreign lunar water-ice exploration missions, the major scientific problems of lunar water-ice are analyzed. From different exploration methods, this paper tentatively puts forward the scientific tasks, payload configuration, functional requirements, and possible scientific outputs of water-ice in China's future lunar exploration projects, which can provide reference for future lunar exploration missions.

High abundance of solar wind-derived water in lunar soils from the middle latitude

Remote sensing data revealed that the presence of water (OH/H2O) on the Moon is latitude-dependent and probably time-of-day variation, suggesting a solar wind (SW)-originated water with a high degassing loss rate on the lunar surface. However, it is unknown whether or not the SW-derived water in lunar soil grains can be preserved beneath the surface. We report ion microprobe analyses of hydrogen abundances, and deuterium/hydrogen ratios of the lunar soil grains returned by the Chang'e-5 mission from a higher latitude than previous missions. Most of the grain rims (topmost ~100 nm) show high abundances of hydrogen (1,116 to 2,516 ppm) with extremely low δD values (-908 to -992‰), implying nearly exclusively a SW origin. The hydrogen-content depth distribution in the grain rims is phase-dependent, either bell-shaped for glass or monotonic decrease for mineral grains. This reveals the dynamic equilibrium between implantation and outgassing of SW-hydrogen in soil grains on the lunar surface. Heating experiments on a subset of the grains further demonstrate that the SW-implanted hydrogen could be preserved after burial. By comparing with the Apollo data, both observations and simulations provide constraints on the governing role of temperature (latitude) on hydrogen implantation/migration in lunar soils. We predict an even higher abundance of hydrogen in the grain rims in the lunar polar regions (average ~9,500 ppm), which corresponds to an estimation of the bulk water content of ~560 ppm in the polar soils assuming the same grain size distribution as Apollo soils, consistent with the orbit remote sensing result.

Lunar Magnetic Anomalies and Polar Ice

AbstractSolar wind ion sputtering is one of several non‐negligible loss mechanisms for water ice in permanently shadowed regions (PSRs) near the lunar poles. Previous estimates of the solar wind ion flux within south polar PSRs have considered only the ambient solar wind flow and effects of topography. Here, improved maps of crustal magnetic fields in the lunar polar regions are constructed, confirming that more anomalies are present near the south pole than near the north pole. These anomalies have moderate amplitudes, occur over at least two permanently shadowed craters, and correlate approximately with the exposed water ice distribution. Because of the low angle of solar wind incidence near the poles, these anomalies are likely effective in reducing the ion flux, and any resulting water ice loss rate. These anomalies may therefore explain why more water ice is found near the south pole than near the north pole.

A study on the characteristic features of the lunar ionosphere using dual frequency radio science (DFRS) experiment onboard Chandrayaan-2 orbiter

ABSTRACT We present ‘first of its kind’ measurements of the enhanced integrated electron density profiles (iEDPs) at the lunar wake and trans-terminator regions using radio occultation (RO) experiments conducted with the dual frequency radio science (DFRS) payload onboard Chandrayaan-2 (CH2) spacecraft. DFRS uses one-way coherent signals at X and S-bands of radio frequencies for RO measurements. Detailed analysis of the results shows that the electron content is large (∼1.5 TECU, with 1 TECU = 1016 m−2) in the lunar wake region compared to the dayside. Large electron content is also seen near lunar polar regions during solar transition periods. These observations are unique in nature as they show post-sunset enhancements in the iEDPs compared to dayside, as reported by earlier missions. These results further confirm recent predictions from the theoretical model for the lunar ionosphere.