The Research on H₂O Adsorption Characteristics of Lunar Regolith Simulants: Implications for the Development and Utilization of Lunar Water Resources

Abstract

This study prepared an adsorption-based water-containing lunar regolith simulant under low-temperature conditions to investigate H₂O behavior in simulated lunar environments. Experiments established that water binds to regolith particles via adsorption rather than existing in liquid/solid states, with critical initial pressure thresholds identified at various temperatures to ensure pure adsorption conditions. Crucially, coexisting substances extend H₂O preservation to −100 °C, suggesting substantial water retention in lunar polar regolith even under extreme cold. Sublimation modeling further revealed phase transition boundaries, indicating water ice likely persists in both permanently shadowed regions and illuminated polar areas. These findings provide fundamental insights into: adsorption-driven enrichment/preservation mechanisms of lunar water, thermodynamic stability thresholds at ultralow temperatures, and water ice distribution patterns across lunar polar terrains. The data advance understanding of lunar water’s stability and extractability, offering critical scientific support for future in situ resource utilization and sustained lunar exploration.

1. Introduction

The development and utilization of lunar resources play a pivotal role in the human exploration of deep space. As humanity ventures further into the solar system, the rich raw materials present on the moon’s surface will serve as a valuable resource base, offering vital support for space exploration endeavors [1,2,3]. Lunar water is of vital interest as an extraterrestrial resource, essential for enabling sustained human presence, deep space exploration, and the development of lunar infrastructure [4,5,6,7]. Molecular water (H₂O) possesses considerable economic potential due to its viability and feasibility as a resource. In the ultrahigh vacuum conditions on the Moon, the adsorption of H₂O by the lunar regolith plays a critical role in governing chemical and physical processes on the lunar surface, thereby influencing its potential for resource exploitation. Cocks et al. investigated the presence of adsorbed water in the lunar regolith of the polar regions [8], and Hodges further confirmed the existence of ice in the lunar polar regions [9], underscoring the significant role of water molecule adsorption by the regolith. The form of lunar water resources that exist is largely governed by temperature conditions on the Moon. Temperature fluctuations induced by comet and micrometeorite impacts trigger surface processes that release gas molecules [10,11,12]. Studies indicate that the water content on the lunar surface can vary by approximately 200 parts per million (ppm) within a day, likely due to mechanisms such as solar wind sputtering and other temperature-dependent processes [11]. Scientists have performed temperature-programmed desorption measurements on the lunar regolith simulants (JSC-1A and albite) using diffuse reflectance infrared Fourier transform spectroscopy. These studies offer a preliminary understanding of the adsorption and desorption behavior of water on the Moon, particularly regarding its interaction with solid surfaces at low temperatures [13,14,15]. A portion of water molecules transported to permanently shadowed regions is anticipated to diffuse into the regolith and remain trapped for prolonged periods [16]. As a result, water is expected to exist primarily in an adsorbed state, with potential storage capacities reaching several hundred ppm. Significant temperature fluctuations on the lunar surface, combined with the low thermal conductivity of the regolith, will affect the rates of heating and cooling experienced by H₂O [17]. Chemical reactions between minerals in the regolith and water molecules may lead to the formation of hydrated regolith minerals, and water could lead to the formation of hydrated minerals, thereby influencing the phase transition behavior of water [18]. Furthermore, the high radiation environment and near-vacuum conditions on the lunar surface will modify the evaporation and sublimation processes of water molecules. The low-gravity environment of the Moon will also influence the movement of water molecules, thereby affecting their phase transitions [19,20].

A method was developed to prepare a water-containing lunar regolith simulant by selecting quartz materials with low adsorption performance to load polar region lunar regolith simulation samples for adsorption tests under vacuum and low-temperature conditions. The unique physical and chemical characteristics of lunar regolith, with its granular structure capable of absorbing H₂O, can alter its phase transition temperature. By adjusting the initial water content in experiments, the adsorption and desorption behaviors of the samples are monitored to determine the initial pressure threshold for simulant samples of different contents at various temperatures. Because the lunar regolith significantly influences the phase transition relationship of H₂O, constructing a ternary phase diagram for the polar region lunar regolith simulant offers reference temperatures for payloads in lunar polar exploration, facilitating the precise determination of water content in the polar regions based on temperature measurements. This investigation not only helps to reveal the adsorption effects of H₂O in lunar regolith but also provides a scientific basis for the future development and utilization of lunar resources.

2. Materials and Methods

2.1. Ground-Based Simulation System of H₂O Adsorption

The experimental setup in this study was interconnected via metal valves (V0, V1, V2) and comprised three primary components: the H₂O Generation System (H₂OGS), the Lunar Regolith Adsorption System (LRAS), and the Vacuum Gauge Measurement System (Figure 1). The system operates under low-pressure conditions facilitated by vacuum pump sets. The main structure of the simulation system is constructed from stainless steel due to the necessity of operating at extreme temperatures. Specifically, adsorption at 100 K requires a material that can adsorb significant amounts of H₂O; hence, stainless steel is utilized. Conversely, desorption at 500 K demands materials resistant to high temperatures, for which sample chambers are constructed from quartz glass tubes.

The H₂OGS used in this research involves placing distilled water in a detachable CF-16 caliber high-vacuum quartz glass tube. The system is controlled with a VAT57036-GE02 all-metal angle valve capable of withstanding extreme temperatures to provide high-vacuum gas output. The LRAS consists of a semiconductor aluminum plate cooler, a liquid nitrogen circulation cooling device, a quartz glass tube, a paperless recorder, valves, and pipelines. A quartz glass tube containing a predetermined weight of lunar regolith simulant samples is positioned above the semiconductor aluminum plate cooler and the liquid nitrogen circulation cooling device to replicate the low-temperature conditions of the lunar surface. The sample temperature is continuously monitored with a paperless recorder. The simulation system maintains a high vacuum level through the operation of mechanical and molecular pumps. The air pressure within the chamber is monitored using a Pfeiffer-CMR362 vacuum gauge, with a measurement error of 0.2% and a range from 1.0 × 10⁻² Pa to 1.1 × 10³ Pa.

2.2. Ground-Based Simulation Test Procedure

In this experiment, stainless steel and quartz glass were selected as low-temperature-resistant materials to minimize adsorption effects from materials other than the samples. Baseline experiments using stainless steel tubes revealed significant adsorption of H₂O on the tube walls, potentially affecting the experimental data accuracy. Conversely, quartz glass tubes exhibited reduced H₂O adsorption compared with stainless steel tubes due to their smooth surfaces. Consequently, quartz glass tubes, with lower adsorption capacity, were chosen to contain the polar region lunar regolith simulants. Before the experiment, the entire system was baked at 120 °C for 2–3 days, and valve V was opened to utilize the molecular pump for degassing to eliminate atmospheric H₂O adsorption in the system. Once the baking process was completed, the system was allowed to return to room temperature. The experimental procedure is as follows:

The experiment begins with pre-treating the samples by placing lunar regolith simulant samples in a vacuum drying oven at 300 °C for 24 h (Figure 2). Once dried, the samples are weighed to the target weights of 40 mg and 130 mg and then transferred to test tube-2. Test tube-1 and test tube-2 are connected to the vacuum system, and valves V and V2 are opened. The ground simulation system, consisting of H₂OGS and LRAS, maintains a vacuum of 1.0 × 10⁻² Pa using pump sets. A vacuum gauge continuously monitors the chamber pressure. After 2 h of maintaining the vacuum, valve V is closed, and valve V1 is manually opened to release water vapor to the preset level for the experiment. Upon reaching the preset value, valve V1 is closed to cease the supply of water vapor. Following this, a semiconductor aluminum plate cooler is activated to lower the sample temperature from 25 °C to 0 °C. Subsequently, the semiconductor aluminum plate cooler is exchanged with a liquid nitrogen circulation cooling device. The liquid nitrogen input is adjusted based on the sample temperature feedback to maintain a stable decrease in temperature of the simulated lunar regolith samples within the quartz tube. Once the sample reaches a low temperature of −60 °C, the liquid nitrogen input is discontinued, concluding the low-temperature treatment of the sample. It is important to disassemble the glass tube containing the sample after each set of experiments. This exposes the system to the atmospheric environment briefly, impacting the system’s background. To mitigate this, the vacuum pump assembly is used to remove interfering gases and minimize environmental effects.

2.3. Sample Handling and H₂O Release

The distilled water and lunar regolith simulant samples used for H₂O adsorption in the study undergo specific pre-treatments. Distilled water undergoes 3 min low-pressure (1.0 × 10⁻² Pa) boiling process through vacuum distillation to eliminate dissolved gases and non-volatile impurities effectively. Pre-treating the lunar regolith simulant is crucial as the initial moisture content impacts its water adsorption capacity. To standardize the sample’s water content accurately, a portion of the simulant is evenly spread in a glass dish and then subjected to vacuum drying at 300 °C for 24 h. Lunar regolith simulants inherently retain some moisture content due to natural hygroscopicity. Removing bound water within simulant crystals poses challenges, requiring prolonged heating at specific temperatures to reach the evaporation point for complete removal of intergranular bound water. During the progressive heating of rock, the water present within it (comprising structural water, bound water, intergranular adsorbed water, etc.) undergoes phase transitions, evaporating and escaping at distinct temperatures. Notably, adsorbed water typically evaporates at approximately 100 °C, while strongly bound water is released within the temperature range of 200–300 °C [21]. Consequently, subjecting polar lunar regolith simulants to 400 °C for 12 h within a glove box serves to mitigate the impacts of adsorbed and structural water. Following the drying process, 40 mg and 130 mg of the lunar regolith simulant are precisely weighed using a semi-micro balance and then carefully transferred to the base of quartz glass tube-2. To prevent the adherence of fine regolith particles to the tube’s inner walls, aluminum foil is utilized during the transfer process.

Figure 2. Schematic diagram of the ground-based simulation test procedure of H₂O adsorption on lunar regolith.

Figure 2. Schematic diagram of the ground-based simulation test procedure of H₂O adsorption on lunar regolith.

2.4. H₂O Adsorption and Environmental Temperature Control

Upon the complete release of H₂O, valve V1 is promptly closed while valve V2 is opened to facilitate the movement of water molecules toward colder regions, leading to their adsorption and capture with the cold trap within the adsorptive lunar regolith simulant particles. Real-time monitoring of the temperature change of the lunar regolith simulant sample inside quartz tube-2 at room temperature (T = 25 °C) is conducted alongside continuous tracking of the vacuum level P within the system chamber. The sample’s temperature is gradually lowered to 0 °C utilizing a semiconductor cooling stage. However, the cooling capacity of the semiconductor cooling stage is constrained by the theoretical limitations of the Peltier effect, resulting in a notable decrease in the cooling rate at lower temperatures. Below 0 °C, the semiconductor cooling stage is substituted with a liquid nitrogen circulation freezing device to efficiently cool the sample, lowering the temperature of the simulated lunar regolith sample to the desired low-temperature threshold for establishing the correlation between temperature and pressure values. The quantity of liquid nitrogen added is modulated based on the temperature change rate, and cooling ceases upon reaching the preset temperature.

2.5. Representativeness of Samples and Characterization of H₂O

Lunar soil, comprising the fine-grained lunar regolith, plays a crucial role as the interface between the Moon’s solid structure and the broader solar system environment, offering valuable insights into both the lunar interior and exterior [22,23]. Predominantly composed of pyroxenes (orthopyroxene, clinopyroxene), plagioclase, olivine, and ilmenite, lunar soil exhibits variations in mineral composition across different lunar surface regions. Subject to meteorite impacts, micrometeoroid collisions, and cosmic ray and solar wind exposure, as well as extreme temperature fluctuations on the lunar surface, lunar soil undergoes a distinct formation process divergent from terrestrial soil, resulting in unique physical properties [24]. Given the limited availability of authentic lunar regolith samples retrieved from the Moon, the utilization of lunar regolith simulants is essential for terrestrial testing and validation purposes. The study utilized representative simulated lunar regolith samples, prepared by Professor Zou Meng of Jilin University, derived from volcanic ash sourced from Huinan County, Jilin Province [25]. These samples closely mimic the characteristics of lunar polar regolith. The properties of the simulated lunar regolith were assessed in accordance with the Chinese National Standard for Soil Testing Methods (GB/T50123-2019) [26]. The highland lunar regolith simulant consists primarily of plagioclase feldspar, with minor quantities of pyroxene, biotite, amphibole, and opaque minerals, maintaining an anorthosite-to-basalt ratio of about 7:3. Particle shapes are mainly angular to subangular, and the particle size distribution closely matches that of Apollo 16 lunar regolith samples. Additionally, the porosity and void ratio closely resemble those of Apollo 16 regolith, facilitating accurate simulation of polar regolith samples [27,28]. In this study, distilled water is placed in a vacuum environment in the H₂OGS to lower its boiling point and release H₂O, as opposed to relying on ice sublimation for H₂O release [29]. Ice sublimation necessitates energy to overcome intermolecular forces for the direct transition from solid to gas, which is less efficient compared with obtaining H₂O through the evaporation of distilled water by reducing pressure. Alternative approaches, such as conductive heating for H₂O release, not only elevate the sample temperature but also markedly increase the temperature of the quartz tube, resulting in temperature variations across different system components. According to the ideal gas equation (PV = nRT), conductive heating can affect the stability of pressure measurements within the system.

3. Results and Discussion

3.1. Selection of Material for Adsorption Vessels

Stainless steel, a chemically stable metal, is commonly used for pipelines in chemical reagent transportation and storage due to its inert nature. Despite its stability, the microstructure of its alloy composition allows for some level of physical adsorption of certain chemicals on its surface. In contrast, quartz glass, renowned for its low reactivity and surface energy, is highly esteemed in chemical analysis and laboratory settings. The exceptionally smooth surface of quartz glass tubes minimizes chemical reactions and physical adsorption, ensuring high purity and preventing cross-contamination during experiments. Additionally, quartz glass exhibits exceptional stability at extreme temperatures and remarkable thermal shock resistance. In this study, the adsorption effects of metal stainless steel and quartz glass tubes were compared. Low-temperature tests were conducted in chambers with varying water content. It was observed that, at identical temperature conditions, stainless steel exhibited a higher adsorption capacity for water molecules compared with quartz glass (Figure 3). The strong adsorption capabilities of metal tubes can lead to substantial interference in adsorption experiments involving lunar regolith simulant samples. This interference can obscure the differentiation between background adsorption and sample adsorption, potentially resulting in the inadvertent adsorption of H₂O by the metal tubes. Consequently, quartz glass tubes, characterized by inferior adsorptive properties, were chosen as the substrate for the sample adsorption experiments in this investigation. This decision renders quartz glass an optimal material for executing experiments with a focus on precision and purity.

3.2. Adsorption Thresholds of Polar Region Lunar Regolith Simulant at Different Temperatures

CE-7 will utilize its scientific instruments to gather and assess lunar regolith samples obtained from sunlit regions and from micro-cold traps within 30 cm of the surface. The sampling mechanism comprises two primary techniques: scooping and drilling. The scoop delivers roughly 130 mg of material per iteration to the analyzer, whereas the drill provides approximately 40 mg for each analysis. Consequently, samples of these specified quantities were chosen for inclusion in this investigation. By manipulating the initial adsorption conditions, this study established initial H₂O release pressures of 540 Pa, 400 Pa, and 240 Pa, utilizing 40 mg and 130 mg of polar region lunar regolith simulants for adsorption assessments across temperatures ranging from 25 °C to −40 °C. Through a comparative analysis of adsorption rate profiles under varied initial pressure conditions and sample quantities (Figure 4a), the empirical findings elucidate the interplay between the adsorption dynamics and the thermodynamic and kinetic attributes of the molecules. Elevated temperatures augment molecular kinetic energy, intensifying the frequency of collisions between adsorbing molecules and the sample surface, thereby amplifying the initial adsorption rate. Conversely, higher initial pressures at lower temperatures facilitate enhanced adsorption owing to heightened intermolecular forces, culminating in increased adsorption quantities. Background adsorption occurs in the no-sample control experiment due to low-temperature adsorption on the quartz tube. Within the experimental temperature range, adsorption rates increase with decreasing temperature. There is a slight variation in the onset temperature of rapid adsorption for different sample quantities, albeit minimal. Comparison of adsorption experiments with and without samples reveals that the quartz glass tube has a lower adsorption capacity than the polar region lunar regolith simulant. Consequently, it is feasible to regulate the initial pressure threshold under specific temperature conditions to exclusively adsorb H₂O with the lunar regolith simulant.

Polar lunar regolith simulants demonstrate a notable tendency to desorb molecular water at low temperatures, with a direct correlation between higher water content and the temperature required for desorption (Figure 4b). In adsorption studies, temperature dictates the ultimate equilibrium state and adsorption capacity from a thermodynamic perspective, whereas time delineates the kinetics that control the rate of adsorption. Temperature fluctuations impact both the equilibrium uptake and reaction kinetics. Simultaneously, the temporal aspect enables the evaluation of the operational efficacy of the adsorption process and furnishes crucial kinetic parameters pertinent to engineering applications, such as the determination of reactor residence times. To address this, the CE-7 Lunar Soil Volatiles Measurement Instrument payload employs a specific operational approach during in-orbit detection. This method involves initially reducing the pressure to below 100 Pa before conducting in situ measurements of lunar regolith with elevated water content. By doing so, the necessary heating temperature is decreased, leading to expedited water content retrieval from the lunar regolith and a reduction in in-orbit power consumption for the payload.

The adsorption properties of a 130 mg polar simulated lunar regolith sample were investigated across a range of pressures and temperatures to assess their impact on the sample’s adsorption capability. Figure 5 illustrates five distinct pressure levels: 119 Pa, 227 Pa, 525 Pa, 877 Pa, and 1026 Pa. Elevated temperatures (ranging from −20 °C to 15 °C) were found to correspond to a notable increase in adsorption capacity, which exhibited a direct correlation with the initial H₂O content. This phenomenon can be attributed to the heightened thermal movement of molecules at higher temperatures, facilitating the ready adsorption of water molecules onto the samples. Conversely, at lower temperatures (ranging from −35 °C to −20 °C), the adsorption capacity decreased with decreasing temperature. This decrease may be attributed to the reduced thermal movement of molecules at lower temperatures, thereby enhancing the adsorption effect. It is worth noting that the adsorption capacity appeared to reach saturation within this temperature range.

At lower pressures (e.g., 119 Pa and 227 Pa), the relationship between adsorption capacity and temperature shows minimal variation, suggesting reduced temperature sensitivity in low-pressure conditions. Conversely, at higher pressures (e.g., 1026 Pa), this relationship demonstrates a more pronounced dependence on temperature, highlighting increased sensitivity in high-pressure settings. Additionally, adsorption isotherms at varying pressures display distinct shapes; specifically, at higher pressures, the isotherms align more closely with the Langmuir model, whereas at lower pressures, they exhibit greater conformity to the Freundlich model [30].

3.3. Effect of the Action of Other Substances on the Temperature of H₂O Preservation

The phase behavior of water in quartz glass, metal materials, and polar lunar regolith simulants can be elucidated by referencing the H₂O phase diagram. By utilizing temperature and pressure data from a vacuum gauge (Figure 6), the boundaries between different states and phase transitions can be determined. The phase diagram of H₂O reveals that the coexistence of solid, liquid, and gas phases occurs at a pressure of 611.73 Pa. This study specifically examines the low-pressure regime below 100 Pa to simulate the phase transitions between the solid and gas phases of water in lunar regolith samples. The aim is to enhance our understanding of the phase transition characteristics exhibited by simulated lunar regolith materials.

Experimental data spanning 0–1000 Pa reveal a significant change in adsorption at a specific temperature, indicating a phase transition in the sample. The solid-gas phase boundary curve is generated through mathematical fitting techniques, specifically polynomial exponential fitting (Figure 6). The phase transition dynamics of H₂O are influenced by the distinct physical and chemical characteristics of various materials. Metals exhibit higher thermal conductivity, enabling water molecules within metals to swiftly achieve thermal equilibrium and undergo phase transitions at lower pressures compared to quartz glass and lunar regolith simulants (as depicted by the yellow fitting curve in Figure 7). In contrast, quartz glass, with lower thermal conductivity, results in a slower cooling rate of water molecules compared with metals, consequently exerting a lesser impact on the phase transition boundary of water molecules (illustrated by the purple fitting curve in Figure 7). Therefore, considering the influence on the phase transition boundary of H₂O, employing quartz glass tubes as sample containers was deemed more appropriate for this investigation.

A 40 mg lunar regolith simulant sample (light blue curve in Figure 7) exerts a more pronounced impact on H₂O phase transition compared to a 130 mg sample (dark blue curve in Figure 7). However, their phase transition boundaries align within a 95% confidence interval. Ground validation experiments, by monitoring phase transition boundary alterations with and without regolith in the quartz tube, can establish an initial pressure threshold at a specific temperature. This threshold corresponds to the phase transition curve within the quartz tube devoid of samples, ensuring exclusive sample adsorption at this temperature, irrespective of sample quantity, and precluding adsorption by the quartz tube.

The experimental three-phase diagram suggests that the presence of additional substances can influence the freezing point of water. It is postulated that a considerable amount of water molecules could potentially exist in the polar regions of the Moon under the influence of regolith at temperatures as high as −100 °C. Findings from water adsorption and temperature-controlled desorption experiments on Apollo lunar samples suggest that water-ice may remain stable on the lunar surface when temperatures are below 110 K [31,32]. This observation aligns with the three-phase diagram presented in this study, indicating that water on the lunar surface remains in the form of ice at temperatures below 110 K (−163 °C).

Experimental measurements and theoretical extrapolation of the water-ice phase transition in lunar soil simulants have provided crucial insights into their physical properties and detection techniques. Figure 7a illustrates that under low-pressure conditions, the simulant material significantly suppresses the sublimation of water ice embedded within it. All observed solid-gas phase transition boundaries lie below and to the right of the theoretical curve for pure ice, as depicted in the pressure-temperature (P-T) relationships. In comparison to the no-sample control, both the 40 mg and 130 mg samples exhibit enhanced suppression. This phenomenon is ascribed to the specific surface area and porous structure of the lunar simulant, which bolsters adsorptive and capillary interactions with the ice, consequently bolstering its stability and heightening the energy barrier for sublimation. Theoretical extrapolations were conducted based on these observations. Figure 7b extends the phase diagram to higher pressure and temperature ranges, demonstrating that in the extremely low-pressure lunar environment, ice sublimation prevails, while the formation of liquid water becomes thermodynamically viable only under increased pressures. These results emphasize that the thermophysical properties of water ice are significantly altered by the surrounding regolith matrix. Therefore, engineering strategies for on-site lunar ice detection must consider these matrix influences. To ensure data accuracy, sampling techniques must preserve the integrity of the ice-regolith system, such as employing scoop sampling for larger sample volumes. It is important to recognize that extracting small samples through drilling may lead to an underestimation of ice stability and an overestimation of sublimation rates. Consequently, a dependable evaluation of lunar water ice distribution and stability necessitates consideration of these variables.

3.4. Prediction of Lunar Regolith Water Content on the Moon

The polar regions of the Moon, characterized by their distinct geographical positioning and environmental attributes, serve as a primary focus for lunar exploration and scientific inquiry. Unlike Earth, the Moon lacks an atmospheric shield, leading to substantial diurnal temperature fluctuations in its polar areas, exceeding 300 °C. These regions feature persistently shaded regions with frigid temperatures, offering optimal conditions for the detection of water ice and volatile compounds, which can remain stable under such extreme cold. Notably, the Shackleton Crater within the lunar polar region emerges as a key candidate for future lunar missions, with NASA’s Commercial Lunar Payload Services program evaluating its exploration potential [33,34]. While precise remote sensing temperature data is lacking, scientists estimate that temperatures in the permanently shadowed regions of the Moon’s south pole could plummet to around −249 °C, nearing absolute zero (−273.15 °C), potentially preserving ancient ice deposits. Rayal et al. have identified a substantial overlap between the cold traps within the Moon’s south pole and the interior of the Shackleton Crater, where most areas experience a maximum annual temperature of −200 °C. Referring to the three-phase diagram in Figure 7, it is suggested that within this lunar south pole region, water molecules are likely predominantly present as ice within the lunar regolith at −200 °C [35].

In the Moon’s polar regions, topographical shielding and seasonal solar elevation angle variations can lead to lower temperatures, creating a conducive environment for the presence of molecular water. The Stratospheric Observatory for Infrared Astronomy (SOFIA) serves as a distinctive platform for observing infrared spectra reflected and emitted from the lunar surface. Analysis of SOFIA data has revealed the presence of molecular water in illuminated regions near the Moon’s south pole, with water concentrations in lunar regolith ranging from 100 ppm to 412 ppm [36]. This discovery offers direct evidence of molecular water distribution on the lunar surface. In these illuminated polar regions, temperatures can plummet to approximately −100 °C [37]. Analysis of the three-phase diagram in this study suggests the possible presence of water ice in the lunar regolith under low-temperature conditions. Ground simulation experiments have corroborated this hypothesis. Specifically, under simulated polar lunar environmental conditions with an initial water molecule pressure of 10 Pa, 130 mg of polar lunar regolith simulants exhibited a water content of 475 ppm. This finding reveals substantial potential for resource exploitation in the illuminated polar regions of the Moon. Unlike Permanently Shadowed Regions, these areas offer more moderate thermal conditions, enhancing their suitability for establishing and sustaining a lunar base. Moreover, continuous solar exposure provides essential energy for base operations and enables sustained resource extraction.

4. Conclusions

The development of water-inclusive lunar regolith simulant is essential for replicating lunar surface conditions, advancing lunar scientific exploration, and supporting sustainable human presence on the Moon. This study developed a method for producing a water-absorbing lunar regolith simulant at low temperatures. We focused on H₂O adsorption and desorption in regolith interactions under low-pressure lunar surface conditions. The findings are crucial for comprehending water accumulation, retention, and potential utilization in the Moon’s polar regions. Our results confirm that water in lunar regolith exists primarily in an adsorbed state on particle surfaces, rather than in liquid or solid form. Experimentally determined pressure thresholds for unobstructed adsorption are 1350 Pa at 20 °C, 175 Pa at 0 °C, and 5.0 × 10⁻¹ Pa at −30 °C. The three-phase diagram shows that the preservation temperature of H₂O is affected by other substances, indicating the potential existence of H₂O at temperatures above −100 °C in the Moon’s polar regions under regolith conditions. By applying mathematical fitting techniques to sublimation curves, we gained further insight into the phase transition characteristics of regolith materials. The results support the hypothesis of a substantial reservoir of water ice in the lunar regolith, particularly in permanently shadowed regions and certain illuminated polar areas. Studying water adsorption in polar lunar regolith simulants is essential for understanding the presence and distribution of water in lunar polar regions. This research elucidates the durability and utility of molecular water within lunar regolith, providing a scientific foundation for future exploration, settlement, and resource utilization. It further informs strategies for extended lunar missions and advances understanding of the lunar surface’s physicochemical environment.