Ice Film Growth Thickness on Simulated Lunar Rock Surfaces as a Function of Controlled Water Vapor Concentration

Abstract

A mathematical model was established to describe the sublimation and diffusion of water molecules and their adsorption onto cold traps. This model was used to analyze the combined influence mechanisms of sublimation temperature and ambient pressure on the vapor deposition process of water ice. Tunable Diode Laser Absorption Spectroscopy (TDLAS) was employed to provide real-time feedback on water vapor concentration within the experimental apparatus. Based on this feedback, the sublimation temperature was dynamically adjusted to maintain the concentration dynamically stabilized around the target value. A dedicated apparatus for generating controlled water vapor flow fields and detecting concentration was constructed. The accuracy of both the mathematical model and Finite Element Analysis (FEA) simulations was verified through comparative experiments. Laser triangulation was utilized as a method to detect the thickness of the adsorbed ice film on the sample surface. Leveraging this technique, a water vapor deposition and adsorption verification system was developed. This system was used to test the differences in water adsorption performance across various materials and to measure the correlation between the thickness of the adsorbed/deposited ice film on the samples and both deposition time and sublimation temperature.

1. Introduction

The detection of lunar water-ice is currently a major international focus in deep space exploration. Water resources on the Moon hold significant importance for sustaining astronaut life, advancing deep space exploration technologies, unraveling the Moon’s evolutionary history, and fostering the development of in situ resource utilization (ISRU) technologies. Within the Permanently Shadowed Regions (PSRs) near the lunar poles, water molecules are adsorbed and deposited onto the surfaces of lunar regolith and rocks, forming water-ice films, primarily through the cold-trapping mechanism. Existing lunar imagery has revealed the presence of heterogeneously distributed surface rocks. To enhance the accuracy and reliability of water-ice detection missions, it is essential to conduct realistic simulation studies in laboratory environments. This involves generating water-ice films on simulated lunar rock surfaces to investigate the growth rate of water-ice on rocky substrates.

Both domestic and international research indicates that lunar water-ice is primarily concentrated within the Permanently Shadowed Regions (PSRs) at the poles, such as Shackleton and Shoemaker craters near the South Pole and specific areas near the North Pole [1]. The extreme cold (as low as ~40 K/−233 °C) and vacuum conditions in these regions enable the long-term preservation of water-ice [2]. Early missions, like the US Clementine mission (1994), first proposed the hypothesis of water-ice existence based on radar signal anomalies (elevated Circular Polarization Ratio, CPR) [3]. Subsequently, the Lunar Prospector mission (1998) detected enhanced hydrogen abundance at the poles using its neutron spectrometer, estimating water-ice reserves on the order of hundreds of millions of tons, although it could not definitively distinguish the source of the hydrogen [4,5,6]. A significant breakthrough came in 2008 when India’s Chandrayaan-1 mission, utilizing the Moon Mineralogy Mapper (M³) instrument, provided the first direct spectroscopic detection of hydroxyl (OH⁻) and water molecule (H₂O) signatures on the lunar surface. This confirmed the dynamic formation of water within the regolith, with stronger signals observed in the polar regions [7,8]. Direct confirmation followed in 2009 with NASA’s LCROSS impact experiment, which unequivocally verified the presence of water-ice (estimated at 5.6 ± 2.9% by mass) in the lunar soil within Cabeus crater near the South Pole [9,10,11,12,13,14]. Challenging the traditional view of an entirely dry Moon, China’s Chang’e-5 mission (2020) detected hydroxyl and even structurally bound water (approximately 120 ppm) in non-polar regolith samples [15,16,17]. Most recently, in 2023, NASA’s Lunar Reconnaissance Orbiter (LRO) mission refined models of polar water-ice distribution, revealing that it exists as a “patchy” mixture within the top layer of the regolith (purity 1–5%), with total reserves estimated at roughly 600 million metric tons–sufficient to potentially support long-term operations at a lunar base [18,19]. Gundlach et al. experimentally confirmed that dust layer thickness is negatively correlated with gas permeability, affecting sublimated gas transport efficiency [20]. Andreas, using a new low-temperature vapor pressure formula, determined that the ice sublimation rate is extremely low in the lunar polar environment of 40–70 K, providing a reference for experimental temperature control [21]. Vasavada et al. revealed through thermal modeling that temperature distribution in polar craters regulates ice sublimation stability [22]. Paige et al. verified with Diviner radiometer data that surface temperatures in permanent shadow regions like the lunar south pole’s Cabeus crater are as low as 29 K, with an annual average temperature at a 2 cm depth of about 38 K, and that ice molecule diffusion and migration are negligible in this environment, further clarifying key parameters for simulating polar cold traps in experiments [23].

This study employs a vapor deposition technique to form water-ice films on rock sample surfaces. We analyzed the factors influencing the sublimation rate of water molecules and their adsorption velocity during this process. A dedicated apparatus for generating controlled water vapor flow fields and detecting concentration was constructed, serving as a system for preparing simulated lunar ice-regolith mixtures. The validity of the theoretical analysis was confirmed through both experimental testing and simulation modeling. This study simulates the adsorption and deposition of water molecules on rock surfaces in the lunar polar permanently shadowed regions (PSRs), and their subsequent formation of a thin ice film. This work provides a validated simulation platform for laboratory studies of lunar ice-regolith materials and establishes a foundation for verifying in situ resource utilization (ISRU) technologies relevant to deep space exploration missions.

2. Water Molecule Generation and Adsorption Model on Particulate Surfaces

2.1. Water Molecule Generation and Adsorption Equations

The sublimation and adsorption processes of water molecules within the extreme lunar environment are governed by the complex interplay of multiple factors. On the one hand, the exceedingly low ambient pressure significantly reduces the resistance that water molecules must overcome to sublimate, leading to a substantially higher sublimation rate compared to terrestrial conditions. On the other hand, dramatic temperature fluctuations cause the saturation vapor pressure of water to undergo drastic variations between lunar day and night, further disrupting its sublimation dynamic equilibrium. Consequently, the developed model primarily focuses on capturing the coupled effects of temperature and pressure.

Equation for the generation of water molecules

During the vapor deposition process, the chamber is initially evacuated to remove air and maintained under low pressure, resulting in an extremely low concentration of air molecules. Furthermore, when the sublimation rate of the water-ice source is slow, the concentration of gaseous water molecules within the chamber is also low. Consequently, intermolecular collisions are relatively infrequent. Therefore, the sublimation process is primarily governed by water molecules overcoming the interaction forces with the solid ice surface, and this process can be approximated as occurring under quasi-equilibrium conditions. Under these circumstances, the Hertz-Knudsen equation is applicable for approximating the water-ice sublimation rate. The sublimation flux J is given by:

$$J=p_v\left(T_1\right)\sqrt{{\displaystyle \frac{M}{2\pi R{T}_1}}}(1-{\displaystyle \frac{p_{s1}}{p_v\left(T_1\right)}})$$

(1)

where J is the sublimation rate, T₁ is the absolute temperature at sublimation, P_v(T₁) is the equilibrium vapor pressure of ice at temperature T₁, M is the molar mass of water (0.018 kg/mol), R is the ideal gas constant (8.314 J/(mol·K)), P_s1 is the actual air pressure on the surface of the sublimate.

Water molecule adsorption equation

Similarly, due to the extremely low concentration of air molecules within the chamber, the deposition rate onto the cold trap is governed by the difference between the vapor pressure and the saturation vapor pressure at the temperature of the deposition surface. The corresponding equation is

$$v_{\mathrm{dep}}=k_{dep}{A}_2\left(P_{s2}-P_0{e}^{-{\displaystyle \frac{\Delta H_{vap}}{R}}\left({\displaystyle \frac{1}{T_0}}-{\displaystyle \frac{1}{T_2}}\right)}\right)$$

(2)

where  $v_{Deposition}$ is the condensation rate of water molecules, $k_{dep}$ is the condensation rate constant, A₂ is the heat conduction area of Ninghua, P_s2 is the actual air pressure on the surface of Ninghua, P_v(T₂) is the saturated vapor pressure of ice at temperature T₂, P₀ is the known reference barometric pressure, and T₀ is the known reference temperature.

2.2. Water Molecule Diffusion Equilibrium Equation

As the chamber constitutes a hermetically sealed independent system, the pressure at the sublimation surface (P_s1) is proportional to the concentration of gaseous water molecules generated by sublimation:

$$P_{S1}={\displaystyle \frac{nR{T}_1}{V_{\mathrm{dev}}}}$$

(3)

where n is the amount of water molecular substances in the device; $V_{dev}$ is the volume of the vapor deposition device.

According to Fick’s first law, the diffusive flux $J_{dev}$ within the chamber is given by:

$$J=-D{\displaystyle \frac{\Delta C}{L}}={\displaystyle \frac{D}{R\overline{T}L}}(P_{S1}-P_{S2})$$

(4)

where D is the diffusion coefficient, T is the rated temperature in the device, and L is the length of the device.

This establishes the relationship between the applied temperature within the chamber, the sublimation rate, and the vapor pressure. Furthermore, when dynamic equilibrium is achieved between sublimation and deposition processes within the system, the sublimation rate equals the deposition rate ( $v_{sub}$ = $v_{dep}$).

2.3. Numerical Calculation of Water Molecule Generation and Adsorption Equilibrium

2.3.1. Numerical Calculation of the Relationship Between Water Ice Sublimation Velocity

(1)

The relationship between sublimation speed and temperature

Figure 1 illustrates the relationship between the initial sublimation rate and the applied external temperature. At constant pressure, the sublimation rate increases significantly with rising temperature, exhibiting a characteristic exponential relationship. At constant temperature, the sublimation rate markedly decreases with increasing gas pressure. This rate is governed by the difference between the saturation vapor pressure corresponding to the current temperature and the ambient gas pressure. Moreover, under low-temperature sublimation conditions, sublimation ceases entirely when the system pressure exceeds the saturation vapor pressure corresponding to that temperature.

(2)

The relationship between sublimation speed and time

Figure 2a shows the relationship between the sublimation-generated water molecule concentration ρ and sublimation time t under different imposed external temperatures, for a system with an initial pressure of 10⁻² Pa, a thermal conduction area of 1 m², and without consideration of the cryogenic trap adsorption end.

Analysis of Figure 2b reveals that within the temperature range of −70 °C to −50 °C, the sublimation rate of water ice is comparatively slow. Upon reaching the saturated vapor pressure, the water vapor concentration within the system exhibits oscillatory behavior, gradually converging toward a stable value. This behavior is attributed to the progressive increase in water vapor concentration and corresponding system pressure during sublimation. As the pressure transiently exceeds the saturated vapor pressure by a small margin, the elevated pressure induces the deposition of a fraction of the vapor molecules back onto the ice surface. Subsequently, when the pressure falls below the saturated vapor pressure, sublimation resumes. This iterative process continues until dynamic equilibrium is established, resulting in the stabilization of both the system pressure and the water vapor concentration.

Within the temperature range of −50 °C to 10 °C, the water vapor concentration in the system exhibits a sharp initial increase during the first few minutes, demonstrating significantly faster kinetics compared to the −70 °C to −50 °C regime. However, as concentration rises, the consequent increase in system pressure progressively suppresses the net sublimation rate, leading to a gradual decline in the concentration growth rate over time.

Within the 10~40 °C range, water vapor concentration in the system increases rapidly. However, as both temperature and pressure exceed the triple point of water, partial melting of water ice may occur, introducing liquid water. This phase transition precludes valid vapor deposition measurements due to fundamental thermodynamic constraints. Consequently, only data corresponding to vapor concentrations below the triple-point pressure of 611.73 Pa (equivalent to a water vapor concentration of 6435.4 mg/m³) were recorded.

2.3.2. Numerical Calculation of Equilibrium Concentration in the System

Upon introducing a simulated lunar regolith cold trap into the system as the adsorption terminal, water ice undergoes sublimation into vapor at the sublimation source while water vapor concurrently adsorbs and deposits onto the simulated lunar regolith, reforming solid ice. A directed vapor flux spontaneously establishes between the sublimation and adsorption terminals due to pressure and temperature gradients.

First, the initial conditions of the system are set, according to the materials consulted, and the accumulation of lunar soil particles in test tubes is simulated; the particles at the bottom of the test tube cannot come into direct contact with gaseous water molecules. Consequently, only the upper surface area of the regolith bed contributes to heat transfer. This configuration yields the ratio of sublimation heat transfer area (A₁) to deposition heat transfer area (A₂) as

$$A_1:A_2=m_{ice}\times SS{A}_{ice}:m_{soil}\times SS{A}_{soil}\times \eta =1:224$$

(5)

where m_ice is the quality of the water ice, M_soil is the simulate the mass of lunar soil particles, SSA_ice is the specific surface area of water ice, SSA_soil is the simulate the specific surface area of lunar soil particles, and η is the coefficient of contact between gas and particles.

Therefore, the sublimation heat transfer area A₁ and the sublimation heat conduction area A₂ were set to 1 m² and 154 m² respectively, the cold trap temperature T₁ was set to −196 °C of the liquid nitrogen bath, and the initial air pressure P_s0 was set to 10⁻² Pa.

Due to the low saturated vapor pressure of water and the slow sublimation rate within the range of −70 °C to −40 °C, a small amount of gaseous water molecules generated will be re-adsorbed and deposited by the water ice at the end of the water molecule flow field. It is difficult to accurately determine the real-time concentration of gaseous water molecules in the system through calculation, which has a significant impact on the regulation of gaseous water molecules. Moreover, the concentration of gaseous water molecules in the device is relatively low. The time required for preparing lunar water ice simulators by vapor deposition is relatively long, which affects the preparation efficiency. The sublimation rate of water ice in the temperature range of 40 °C and above is too fast, and the concentration of water molecules in the system increases rapidly within a short period of time. The rapid increase in concentration poses a significant challenge to the response speed of water molecule control, which is not conducive to the real-time adjustment of the control system based on the data from the detection equipment. Moreover, the rapid increase in concentration also leads to a rapid rise in the air pressure within the system. There is a risk that in a short period of time, the pressure in some areas will be higher than the triple point of water, causing solid water ice to liquefy and affecting the accuracy of vapor deposition, as shown in Figure 2. Therefore, only the occurrence and control of water molecule processes will be studied in the range from to −30 °C to 30 °C.

Substituting the above conditions into the program, the program was used to calculate the relationship between the water molecule concentration ρ in the system and time t within the range of −30 °C to 30 °C. The results are shown in Figure 3.

Calculations indicate that sublimation initiates rapidly at the water source. Conversely, deposition at the cold trap proceeds slowly due to the initially low water vapor concentration within the system. Consequently, the system’s water vapor concentration increases significantly. As concentration rises, pressure builds, leading to an accelerated deposition rate and a deceleration of sublimation. These opposing processes ultimately reach a dynamic equilibrium where the water vapor concentration stabilizes, exhibiting only minor fluctuations around a specific value.

By comparing the changes in water molecule concentration over time at different temperatures, it can be found that the higher the temperature, the faster the sublimation rate, and the higher the water molecule concentration in the system at equilibrium, and the time required to reach equilibrium also increases accordingly. It indicates that the sublimation rate of water ice can be controlled by temperature, thereby controlling the concentration of gaseous water molecules within the system, as well as the sublimation rate and concentration value at the sublimation adsorption equilibrium.

3. Control Model of Water Molecule Flow Generation Rate and Its Detection Method

3.1. Simulation of Water Ice Sublimation Process and Concentration Control

3.1.1. Finite Element Model Construction and Parameter Setting

The parameters required for simulation (Comsol 5.0) are shown in

Table 1. Simulation Parameter Table.

Parameter Name	Numeric Value
Sublimated heat conduction area, A1	3.96 × 10−3 m2
The heat conduction area of sublimation, A2	0.9765 m2
The total volume of gas in the device, V	2.646 × 10−3 m3
Thermal conductivity of aluminum, kalu	220 W/(m·K)
The thermal conductivity of borosilicate glass, kglass	0.9 W/(m·K)
Simulate the thermal conductivity of lunar soil, klunar soil	0.3 W/(m·K)
The initial air pressure inside the device, P0	10−2 Pa
Simulate the temperature of the lunar soil cold trap, T2	−196 °C

below.

In the simulation software, the thermal and flow field relationship formulas were set, and the surface of the solid water ice in the test tube was meshed, as shown in Figure 4, where the water ice was finely meshed according to volume changes and thermal conditions. The simulated lunar soil is divided into grids based on the thermal and flow fields as a whole, while the rest only considers the flow field. Therefore, the grids are divided only based on the flow field to reduce the computing power burden, as shown in Figure 5.

Figure 5. Overall grid division diagram of the device.

Figure 5. Overall grid division diagram of the device.

Normal displacements are set at each point of the grid, which are associated with the mass loss of water ice, that is,

$$\Delta h=\frac{\Delta m}{{\rho }_{ice}}=\frac{v_{sub}\times \Delta t}{{\rho }_{ice}}$$

(6)

By combining the above equation with the sublimation rate equation of water ice, the real-time volume change of water ice in the test tube can be obtained. Through the volume change, the accurate dynamic sublimation heat transfer area can be obtained. The simulation temperatures were set to −30 °C, −15 °C, 0 °C, 15 °C and 30 °C, respectively, for calculation and analysis.

3.1.2. Analysis of Finite Element Simulation Results

(1)

Volume change of water ice

During the water vapor flow field development, the water ice volume progressively decreases due to sublimation, as depicted in Figure 6a. After 15 min of sublimation, volumetric changes initiate preferentially at the upper section of the ice within the test tube, forming a protuberance-like shape. This occurs because water molecules at the upper surface detach more readily. By 30 min, sublimation progresses at the lower section near the glass, resulting in a conical configuration. This morphology arises from the low gas molecule concentration and system pressure, which reduce gas-phase heat transfer efficiency. Once the outer ice surface sublimates, it loses direct contact with the glass heat source. Consequently, heat transfer slows, significantly reducing the sublimation rate at the core compared to the outer surface, leading to the formation of a protruding conical structure. After 60 min, the ice initially contacting the glass has substantially sublimated, allowing gas to escape through the formed gaps. Sublimation of the ice contacting the lower tube section creates a void beneath the remaining ice. This experimental validation confirms the accuracy of the finite element simulation regarding water ice volume reduction, as shown in Figure 6b.

Simulation results depicting water ice volume reduction over time are presented in Figure 7. At 30 °C, the ice completely sublimates within 147 min, while at −30 °C, sublimation remains incomplete even after 300 min. This demonstrates that a higher external heat source temperature accelerates the sublimation rate, resulting in greater volume reduction within the same timeframe. At a constant temperature, the sublimation rate progressively decreases as the ice volume diminishes. Notably, once the ice volume falls below 1 cm³, the effective heat transfer area for sublimation significantly contracts, causing a marked reduction in the sublimation rate. Under these conditions, actively increasing the temperature can accelerate sublimation.

(2).

Changes in the concentration of water molecules inside the device

Simulation results for water vapor concentration over time are shown in Figure 8. At the onset of sublimation, the sublimation rate substantially exceeded the deposition rate. Consequently, the water vapor concentration within the apparatus increased rapidly until reaching an equilibrium, which persisted for a period. Subsequently, as the water ice volume diminished, the deposition rate surpassed the sublimation rate, leading to a gradual decrease in internal concentration. Upon complete depletion of the water ice, the vapor source within the apparatus ceased. This caused a pressure drop and reduced collision frequency among water molecules, resulting in a sharp decline in the deposition rate. Consequently, the concentration decreased slowly thereafter.

3.2. Construction of Water Molecule Flow Field Generation Control and Concentration Detection System

Through the above finite element analysis, it was found that the sublimation rate of water ice and the real-time concentration of water molecules in the device are positively correlated with the sublimation temperature applied externally. Therefore, the concentration of water molecules generated in the device can be controlled and adjusted in real time by changing the sublimation temperature applied externally, as shown in Figure 9.

For the required quantity of lunar water ice simulant prepared via vapor deposition, the target water molecule concentration ρ and its fluctuation range are first designed. An initial temperature T₁, corresponding to the target concentration at equilibrium, is then selected for heating and sublimation. During sublimation, the current water molecule concentration ρ within the apparatus is continuously monitored. This measured concentration is input into simulations to calculate the corresponding sublimation rate $v_{sub}$. Based on this calculated value, the sublimation temperature is proactively adjusted to maintain the water molecule concentration within the target range.

If the concentration of water molecules in the system exceeds the target range during preparation, the sublimation temperature T₁ should be rapidly reduced to bring the concentration in the device back to the target concentration ρ; otherwise, the sublimation temperature T₁ should be rapidly increased. This ensures that the concentration of water molecules within the device is always maintained within a fixed range of the target water molecule concentration ρ.

The test setup built based on the schematic diagram in Figure 9 is shown in Figure 10.

3.3. Analysis of the Test Results of Flow Field Generation Control and Concentration Detection

3.3.1. Accuracy Verification of Water Molecule Detection by TDLAS

Before the test begins, it will be necessary to check the airtightness of the device. The helium mass spectrometer leak detector was used to check for leaks in the device. The helium concentration on the display of the helium mass spectrometer did not change by an order of magnitude, indicating that no helium gas had entered the device. The device has good air tightness and can be used for the test.

During the experiment, the concentrations of gaseous water molecules in the device were detected when the vacuum gauge readings were 10⁵ Pa, 10 Pa, and 10⁻² Pa, respectively. The second harmonic waveforms obtained are shown in Figure 11.

The obtained harmonic signal is compared with the calibrated harmonic signal to obtain the corresponding parts per million concentration (ppm). Then, the ppm is converted using Equation (7) to obtain the water molecule concentration at the corresponding pressure and temperature.

$${\rho }_{water}=c_{water}\times \frac{M_{water}}{22.4}\times \frac{273.15}{273.15+T}\times \frac{P}{101325}$$

(7)

where ρ_water is the mass concentration of gaseous water, C_water is the volume concentration of gaseous water (PPM), M_water is the molecular mass of water (18.0152 g/mol), T is the temperature of gaseous water, and P is the pressure of gaseous water.

The calculated water molecule concentration was converted into pressure through the ideal gas equation and compared with the reading of the vacuum gauge. The results are shown in Figure 12 below. When TDLAS detected three different pressure gradients of 10⁵ Pa, 10 Pa, and 10⁻² Pa, the readings were basically consistent with those of the composite vacuum gauge. The calculated deviation rate was 1.8%. Therefore, it can be considered that the concentration of gaseous water molecules in the device detected by TDLAS is accurate and can be used as the basis for subsequent judgment.

3.3.2. Comparative Analysis of Water Ice Sublimation Theory and Simulation

After the TDLAS detection accuracy test was completed, theoretical and simulation analysis and comparison verification tests were conducted, respectively, at temperatures of −30 °C, −15 °C, 0 °C, 15 °C and 30 °C. Firstly, the sublimation rate was detected without cooling the simulated lunar soil in a liquid nitrogen bath. The comparison between the sublimation rate detected in the experiment and the theoretical calculated sublimation rate is shown in Figure 13. When the sublimation temperature is relatively low, the theoretical rate is close to the detected rate, but when the sublimation temperature is relatively high, a difference of 6.9% appears. This is because the volume change caused by the sublimation of water ice was not taken into account in the theoretical calculation, resulting in the sublimation rate calculated theoretically being higher than that actually detected.

After the sublimation rate comparison test was completed, the concentration of gaseous water molecules in the device was detected under the condition of liquid nitrogen bath cooling of the simulated lunar soil. The concentration-time relationship obtained at different sublimation temperatures was compared with the simulation results, as shown in Figure 14. After considering the volume change caused by the sublimation of water ice, it could be seen that the curves obtained through simulation calculation are close to those measured in actual experiments at different sublimation temperatures. The error rate range of the simulation calculation is ±1.17%, and the simulation results are accurate.

3.3.3. Performance Test of the Water Molecule Concentration Detection System

After verifying the accuracy of TDLAS and the simulation analysis, the concentration of gaseous water molecules in the device was controlled in real time based on the relationship between TDLAS concentration detection and sublimation temperature in the simulation analysis. Experiments were conducted under two control target concentrations of 100 mg/m³ and 200 mg/m³. The fluctuation range between the water vapor concentration inside the closed-loop controlled apparatus and the target concentration was verified.

As shown in Figure 15, in the experiment with a target of 200 mg/m³, the concentration of water molecules fluctuated within the range of 200 ± 7.45 mg/m³, with a fluctuation range of 3.72%. The maximum concentration was 207.45 mg/m³ and the minimum was 194.45 mg/m³. In the experiment with a target of 100 mg/m³, the concentration of water molecules fluctuated within the range of 100 ± 3.09 mg/m³, with a fluctuation range of 3.09%. The maximum concentration was 102.53 mg/m³ and the minimum was 96.91 mg/m³.

4. Test on the Thickness of Water Molecule Adsorption Deposition on the Surface of Simulated Lunar Rock

4.1. Design and Construction of Water Molecule Adsorption Deposition Inspection System

The composition and principle of the test system are shown in Figure 16. The water molecule adsorption and deposition testing system consists of two parts: the adsorption and deposition module and the information collection and detection module. Among them, the adsorption and deposition module is composed of a vacuum pump group formed by the series connection of a mechanical pump and a molecular pump, a vacuum tank, metal pipes, flange test tubes, a vacuum baffle valve, a constant temperature bath table, a cold table, the sample to be tested, a liquid nitrogen tank and a liquid nitrogen pump. The information collection module consists of a vacuum gauge, a temperature sensor, a temperature sensor recorder, an electric lead screw and a laser distance sensor.

As shown in Figure 17, the liquid nitrogen pipeline is connected to the cooling platform inside the vacuum tank through the KF flange pipe passing through the chamber. The electrical components of the detection module are connected to the external power supply and control system of the vacuum tank through the KF flange aviation plug to ensure the airtightness of the vacuum tank.

After the external connection is completed, wrap the liquid nitrogen copper tube, lead screw and lead screw bracket with insulation layers on the outside to prevent water molecules from adhering to parts outside the cold table. Then, complete the connection of the internal pipelines and circuits of the vacuum tank, as shown in Figure 18. After the connection is completed, test whether each electrical component of the detection module is working normally to ensure the accuracy of the deposition adsorption detection. Finally, the block-shaped samples to be tested that have been dried at 200 °C for 1 h are placed in the grooves on the surface of the cold stage, the liquid nitrogen inlet and export keep the temperature of cold platform near 80 K range, to allow them to cool down fully, ensuring that gaseous water molecules deposit on the surface of the samples to be tested.

4.2. Test Results of Water Molecule Deposition Adsorption

4.2.1. The Thickness of the Deposited Adsorption Ice Film

After the test was completed, the block-shaped samples were taken out for visual observation, and it was found that there was an ice film on the surface of the samples to be tested. The recorded distance data was imported into the data analysis software for processing to obtain the variation of ice film thickness on different materials at intervals of 30 min, as shown in Figure 19. By observing and comparing the average thickness of ice films formed on different material samples at the same temperature, it can be found that different materials have different adsorption capabilities for gaseous water molecules. The ice film thicknesses from high to low are small-pore basalt blocks, large-pore basalt blocks, smooth aluminum sheets, and smooth glass sheets. Moreover, the thickness of the ice film on the two samples of the same material also varies. The thickness of the ice film between the two smooth aluminum sheets is close to that between the two smooth glass sheets, while the thickness of the ice film between the two large-pore basalt blocks and the two small-pore basalt blocks differs significantly.

The formation rate of ice film is mainly related to the sublimation heat transfer surface area, surface temperature and the volume of gas in contact. Small-pore basalt has many small pores and the largest specific surface area, and water molecules can penetrate into the pores to sublimate, forming the thickest ice film. Macroporous basalt has larger pores. Although its specific surface area is smaller than that of microporous basalt, it is still significantly higher than that of smooth surface. The thickness of the ice film is second. Smooth aluminum sheets have excellent thermal conductivity, a lower surface temperature, and a faster sublimation rate. However, only the outer surface is covered, and the ice film thickness is less than that of porous materials. Smooth glass sheets have poor thermal conductivity. The heat released by sublimation may cause the surface temperature to be slightly higher, which inhibits sublimation. Moreover, they have the highest surface smoothness, so the ice film is the thinnest. Therefore, when preparing water ice by vapor deposition, the ice film adsorbed on other materials in the device will be much smaller than that adsorbed on the simulated lunar soil particles due to the reasons of temperature and porosity.

The differences among samples of the same material may stem from the differences in porosity, as shown in Figure 20. From the figure, it can be found that the porosity of large-pore basalt 1 and small-pore basalt 1 is different from that of large-pore basalt 2 and small-pore basalt 2. To obtain an accurate porosity, the porosity of the above-mentioned samples was measured by the immersion method.

First, weigh the dry sample to obtain the dry weight, m, of the sample. Then, completely immerse the sample in water, expel the air in the pores until it is saturated, and measure the buoyant weight m of the sample in the liquid. After completion, take it out and weigh the saturated weight, m. Through the above steps, the porosity Φ of the sample can be calculated:

$$\phi =\frac{m_{sat}-m_{dry}}{m_{sat}-m_{float}}$$

(8)

The porosity of small-pore basalt 1 was measured to be 21.2%, that of small-pore basalt 2 was 19.9%, that of large-pore basalt 1 was 17.7%, and that of large-pore basalt 2 was 17.2%. Therefore, the greater the porosity of the same material sample, the stronger the ability to adsorb and deposit gaseous water molecules, thus resulting in differences in the thickness of the ice film. The surfaces of the two aluminum sheets and the two glass sheets are relatively smooth, and the pore difference is much smaller than that of basalt, resulting in a small difference in the thickness of the ice film on the surfaces of the two aluminum sheets and the two glass sheets.

By observing and comparing the average thickness of the ice film formed on the same sample at different temperatures, it can be found that as the temperature of the constant-temperature ethanol bath increases, the thickness of the ice film formed by the adsorption and deposition of gaseous water molecules on the sample increases faster, and as the volume of water ice decreases, the sublimation rate gradually slows down, as shown in Figure 21.

The average deposition rate value during this period can be obtained by calculating the relationship between the thickness of the ice film adsorbed and deposited by small-pore basalt 1 under constant-temperature ethanol baths at different temperatures and time. After multiplying the obtained average deposition rate by the corresponding volume ratio and comparing it with the average sublimation rate of water ice in constant-temperature ethanol baths at different temperatures obtained, Figure 22 is obtained. The average sublimation rate of water ice at the occurrence end measured in the previous text is relatively close to the average deposition rate of basalt detected at different temperatures. The calculated error rate is 2.7%, which confirms the accuracy of the control of the water molecule flow field occurrence.

4.2.2. The Total Mass of the Deposited Adsorption Ice Film

By multiplying the thickness of the ice film on the surface of different material samples and the surface of the cold table by their corresponding areas, the volume of the corresponding ice film can be calculated. Then, by multiplying the volume by the density, the total mass of the generated ice film can be calculated, as shown in Equation (9).

$$m_{ass}=\left[h_{alu}\times s_{alu}+\sum \left(h_{picece}\times s_{piece}\right)\right]\times {\rho }_{ice}$$

(9)

Equation (9) can be used to calculate the relationship between the total mass of the ice film and time at different temperatures in the above experiment and obtain the mass-time relationship graph of the ice film, as shown in Figure 23. The total mass of all ice films on the cold stage increases with the rise of sublimation temperature, and the rate of increase gradually slows down over time. When the temperature of the constant-temperature ethanol bath was 30 °C, the water ice in the test tube had completely disappeared within 150 min. At this time, the total mass of the ice film on the cold stage was 2.956 g, which was less than the mass of the deionized water injected into the test tube of 3.2 g, with a loss of approximately 7.6%. It is speculated that this part of the loss might be due to the adsorption of some gaseous water molecules on the various insulation layers in the device. Some gaseous water molecules have not yet deposited to form an ice film. Therefore, when preparing the lunar water ice simulation using the above-mentioned method, the delay loss should be taken into account, which is approximately 7.6%.

5. Conclusions

Based on the mathematical models of water molecule sublimation, diffusion and cold trap adsorption, this paper analyzes the influence mechanism of water ice meteorological deposition. The results can be utilized not only to elucidate the adsorption and deposition behavior of water molecules on rock surfaces within the lunar polar permanently shadowed regions (PSRs), but also to analyze the deposition of water molecules at the night lunar equatorial regions. The findings of this study can provide a fundamental reference for understanding the occurrence state of water ice on rock surfaces, the abundance of water ice resources, and the distribution characteristics of water ice on the surfaces within PSRs. Moreover, it offers preliminary insights for future exploration, development, and utilization of lunar water ice resources. Through simulation and experimental verification, the following conclusions are obtained.

TDLAS technology can be applied to the precise detection of water molecule field concentration in vacuum low-temperature reaction chambers. By using its feedback data as a reference and adjusting the sublimation temperature of water ice, the water molecule concentration in the reaction chamber can be dynamically stabilized around the target concentration value.

The higher the porosity and specific surface area of basalt blocks, the easier it is for water molecules to adsorb and deposit on their surface to form an ice film, and the thicker the ice film will be.

Compared with the surfaces of metals and glass, water molecules are more likely to adsorb and deposit on the surface of rock blocks to form ice films.