Karst Cave as Terrestrial Simulation Platform to Test and Design Human Base in Lunar Lava Tube

Developing efficient approaches to building a suitable environment for humans on the moon plays a key role in future long-term sustainable lunar exploration activities, which has motivated many countries to propose diverse plans to build a lunar base. The lava tubes discovered by the Kaguya mission offer huge potential sites to host such bases. Through computation and analysis, we show that lunar lava tubes offer stable structures, suitable temperatures, low radiation doses, and low meteorite impact rates. We summarize previous research results and put forward the conditions to find and use a suitable lunar lava tube for human habitation on the moon. The establishment of extraterrestrial bases still faces many technical bottlenecks, many countries have begun to use the earth's environment for extraterrestrial exploration and simulation missions. In this regard, we proposed the idea of using the Earth's karst caves to simulate extraterrestrial lava tubes, selected caves in Chongqing as the simulation site, and demonstrated the feasibility from both structural and environmental aspects. Finally, we proposed a karst cave simulation platform with three main research directions: cave sealing technology, efficient daylight system, and internal circulation research of artificial ecosystems containing natural soil and rock. We hope to promote the development of related research on extraterrestrial bases through simulation experiments.


Introduction
similar cave system on the earth. The widely distributed karst caves on the earth become one of the best choices. Karst caves are developed due to the excavation of soluble limestone by groundwater which is usually characterized by rocky floors, sinkholes, and underground rivers.
Biosphere-2 artificial ecological system is the most famous attempt. Biosphere-2 is a steel frame type with double-glazed window panels. It prohibits material transformation with the outside atmosphere and underground soil through engineering means but allows sunlight to pass through the glass structure for the photosynthesis of plants. However, it ultimately failed, the common view on this failure is that the oxygen content of Biosphere-2 decreased with time, which led to the inability of organisms to survive stably for a long time.
The soil and rocks in nature play a very key role in the ecosystem. Sealing the caves that exist in nature and preserving the role of soil and rocks in the ecosystem enable people to better understand the circulation of substances in nature, also can enhance the stability of the artificial ecosystem.
The objective of this paper is to introduce a research project that will establish on solid grounds the suitability of using lunar lava tubes for human habitation, using parallel studies of the Lunar habitat itself and realistic simulation studies on Earth using Karst caves as an Earth analog of lunar lava tubes. In Section 2, we will address the rationality of the choice of lunar lava tubes for hosting a future lunar base, focusing on mainly two aspects: 1) energy budget of the base required to maintain both the needs of human nutrition and respiration, and a stable environment; 2) the safety conditions offered by lava tubes. In Section 3, We analyzed the feasibility of the transformation from both the structure and the environment and proposed a transformation plan. In section 4, we will analyze the significance of using the karst cave as a simulated platform.

The Rationality of Using a Lunar Lava Tube to
Establish a Lunar Base

The Energy Estimation of Lunar Base
The lunar base needs to provide adequate food and protection with low power consumption.
Water and food supply within artificial systems have been studied before [8,28] , and here we focus on energy consumption and protective effects that can be provided by lunar lava tubes. The external energy required by the closed system involved in the operation of a human Lunar base mainly consists of three parts: human respiration, plant photosynthesis, and internal temperature control. Our estimates consider the basic survival needs of eight astronauts. Their living space is assumed to be a semi-circular tunnel cave with an area of about 8,000 square meters and a height of 50 meters. These data can be used to help us predict the ground simulation system. More accurate data can be obtained from the simulation system and finally applied to the lunar base.

Energy Required to Maintain Basic Human Life
Oxygen, food, and suitable temperature need to be provided on bases for members to live.
Studies have shown that the oxygen requirement of a normal adult is about 3.51 ml/min/kg [10] .
Consider 8 astronauts with an average weight of 70 kg in a day and night on the moon (about 27 days, precise value is 2360591.47 seconds). The volume of oxygen required is 77333L, and the weight is 101.155kg according to the ideal gas equation (at 25℃, 1 atmospheric pressure).
Regarding the calorie needs of adults, related studies have shown that human's recommended daily calorie intake is about 2900 kcal, while women's is about 2200 kcal [8] . Then for 8 astronauts, the heat required in a day and night on the moon is about 557361.91 kcal (233034.337 kJ).
According to the biomass of soybeans in NASA's Biomass Production Chamber (BPC) project, dry matter and oxygen production of soybean in closed ecosystems were shown in Table 1 [34] . Based on Wheeler's study about the nutrient composition of soybean, 20.253 kJ of energy per gram of soybeans in a closed system can be estimated [33] . Combining these two equations, its clear that the wheat's units must be larger than 45(900 m 2 ). It means that 900 m 2 of soybeans can meet the food and oxygen needs of eight adults in one month (27 days). With the planted area, we can calculate the minimal energy that the system needs to obtain from the sun. From the light quantum energy equation, the energy provided to the crop during the production cycle of a day and night on the moon is: Where W is totally required energy, J; h is Planck's constant, approximately 6.63 ×10 34 J·s; ν is the frequency of the light source irradiated on the crop canopy, according to the wavelength of the high-pressure sodium lamps, the value is 5.087 ×10 14 hz; PPF is the intensity of light irradiated on the crop canopy, (umol/m 2 ·s); S is the total area of the crop canopy in the device, m 2 ; t is the production time of the crop(27day). Therefore, the total energy required to support eight astronauts living is 1.76×10 11 J (48889 kW·h).

Energy Required to Maintain a Stable Internal Environment
There have been some previous studies about the temperature in lunar lava tubes [3,5] . The average temperatures found at depths of 1.3 m and 2.3 m below the lunar surface of the Apollo 15 and 17 landing sites are -17 ℃ and -16 ℃ [12] , and temperature also increases with depth [9] .
Haruyama and others analyzed the thermal environment of Marius Hills Hole (303.3°E, 14.2°N), which has a diameter and depth of 50 m (Figure 2). The temperature change is relatively mild in areas not exposed to direct sunlight. Assuming that the internal temperature of the lunar lava tube base is 20 ℃ and the humidity is 85% [34] , the specific heat capacity of the air in the system can be calculated [36] . The equation of specific heat capacity of humid air is as follows: weathered layer of about 2 cm, and its albedo is equal to 0.1) [12] .
Where Cp, ma is the specific heat capacity of humid air at constant pressure, J/(kg·K); Cp, da is the specific heat capacity of dry air at constant pressure, J/(kg·K); Cp,v is the specific heat capacity of water vapor at constant pressure, J/(kg·K) ; d is air moisture content, kg/kg; Pma is wet air pressure, pa; Pv is the partial pressure of water vapor, pa. The equations of water vapor pressure and relative humidity [9] are as follows: Where E means saturated vapor pressure of air at a certain temperature, pa; e is the water vapor pressure of air at a certain temperature, pa; t is air temperature, K; φ is relative humidity, %; Then the specific heat capacity of the air inside the system is 510.89 J/(kg·K). Fitting the blue curve in According to the previous assumptions, the volume of the system is 3.14 x 10 7 m 3 , and the contained air quality (normal pressure at 20 ℃) follows the equation [36] : Where ρma is the density of humid air, in kg/m 3 ; Rda is constant, 287.06 J/(kg·K); Ps is water vapor saturation pressure corresponding to temperature, pa. Based on this formula, the mass of air that can be contained in the system is 6.8×10 7 kg, and the energy required to maintain 20 ℃ over a daily cycle on the moon is 2.05×10 13 J(5.7×10 6 kW·h). Therefore, the energy value required to maintain the basic survival requirements of 8 astronauts in 27 days on the moon should be at least 2.07×10 13 J (5.75×10 6 kW·h).
Fitting the red curve in Figure 2 (temperature of the lunar surface), the energy required to maintain the temperature in the system at 20°C is 1.04×10 14 J (2.89×10 7 kW·h). Results show that it takes much less energy to maintain a human base in a lunar lava tube than on the lunar surface ( Table 2). Note: *1 use the data of the temperature change of the lunar surface in Haruyama's simulation [6] *2 the shadow area of the Marius hole

Safety Assessment of Lunar Base in Lava Tubes
Compared with the lunar surface, lunar lava tubes have the advantages of small radiation and less extreme conditions, more suitable for a human lunar base. We now quantitatively analyze these advantages concerning the Lunar surface.

Radiation Simulation
A numerical simulation study performed in 2002proved that lava tubes buried more than 6 m below the lunar surface can be protected from cosmic rays and solar wind [1] . More specifically, Figure 3 shows the variation in depth of radiation intensity of different particles in the lunar soil.
One can see that this intensity at 6 m below the lunar surface is almost zero [30]. Even at a depth of less than 1m, solar flares and solar particle events (SPE) do not affect the interior of the lava tube.

Meteorite Impact
Because lunar lava tubes are deeper below the surface of the Moon, they also have the advantage of avoiding the impact of the meteorite. This advantage can protect the humans and their base. For a suitable tube, it should not be broken down by meteorites. This means that the possible influence radius of the meteorite must be smaller than the tube's burial depth. According to experimental and simulation studies, the most safety condition the scientist calculated now is the ratio of target thickness-to-crater diameter. That is, if it is smaller than 0.87 then a "skylight" (crater) will appear [20] .
For example, the Mare Tranquillitatis hole (MTh) might be produced by a 7.2 m impactor with a 45° angle and a speed of 18 km/h in a 47 m thick target [13] . The projectile's material is basalt and its density is 2.86 kg/m 3 . The target is overlaid by 3 m of regolith. The crater diameter Dc is 75.67±3.0 m, and the depth is 30.07±3.6 m. Then the ratio target thickness-to-crater diameter is 0.85 which is around and smaller than 0.87 [20] .
Also, to build a habitat in a lunar lava tube, it is important to build the habitat accordingly deployed at some distance to the skylight itself, to avoid possible subsequent collapse related to the same mechanism that formed the hole's structure.

Moon Dust
There is very little moon dust inside lunar lava tubes. Moon dust is the charged dust formed by small debris on the lunar surface under the action of the solar wind. Particles of less than 20 µm in size of the debris on the lunar surface (the regolith) account for 20% of the total mass of moon dust.
Being charged, they can be adsorbed on any equipment. In addition, moon dust is abrasive, and very harmful to people and machines, particularly below a size of 24 microns [3] . But the interior of lava tubes is a permanent shadow zone, generally inaccessible to the solar wind, where charged moon dust is hardly produced. It is also well protected from the impact of micrometeorites [20] .

Moon Quake
Decades of analysis of the seismic data collected by the lunar seismic network revealed that vibrations on the moon mostly fall into four types, each with a distinct cause rooted in the moon's structure and its position in our solar system: -Deep moonquakes, quakes originating deep (over 700 kilometers deep) within the moon, caused by the stretching and relaxing of the gravitational pull between the Earth and the moon, the same force that drives our ocean tides.
-Shallow moonquakes, quakes at the surface of the moon (20-30 kilometers deep), are likely caused when the moon's crust slips and cracks due to the gradual shrinking or "raising" of the moon as it cools.
-Meteor impacts, vibrations caused when meteors crash into the surface of the moon.
-Thermal quakes, quakes caused by the short-term thermal expansion and contraction of materials on the surface of the moon as it is warmed by, and shaded from the rays of the sun.
-Bonus! Mission "quakes", were caused by the force of the later Apollo mission landings on the moon's surface, which helped guarantee the accurate measurements of the first seismic stations.
The moon quakes observed by the lunar seismic network between 1969 and 1977 varied in where, how often, and how severely they shook the moon, like Table 3. The fact that the lunar lava tube has survived on the moon for so many years suggests that its structure is stable enough to withstand the effects of moon quakes. This is a natural advantage over bases built on the moon's surface.

Conclusions on the use of Lava Tubes for Lunar Bases
Building a Lunar base into lunar lava has many advantages, some of which we reviewed in this section. Lunar lava tubes can give us a natural, ready-made infrastructure that eliminates the need for additional construction. It only needs to be closed, reinforced, and transformed into functional areas, and there is no need to launch the building materials and equipment from Earth to the surface of the moon. Second, as we just showed, a lunar lava tube offers a relatively stable environment.
The changes in various environmental parameters are relatively regular, easy to control and facilitate the adaptation of host organisms. The buried depth of lunar lava tubes protects them from the risk of moon dust, moon quake, cosmic radiation, and meteorite impact. Finally, thanks to suitable temperatures and small temperature variations, the energy required to maintain a temperature suitable for humans and organisms is very low.
In summary, compared to the lunar surface, lunar lava tubes may offer a more stable, more suitable, safer, and less energy-consuming environment. Due to these benefits, lava tubes seem to be more suitable for a lunar base than on the lunar surface. Whilst the Moon's surface has been well documented by orbital spacecraft, it hides an underground world that remains a mystery [27] . The shelter that lunar caves provide, as well as the access to water and other resources, could be vital for our future human or robotic exploration of the Moon.

Formation and Structure of Kester Caves on the Earth
Karst caves refer to the underground space formed by the dissolution, invasion, and collapse of soluble limestone under certain conditions. It is the combination of CO3 2ions produced by the ionization of CaCO3 and H + produced by the interaction of CO2 and H2O to produce HCO 3-, which destroys the ionization balance of CaCO3 so that CaCO3 continues to be ionized and continuously dissolved.
The formation of karst caves is mainly divided into three stages：(1) The initial stage, at this stage, the water seeps along the cracks existing in the limestone, and the slow chemical dissolution is mainly Due to the special formation process, the internal structure of the karst cave is very complicated, there are usually accumulations in the caves, resulting in not smooth. Its length can reach hundreds of kilometers, the width is between several meters to tens of meters, and some caves also have a layered structure. Temperatures deep in the caves are similar to the local average annual temperatures [15] . The air in the cave is relatively clean, and some areas also use the cave as a treatment place.
Karst areas are widely distributed in Chongqing, such as Wulong, Nanchuan, Youyang and Wansheng. Including karst caves, skylights, earth cracks, canyons, and other landforms. We have explored some caves in these areas, the height of the hall in caves can reach 18 m (Figure 4 a), and some caves exist rivers (Figure 4 c). The volume of the hall reaches tens of thousands of cubic meters, which is enough to accommodate man-made buildings of various sizes.

The Feasibility of Chongqing Karst Caves Reconstruction
Taking karst caves in Chongqing as an example, we show now that Earth karst caves can be considered as good analogs of lunar lava tubes under three aspects: structure, environment, and insulation.
(a)Structural aspects: Karst caves and lava tubes have different formation mechanisms and environments, but they are comparable in size.
They are both curved semicircular caves, though their formation mechanisms are different.
Lunar lava tubes can collapse due to meteorite impacts, moonquakes, and tectonic activities [23] , just like karst caves can collapse on Earth. The figure below illustrates the similarities between the two structures.
Although the caves on the moon are expected to be much larger than those on the earth due to special environments such as low gravity, karst caves on the earth still provide us with the best choice, and the space it provides can meet the research needs at this stage. Moreover, the largest known karst cave in the area can reach 77020 meters in length [31,38] , this range of sizes is sufficient to simulate various types of lunar lava tubes.
(b) Environmental aspects: Karst caves and lava tubes have relatively mild environments.
In terms of temperature, The temperature range in the cave is between 18-25 ℃, even for caves with good ventilation, the internal temperature and external temperature are less than 5 ℃. Other literature points out that Karst Cave in Wulong has a small temperature difference throughout the year, ranging from 14～21 ℃ [19] . For the shadow area at the bottom of the collapsed lava tube in Marius hill, the internal temperature range is between -20～30 ℃, and the temperature difference is small [12] . The mild temperature inside the lava tube can reduce energy consumption in terms of heat control.
In terms of acidity and alkalinity, the lunar lava tube is mainly composed of lunar basalt. The rock composition is mainly composed of iron and titanium compounds. The hydrolysis product is weakly alkaline. The karst cave is mainly composed of carbonate rock, and most of the hydrolysis product is hydrogen carbonate, also weakly alkaline.
In terms of air quality, one advantage of the lunar lava tube is that there is almost no moon (c) Insulation aspects: Karst caves and the lava tubes can provide an environment relatively isolated from the outside world.
The lunar lava pipeline is protected by the weathered layer, and the internal and external environments are isolated. Due to the lack of conductive media, the internal environment is very independent. As for karst caves, since they have almost no communication with the external environment during their formation process, they are rarely affected by the external environment.
Their internal humidity is relatively high, and the density of aerosol particles is very low.
Due to the presence of the air, the connectivity between the caves on the earth and the external environment is greater than that of the extraterrestrial lava pipeline, which is very unfavorable for the study of closed artificial ecosystems, so it must be reformed. At present, there are few of research on cave sealing, and research on lunar soil building materials rarely pays attention to the sealing performance of materials. The current lunar sintering and extrusion technologies tend to focus only on the mechanical properties of the material (compressive strength, brittleness, etc.). In our opinion, the substrate for the lunar base should have good sealing properties while meeting the strength requirements to reduce the extra work. Some of our preliminary work so far indicates that extruded lunar soil appears to have better sealing properties. We hope to use this example to explore the technology of sealing extraterrestrial caves economically using in-situ resources by sealing caves on the earth.
This analysis shows that the Chongqing karst cave can provide a relatively closed ecosystem containing soil, rocks, and organisms. We can use this special environment to simulate the situation that human beings may face in the lunar lava tubes in the future. Experience gained through this project could provide valuable lessons for humans to build ecosystems in moon caves.

Using Karst Cave as a Simulation Location
We intend to use karst caves to simulate the internal environment of extraterrestrial caves and isolate them from the external environment by sealing the caves. The cave experiment is divided into two parts. The first part is the automatic construction technology in the cave and the in-situ utilization technology. At present, people have conducted a lot of research on automatic construction and in-situ resource utilization in the cave. The adaptability of these technologies has not been studied yet. In addition, the experiences gained from Biosphere 2 allow us to conduct a separate study on sealability [30] .
The other part is the artificial ecosystem experiment. Many ideas have been proposed for building bases using extraterrestrial caves. However, due to insufficient research on the internal circulation theory of ecosystems, it is impossible to effectively control large-scale ecosystems.
Therefore, the second part of the experiment will select a section of the cave to seal it, and through the study of the interaction of animals, plants, microorganisms, and limestone in the system, team members will enter after the laboratory has developed to the stable functioning stage.
Regarding energy supply and light, we tried to use light pipes to introduce sunlight into the ecological laboratory and provide it for crop growth. Develop a set of automatic control energy control systems, use solar energy for charging during the day, use the stored energy supply system at night, and artificially provide the plants with light to ensure their growth when the light is insufficient. The conceived laboratory structure is shown in Figure 5.

Simulate Extraterrestrial Lava Tubes
The United States has launched the Artemis plan to return to the moon. Its goal is to send astronauts to the moon and return safely before 2024, and establish a normalized residency    [25] . It is used to simulate the surface environment of Mars to train people to solve the psychological and technical problems that may be faced by landing on Mars in the future. The use of extraterrestrial lava tubes to build human bases mainly faces the following challenges: ➢ Lack of internal measurement data of the lava pipeline, unable to accurately assess； ➢ Large artificial closed ecosystems have not yet been a successful case; ➢ Low-cost sealing and maintenance solutions for lava tubes; ➢ In-situ resource utilization of the moon is still in the laboratory stage; ➢ Many technologies and equipment on the earth need to be improved due to low gravity; ➢ The psychological impact of long-term appearance survival needs to be alleviated; Simulation through caverns can simulate the internal environment of the lunar lava tubes, and the construction conditions inside the tubes. For example, how to use the in-situ resources on the moon to seal the tubes and construct them in a limited space. In addition, emerging technologies such as 3D printing, and robotics will also be applied to the construction of the system. Concepts such as lunar soil sintering, 3D printing, and cave robots have been proposed to build lunar bases [16,17,35] , but there are few examples of applications in the natural environment. These technologies are indispensable for mankind to build a lunar base in the future. But at this stage, they are all used in other industries that cannot be directly applied to the construction of lava tube bases, and the karst caves provide an example with analogous environment. Using this platform, many interesting extraterrestrial base ideas can be realized on the earth, preparing for their future realization on the moon.

Conclusion
Lunar lava tubes have large overlying depths and wide internal widths. Their internal temperature change, radiation exposure, and meteorite impact are relatively small, so their internal environment is relatively stable. This makes them good candidates to host a lunar base where humans could live and work. We showed that the minimum energy consumption for maintaining a lunar base hosting 8 astronauts deep inside a lava tube cave is about 145.5 kW in a semi-circular tunnel cave with an area of about 8,000 square meters and a height of 50 meters. At this stage, a lava tube is the best choice for establishing a lunar base.
Compared with artificial construction, Karst cave provides a natural ecosystem that includes soil and rocks. The feature of convenient sealing allows us to build a well-structured ecosystem at a low cost. The feasibility of using karst caves to transform extraterrestrial caves is analyzed from two aspects: structure and internal environment. The results show that the structure and size of the karst cave can meet the research needs at this stage. The internal environment of the cave is comparable to that of the extraterrestrial lava tubes, but due to the existence of air convection, karst caves need to be sealed first.
Karst cave as a platform to study how to use lunar lava tubes to build a lunar base is promising.
We conceived a cave simulation platform that is relatively isolated from the outside world and tried to carry out automatic construction and in-situ resource utilization. The use of in-situ resources to seal caves, light supply system, and material circulation relationship of soil and rock in the artificial ecosystem is very important for the construction of extraterrestrial bases. We have carried out relevant pre-experiments in this area and will continue to advance them.
We shall call for increasing attention to the extraterrestrial lava tubes, using satellite remote sensing technology and autonomous robots to detect the tubes and measure their internal size and structure. Building settlements in an earth cave system is worth it to simulate and evaluate extraterrestrial bases while promoting the development of earth cave systems.