Where will the future moon base be built? In a volcanic cave →

In science fiction movies, we often see moon bases full of modern industrial textures. For example, the following:

Figure 1 The lunar base in the movie "Alone on the Moon" Source: Movie "Alone on the Moon"

But does a lunar base really look like this? Not necessarily. Future lunar explorers may live in caves.

At the end of 2022, a research team from China (a joint research by Xie Gengxin, Dean of the Advanced Technology Research Institute of Chongqing University and Executive Deputy Director of the Joint Research Center for Deep Space Exploration of the Ministry of Education, and Guo Linli, a researcher at the Beijing Institute of Space Mechanics and Electronics) proposed the idea of ​​building a large base in the "volcanic cave" on the moon, which attracted great attention.

The "volcanic hole" on the moon is actually a lava tube. The concept of lava tube may be a little unfamiliar to everyone, but its formation principle is actually quite simple.

When the overwhelming magma spewed out by the volcano flows on the surface, the outer surface of the magma cools down first due to the rapid heat dissipation to form a shell of a certain thickness, while the hot magma inside continues to flow, forming a hollow pipe. Under the continuous action of this process, the magma keeps "shedding its shell" as it flows, creating a long lava tube.

Figure 2 Flowing magma Source: Wikipedia

Lava tubes are not unique to the moon, they are found on many planets including the Earth. The largest lava tube cave on Earth is the "Manjang Cave" located on Jeju Island, South Korea. It is more than 13 kilometers long, tens of meters high, and more than ten meters wide. It was formed when a volcano erupted 200,000 to 300,000 years ago. Lava tubes are also developed in the Jingbo Lake volcanic rock area in Heilongjiang, my country. The Northeast Anti-Japanese Allied Forces used it as an underground fortification to fight against the Japanese invaders.

Figure 3. Manjanggul lava tube in Jeju Island, South Korea (left), Gyeongbok Lake volcanic lava tube (right) Source: Reference [2]

Because the moon's gravity is smaller than that of the earth, magma is less likely to collapse when it flows, so the volume of lunar lava tubes is often larger than that of the earth.

In 2009, a lunar orbiter launched by Japan discovered three caves with a diameter and depth of several tens of meters. These caves are believed to be skylights formed after the partial collapse of lava tubes.

Figure 4: Lunar lava tube skylight (left), lunar crater (right). It can be seen that the lava tube skylight is quite different from the crater. Compared with the crater, the lava tube skylight has no obvious protrusions around it and no slope inside. (Image source: Reference [2], Wikipedia)

In 2011, India's Chandrayaan-1 probe explored the lunar creeks and found that the top of the lava tube here was more than 300 meters wide; in 2017, numerical simulation results from Professor Blair of Purdue University in the United States showed that a 5-kilometer-wide lunar lava tube can still remain stable under certain conditions; in 2020, Professor Riccardo Pozzobon of the University of Padua in Italy said that the length of the lunar lava tube can exceed 40 kilometers under certain conditions.

Assuming that a huge lunar lava tube can reach a width of 5 kilometers and a length of 40 kilometers, its bottom area can reach an astonishing 200 square kilometers, which can accommodate 6 Macaus.

Figure 5 Numerical simulation results of Professor Blair Source: Reference [3]

Why choose lava tubes when building a base?

After understanding the concept of lava tubes, the second question comes up: why choose to build a lunar base in a lava tube? Imagine if you were suddenly sent to a deserted place with no houses, where would you choose to spend the night? In the past, human ancestors chose caves that could shelter from wind and rain to live in, and on the moon, a lava tube cave can also play a good protective role. Let's take a look at its five major functions.

1. The environment is relatively constant temperature

There is no actual data on the temperature inside the lunar lava tube, but the temperature measurement of the illuminated and shadowed areas of the collapsed cave shows the temperature variation between the lunar surface and the inside of the cave (as shown in the figure below). The figure shows that the temperature on the lunar surface is between -170℃ and 110℃, with a temperature difference of 280℃; while the temperature inside the unilluminated area is between -20℃ and 30℃, with a temperature difference of only 50℃.

According to this temperature change trend, the research team calculated that when the temperature inside the lunar base is maintained at around 20°C, which is enough for humans to live and work normally, the energy consumption in the lava tube will be only 1/5 of that on the lunar surface.

Figure 6 Temperature measurements of illuminated and shadowed areas of a collapsed cave Source: Reference [2]

2. Protect against cosmic rays

Some scientists estimate that the amount of solar radiation on the lunar surface in one day is 2 Sv, which is more than 1,000 times the limit that humans can withstand, and will seriously threaten the lives of base workers. The lunar soil on the top of the karst tube can effectively resist solar rays. Studies have shown that 6 meters of lunar soil can reduce the radiation dose to nearly zero. The burial depth of lava tubes often reaches tens of meters, so it has an advantage in resisting cosmic rays.

Figure 7 The weakening effect of lunar soil at different depths on cosmic rays Source: Reference [2]

3. Avoid the solar wind

The sun is constantly ejecting high-speed charged particles, mainly protons and electrons, a phenomenon known as solar wind. When the solar wind reaches the moon, it causes the fine matter on the back of the moon to form negatively charged lunar dust and move toward the positively charged sunward side. This phenomenon may occur on a large scale at sunrise and sunset, and will have a very adverse effect on the human body and instruments. In "The Wandering Earth 2", the 550C computer that humans finally brought to the moon to ignite the engine of the moon-chasing satellite was completely scrapped after experiencing the solar wind.

Figure 8 The damage of solar wind to electronic equipment in "The Wandering Earth 2" Source: "The Wandering Earth 2" movie

The interior of the lava tube is a permanent shadow area, so no moon dust will be affected by the solar wind and hit here. In addition, the skylight of the lava tube is also a good place to monitor the migration of moon dust.

4. Prevent meteorite impact

Meteorite impacts pose a great threat to lunar bases. Studies have shown that the probability of a space suit and a 1-cm-thick aluminum plate being penetrated by particles after being exposed on the lunar surface for one year is 8% and 30% respectively. The lunar soil and rocks at the top of the lava tube can intercept most meteorites on the lunar surface.

Figure 9: Fragments of the "Cosmic Hammer" in "Alone on the Moon" hit the lunar base (most meteorites are celestial fragments) Source: "Alone on the Moon" movie

5. Possible water ice

Although the lunar surface often appears to be a barren wasteland, exploration has discovered that there is water ice on the moon, which may be formed by the reaction of hydrogen ions in the solar wind and oxygen-containing substances on the lunar surface, brought in by comet impacts, or carried by magma activity.

In 2023, scientists analyzed the data collected by Chang'e 5 and published an article in Nature Geoscience, saying that the water content on the moon's surface may be as high as 270 billion tons. Since the lava tube is a permanent shadow area with very low temperature and low terrain, it is possible to collect water ice on the moon. Once a large amount of water ice is really found, it will greatly facilitate the construction of a lunar base and provide full protection for human production and life on the moon.

Figure 10 Scientists discovered water ice in the Cabeus crater on the moon (Cabeus, upper left) Source: Wikipedia

Building a lunar base? Experiment in Earth's caves first

Since humans currently lack the conditions and experience to build a base in a lunar lava tube, the research team took a unique approach and proposed using the Chongqing karst cave to simulate a lunar lava tube and conduct experiments and engineering construction inside.

Although the formation mechanisms of karst caves and lava tubes are different, the former is generally formed by groundwater eroding carbonate rocks, which can be understood as groundwater finally dissolving a hole in the rock mass after working day after day and year after year; while the latter, as mentioned above, is formed by the cooling of the outer layer of hot magma during its flow. However, the two are very similar in structure, both are curved semicircular caves; both have relatively constant temperatures in the environment; and both are relatively closed spaces. Therefore, it is worth trying to conduct experimental simulations in karst caves.

Figure 11 Karst caves in Chongqing Image source: Reference [1]

The research team plans to conduct experiments in three aspects.

First, considering that it is not realistic to send a large amount of manpower and material resources to the moon, is it possible to send only a few robots that can automatically use lunar soil and rocks to build a lunar base? Therefore, experiments in intelligent construction technology and in-situ resource utilization technology are needed.

Second, considering that there is no air on the moon, is it possible to introduce suitable animals, plants and microorganisms into a closed lunar base to build a stable ecosystem so that humans can breathe fresh air freely in the base like on Earth? Therefore, it is also necessary to carry out artificial ecosystem experiments.

Third, considering that the operation of the base requires a steady supply of energy, it would be too costly to send it from the Earth to the Moon. It is also impossible to fully tap into the solar energy and other energy sources on the Moon, so it is also necessary to try to establish an automatic energy control system in the cave.

Figure 12 Karst cave simulation platform Image source: Reference [1]

Science fiction or reality? Believe in the future

Although the idea is beautiful, there are indeed many difficulties and challenges in actual implementation. For example, no probe has ever entered the inside of a lava tube, so there is still a lack of detailed data inside; for example, there has been no successful case of large-scale closed ecosystem experiments, and the vigorous "Biosphere 2" experiment in the 1990s ended in failure; for example, the development of intelligent construction technology is not so mature, and it is currently unable to automatically build large and complex engineering systems.

Figure 13 Principle prototype of lava tube exploration vehicle Image source: Reference [4]

Although there are still many key technologies to be broken through, humans have been working hard. For example, the United States plans to send a probe into a lunar lava tube in 2025 to conduct relevant experiments. Exploration of ecosystems and intelligent construction is also in full swing around the world.

Once a large lunar base in a lava tube is successfully built, it means that large-scale sustainable lunar development plans will be possible: the rich resources on the moon will have the opportunity to be used by humans, Earth-Moon travel may also be accessible to ordinary people, and the moon can also become a base for Mars exploration. This is of vital importance to the survival and development of mankind, and we can choose to believe in the future.

References
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[2] Xiao Long, Huang Jun, Zhao Jiawei, et al. Significance and preliminary ideas of lunar lava tube cave exploration [J]. Science in China: Physics, Mechanics and Astronomy, 2018, 48(11): 87-100.

[3]Blair, DM, Chappaz, L., Sood, R., Milbury, C., Bobet, A., Melosh, HJ, Howell, KC, Freed, AM, 2017. The structural stability of lunar lava tubes. Icarus 282, 47–55. https://doi.org/10.1016/j.icarus.2016.10.008.

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[6] Shi Quanqi, Zong Qiugang, Le Chao, Li Lei. Response of the lunar surface and space environment to solar wind and earth wind[J]. Chinese Science Foundation, 2022, 36(06): 871-879. DOI: 10.16262/j.cnki.1000-8217.20221208.006.

[7] Chen Haibo. my country will build a basic lunar research station around 2028. Guangming.cn [EB/OL]. 2022-11-25. https://news.gmw.cn/2022-11/25/content_36186673.htm

[8]Università di Bologna. "Lava tubes on Mars and the Moon are so wide they can host planetary bases." ScienceDaily. ScienceDaily, 5 August 2020. <www.sciencedaily.com/releases/2020/08/200805110118.htm>.

[9] Lunar lava tubes, lunar craters. Wikipedia.

[10] Li Jinyang. A brief analysis of the “safe zone” in the US Artemis Accords[J]. International Space, 2020(12):57-60.

[11]Huicun He, Jianglong Ji, Yue Zhang, Sen Hu, et al. A solar wind-derived water reservoir on the Moon hosted by impact glass beads, Nature Geoscience, 2023, DOI: 10.1038/s41561-023-01159-6.

Author Information

Author Name: Xiao Yidong Affiliation: Institute of Geology and Geophysics, Chinese Academy of Sciences