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How much do rock layers affect the stability of lunar lava caves? The question is important considering future exploration of the Earth's natural satellite, to which Marcin Chwała, PhD, DSc, Eng of Wrocław University of Science and Technology in collaboration with researchers from Italy and Japan seek the answer.
Depressions on the Moon's surface were discovered in 2009, and it is speculated that some of them are collapsed lava passages that may lead to extensive cave systems beneath the Moon's surface. They were probably formed during lava flows and can reach widths of up to several hundred metres due to the low gravitational force. According to many scientists, these types of caves can be used as bases and shelters for human exploration of the Moon.
The problem of the strength of lunar lava tubes is being investigated at Wrocław University of Science and Technology by Marcin Chwała, PhD, DSc, Eng from the Faculty of Civil Engineering. The scientist presented the first results of his research early this year and has now published another paper entitled “Effects of Layered Roof for Stability and Exploration of Lunar Lava Tubes”, which looks at the stability of lunar cave ceilings.
In addition to the scientist from the Department of Geotechnics, Hydrotechnics, Underground and Hydraulic Engineering, the research project involved Prof. Goro Komatsu (International Research School of Planetary Sciences, Universita d'Annunzio, Pescara, Italy) and Junichi Haruyama, PhD (Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan).
Lunar basalt layers exposed on the inner walls of the (a) Dawes crater and the (b) Bessel crater with visible stratification. (c) Photograph of the surface of the inner wall of Hadley Rille, taken from Apollo 15. (d, e) Mare Tranquillitatis Pit with visible thin stratification. (f, g) Mare Ingenii Pit.
The stratification of ceilings in lunar caves
“In our analyses, we took into account, for the first time in the literature, the stratification of the rock ceiling, which is very visible in the images sent by the Lunar Reconnaissance Orbiter (LRO), both at the lunar collapse pits and crater rings," says Marcin Chwała, PhD, Dsc, Eng.
The scientists studied the impact of the presence of a weaker layer, the so-called palaeoregolith, which most likely formed between successive lava flows on the lunar surface. It is a regolith, i.e. lunar soil characterised by poorer mechanical properties, which was trapped between two layers of basalt rock.
“Based on previous work, the thickness of the rock layers was found to vary between 2 and 13 metres. To date, however, no one has studied the effect of these layers on the stability of cave ceilings on the Moon. Our results showed that with a stratified ceiling, the characteristics of the collapses clearly change – there’s an increase in the number of smaller collapses, so-called local collapses, which don’t reach the lunar surface and thus cannot be observed from Earth,” the scientist explains.
Importantly, the possible multiple occurrence of collapses of this type is important for planning exploration missions. Primarily because of the large amount of rock rubble deposited at the bottom of the cave (the need to adapt the technology to such an environment arises) and bearing in mind the possibility of their radar detection from the Moon's orbit (difficulties in radar echo detection in the case of a bottom covered with rubble and boulders).
“Another very important result is that, due to the presence of local collapses, we can have direct access to exposed ancient layers of palaeoregolith from inside the caves, which scientists believe could be an invaluable source of astrophysical data, e.g. on cosmic radiation and solar activity, or even information about the chemical composition of the Earth's atmosphere. The results of the work therefore indicate the validity of designing reconnaissance missions to lunar collapses and caves because of their enormous potential for research into the history of the Solar System,” emphasises Marcin Chwała.
The just-published paper is an introduction to the scientific project entitled “Probabilistic boundary analysis in collapse reconstruction and stability assessment of lunar lava tubes (PROMISE)”, which has received funding from the National Centre for Science as one of the winning entries in the Sonata 19 competition. The grant-funded project already involves Dr Marcin Chwała already assembling a research team at Wrocław Tech with which he will tackle the topic of the stability and size of lunar lava caves.
The existence of caves on the moon has been confirmed
Scientists’ interest in lunar caves is growing steadily. In mid-July 2024, a team of Italian and American researchers confirmed the existence of a cave on the Moon in the region of the Sea of Tranquillity, where Neil Armstrong and Buzz Aldrin landed 55 years ago. They suspect that there may be over 200 similar cavities in the area, where astronauts could take shelter in the future.
The cave described by them is located 400 km from the Apollo 11 landing site. Only the initial part of the underground cavity is visible, but according to the researchers, it is at least 40 m wide and several tens of metres long.
Dr Marcin Chwała stresses that the discovery of this section of the cave, which is most likely connected to an entire cave system, is vital because it means that we could enter the caves from the Mare Tranquilitatis Pit collapse. Until now it has not been clear whether these collapses offer this possibility.
“The very fact that such a connection exists isn’t surprising to me. Interestingly, it was also indicated by my earlier work and the one just published, but these were only the results of numerical analyses. It’s now been confirmed, and the shape of the cave entrance is consistent with our interpretation of the results of the numerical analyses,” explains the scientist. “On the other hand, I was very positively surprised that the authors of this work managed to extract new information from radar data that has been available for several years. The discovery means increased interest from scientists, space agencies, and private companies in these objects,” he adds.
Interpretation of collapse geometry based on stability analyses for a half-filled lava cave.
What’s inside the lunar caves?
In their publication, the Italian-American team also pointed out that similar cavities may also be located at the Moon's south pole, where NASA astronauts are planned to land in the future. It is anticipated that shaded craters, as well as cavities, may contain frozen water.
“It’s possible, but from my conversations with people researching water migration on the lunar surface, the best chances of finding frozen water are near the poles of the Moon in the so-called perpetually shaded areas, where the temperature is much lower (even < -200 degrees C) than the expected temperature in the interiors of the caves (-15 to -20 degrees C),” explains Dr Chwała.
Researchers from the US and Italy agree that natural caves could serve as shelter for astronauts in the future and estimate that building settlements on the lunar surface will be more time-consuming and difficult than reinforcing cave walls. Without a thorough examination of their interiors, it is difficult to confirm these speculations unequivocally.
“The research carried out to date, which indicates that there is a large amount of debris at the bottom of the caves, is certainly an element that hinders the possible establishment of bases. On the other hand, if this is indeed the case, it means, according to the results of our research, that we have a good chance of exploring the palaeoregolith exposed by the local collapses, which is a great incentive for their exploration,” admits the researcher. “We can only keep our fingers crossed for the first mission to explore the interiors of the lunar collapses. Here, we have a large space for future research under the PROMISE project, where we will want to better understand the process of collapse formation and estimate the size of lunar caves,” he adds.
