Probability Distributions for Geometrical Characteristics of Lunar Lava Tube Collapses

There is strong evidence for the existence of lunar lava tubes (Sauro et al., 2020). Particularly astonishing examples are collapse pits discovered by Haruyama et al. (2009) and imaged with high resolution by the Lunar Reconnaissance Orbiter (LRO). Unfortunately, there is still a lack of comprehensive studies on their stability. Due to their natural formation, their geometry is expected to be irregular, similar to terrestrial lava tubes (e.g., Tomasi et al., 2022). The impact of irregular geometries was recently investigated for homogeneous roofs (ChwaBa et al., 2024a), and the results suggest that it is an important feature for lava tube stability. Finite Element Limit Analysis (FELA) is used for analysing lava tube collapses. Its numerical efficiency, allows for the analysis of tens of thousands of cross-sections.This study uses results obtained for different probabilistic characteristics used in lava tube section generation to approximate probability density functions (PDFs) for the resulting data. Specifically, PDFs are approximated for gravity multipliers, which are used to measure the stability level of lava tube roofs, the maximum horizontal extent of the collapse mechanism, and the area of the collapse. The latter is particularly important in assessing the potential volume of the collapses.It is found that the collapses can be distinguished into two groups, which are important for practical detection possibilities. One group includes those that reach the surface of the Moon, while the second group, called local collapses, do not reach the lunar surface. This means they cannot be seen directly from the lunar orbit. The found PDFs may be used to apply reliability-based design in lunar lava tube safety and to predict some lava tube characteristics based on observed collapses.

Keywords: Lava tubes, Moon exploration, Future of geotechnical engineering, Stability, Collapse pits.