Lunar lava tubes: Morphology to structural stability

Abstract

Future permanent habitats on the Moon may facilitate space exploration by serving as an outpost for manned missions to other planets. Safety and resilience of those habitats are the main concerns, especially given the existing dangerous conditions and hazards such as temperature fluctuation, radiation, seismic activity, and meteorite impacts. Underground habitats in the form of “lava tubes” are good candidates for permanent human shelters because they provide immediate protection from such hazards. Evidence for their existence under the surface of the Moon is provided by GRAIL, SELENE spacecraft, and the LRO. Data from GRAIL suggests that the width of the lava tubes can be as large as 1–2 km. However, the size and geometry configurations under which the lava tubes are stable are not well-defined and there are no well-established criteria for their stability. The paper provides analytical and numerical solutions to estimate the size and stability of lunar lava tubes. Analytical solutions use the flow characteristics of molten lava and the limit equilibrium analysis of the collapsed pits to estimate the size of the tubes. Numerical analyses are conducted for different geometries and for different material properties of the lava. Different sizes are considered, from 300 m, up to 4000 m in width. The results show that the key parameters for the stability of the tubes are the roof thicknesses and the strength of the lava, in particular, its tensile strength. As the lava tube gets wider, a larger tensile strength and roof thicknesses are needed for the tube to remain stable. The analyses show that 1 km wide lava tubes are likely to exist and remain stable, which agree with GRAIL observations. For larger widths, minimum roof thicknesses of few hundred meters might be necessary to preserve their stability.