Slight variations in the moon’s gravitational tug have hinted that kilometers-wide caverns lurk beneath the lunar surface. Like the lava tubes of Hawaii and Iceland, these structures probably formed when underground rivers of molten rock ran dry, leaving behind a cylindrical channel. On Earth, such structures max out at around 30 meters across, but the gravitational data suggest that the moon’s tubes are vastly wider.
Other satellites had found the openings of large lunar lava tubes and caves
- The Lunar Reconnaissance Orbiter has now imaged over 200 pits that show the signature of being skylights into subsurface voids or caverns, ranging in diameter from about 16 feet (5 meters) to more than 2,950 feet (900 m), although some of these are likely to be post-flow features rather than volcanic skylights.
- The Chandrayaan-1 orbiter imaged a lunar rille, formed by an ancient lunar lava flow, with an uncollapsed segment indicating the likely presence of a lava tube near the lunar equator, measuring about 2 km (1.2 mi) in length and 360 m (1,180 ft) in width
Lunar lava tubes may potentially serve a role as enclosures for human habitats. Tunnels up to 5000 meters in diameter may exist, lying under 40 to 500 meters (130 ft) or more of basalt with a stable temperature of −20 °C (−4 °F). These natural tunnels provide protection from cosmic ray radiation, meteorites, micrometeorites, and ejecta from impacts. They are shielded from the variations in temperature at the lunar surface, which would provide a stable environment for inhabitants. Lunar lava tubes are typically found along the boundaries between lunar mares and highland regions. This would give ready access to elevated regions for communications, basaltic plains for landing sites and regolith harvesting, as well as underground mineral resources
• We explore the stability of lava tubes on the Moon using finite element modeling.
• Lava tube stability depends on roof thickness, width, and regional stresses.
• Kilometer-wide lava tubes may remain stable with a roof only 2 m thick.
• Given certain conditions, lava tubes up to 5 km wide may be plausible.
• The large lava tubes inferred from GRAIL data are therefore mechanically plausible.
Mounting evidence from the SELENE, LRO, and GRAIL spacecraft suggests the presence of vacant lava tubes under the surface of the Moon. GRAIL evidence, in particular, suggests that some may be more than a kilometer in width. Such large sublunarean structures would be of great benefit to future human exploration of the Moon, providing shelter from the harsh environment at the surface—but could empty lava tubes of this size be stable under lunar conditions? And what is the largest size at which they could remain structurally sound? We address these questions by creating elasto-plastic finite element models of lava tubes using the Abaqus modeling software and examining where there is local material failure in the tube’s roof. We assess the strength of the rock body using the Geological Strength Index method with values appropriate to the Moon, assign it a basaltic density derived from a modern re-analysis of lunar samples, and assume a 3:1 width-to-height ratio for the lava tube. Our results show that the stability of a lava tube depends on its width, its roof thickness, and whether the rock comprising the structure begins in a lithostatic or Poisson stress state. With a roof 2 m thick, lava tubes a kilometer or more in width can remain stable, supporting inferences from GRAIL observations. The theoretical maximum size of a lunar lava tube depends on a variety of factors, but given sufficient burial depth (500 m) and an initial lithostatic stress state, our results show that lava tubes up to 5 km wide may be able to remain structurally stable.