Lunar lava tubes are tunnels on the Moon that are thought to have formed during basaltic lava flows. When the surface of a lava tube cools, it forms a hardened lid that channels the lava as it continues to flow beneath the surface in a conduit-shaped passage. Once the flow of lava diminishes, the tunnel may drain, forming a hollow void. Lunar lava tubes are formed on sloped surfaces that range in angle from 0.4° to 6.5°. These tubes may be as wide as before they become unstable against gravitational collapse. However, stable tubes may still be disrupted by seismic events or meteoroid bombardment. The existence of a lava tube is sometimes revealed by the presence of a "skylight", a place in which the roof of the tube has collapsed, leaving a circular hole that can be observed by lunar orbiters.
An area displaying a lava tube and rilles is the Marius Hills region. In 2008, an opening to a lava tube in this area may have been discovered by the Japanese Kaguya spacecraft. The skylight was photographed in more detail in 2011 by NASA's Lunar Reconnaissance Orbiter, showing both the 65-meter-wide pit and the floor of the pit about 36 meters below. Additionally, the Hadley Rille may have been a partly roofed lava channel, some parts of which have since collapsed. There may also be lava tubes in the Mare Serenitatis. The Lunar Reconnaissance Orbiter has imaged over 200 pits that show the signature of being skylights into subsurface voids or caverns, ranging in diameter from about to more than, although some of these are likely to be post-flow features rather than volcanic skylights. The ISROChandrayaan-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. The tunnel measures about in length and in width. Gravitometric observations by the GRAIL spacecraft suggest the presence of lunar lava tubes with widths of over 1 km. Assuming a width-to-height ratio of 3:1, such a structure can remain stable with a ceiling that is thick.
Proposed exploration
Several groups have proposed robotic missions to explore lunar and Martian lava tubes. The "Moon Diver" mission led by Laura Kerber proposes to send the two-wheeled AXEL extreme-terrain rover developed at NASA-JPL into a lunar pit in order to investigate the history of the lunar mare and flood basalt eruptions.
Sites for human habitats
Lunar lava tubes may potentially serve as enclosures for human habitats. Tunnels larger than in diameter may exist, lying under or more of basalt, with a stable temperature of. These natural tunnels provide protection from cosmic radiation, solar radiation, meteorites, micrometeorites, and ejecta from impacts. They are insulated from the extreme temperature variations on the lunar surface and could 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; and underground mineral resources.
