Lunar lava tube

Lunar lava tubes are sub-surface tunnels on the Moon that are believed to have formed during basaltic lava flows. When the surface of a lava tube cools, it forms a hardened lid that contains the ongoing lava flow beneath the surface in a conduit-shaped passage. Once the flow of lava diminishes, the tunnel may become drained, forming a hollow void. Lunar lava tubes are formed on surfaces that have a slope that ranges in angle from 0.4–6.5°. Lunar lava tubes may be as wide as 500 metres (1,600 ft) 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.

One such area containing lava tubes and rilles is the Marius Hills region. In 2008, an opening to such 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 the Lunar Reconnaissance Orbiter, showing both the 65-meter pit and the floor of the cave about 36 meters below. 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 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.

Gravitometric observations by the GRAIL spacecraft suggest the presence of lunar lava tubes with widths of over a kilometer. Assuming a width-to-height ratio of 3:1, such a structure can remain stable with a ceiling that is 2m thick. Lava tubes at least 500 m underground can theoretically remain stable with widths of up to 5 km.

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Wikipedia