Lunar lava tubes are lava tubes on the Moon formed during the eruption of basaltic lava flows. When the surface of a lava flow cools, it hardens and the lava can channel beneath the surface in a tube-shaped passage. Once the flow of lava diminishes, the tube may drain, forming a hollow void. Lunar lava tubes are formed on sloped surfaces that range in angle from 0.4° to 6.5°.[1] These tubes may be as wide as 500 metres (1,600ft) before they become unstable against gravitational collapse. However, stable tubes may still be disrupted by seismic events or meteoroid bombardment.[2]
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.[3][4]
Observational evidence
Sinuous 50‑km‑long chain of collapse pits transitions into a continuous uncollapsed segment of a lunar lava tube.
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 5m (16ft) to more than 900m (3,000ft),[12] although some of these are likely to be post-flow features rather than volcanic skylights.[13]
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 2km (1.2mi) in length and 360m (1,180ft) in width.[14]
Gravitometric observations by the GRAIL spacecraft suggest the presence of lunar lava tubes with widths of over 1km. Assuming a width-to-height ratio of 3:1, such a structure can remain stable with a ceiling that is 2m (6.6ft) thick.[15]
In 2023-2024, radar imaging of the Mare Tranquillitatis pit from NASA's Lunar Reconnaissance Orbiter Camera was analyzed and determined to have been formed by the collapse of a lava tube that resulted in the formation of a cave conduit at least tens of meters long, proving the existence of lunar caves.[16]
Oblique view of the Marius Hills "Haruyama Skylight" pitPossible post collapse geometry interpretations of stability analyses for half-filled lava tube.
In 2019, the European Space Agency launched a campaign through the ESA's Open Space Innovation Platform (OSIP) to evaluate innovative proposals aimed at the exploration, documentation and 3D mapping of volcanic cavities on the Moon. Two complementary studies have been selected, the Descent And Exploration in Deep Autonomy of Lava Underground Structures (DAEDALUS) Sphere and the RoboCrane.[20] DAEDALUS is a prototype designed by the University of Wurzburg (Germany), the Jacobs University (Germany), the University of Padua (Italy), the INAF-Osservatorio di Padova (Italy) and the VIGEA-Virtual Geographic Agency (Italy). This prototype is equipped with several components capable of performing a high-definition 3D mapping during the descent and moving autonomously within a lava tube. This system is in fact equipped with LIDAR and stereoscopic cameras to guarantee almost total coverage in order to acquire data in any condition.
The temperature of the Moon's interior increases with depth
Lunar lava tubes may potentially serve as enclosures for human habitats.[5][8][21] Tunnels larger than 300 metres (980ft) in diameter may exist, lying under 40 metres (130ft) or more of basalt, with a stable temperature of −20°C (−4°F).[22] 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.[23]
Lava tubes with a persistent temperature below freezing could act as a cold trap for water molecules, although any such spelean ice is likely to sublimate over geologic time unless the temperature is below 120K. This is only likely in lava tubes near the lunar polar regions.[24]
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.[25]
1 2 Coombs, Cassandra R.; Hawke, B. Ray (September 1992), "A search for intact lava tubes on the Moon: Possible lunar base habitats", In NASA. Johnson Space Center, The Second Conference on Lunar Bases and Space Activities of the 21st Century (SEE N93-17414 05-91), vol.1, pp.219–229, Bibcode:1992lbsa.conf..219C
↑ Ximenes, S. W.; Elliott, J. O.; Bannova, O. (2012). "Defining a Mission Architecture and Technologies for Lunar Lava Tube Reconnaissance". Earth and Space 2012. p.344. doi:10.1061/9780784412190.038. ISBN978-0-7844-1219-0.
↑ York, Cheryl Lynn; etal. (December 1992), "Lunar lava tube sensing", Lunar and Planetary Institute, Joint Workshop on New Technologies for Lunar Resource Assessment, pp.51–52, Bibcode:1992ntlr.work...51Y
↑ De Angelis, G.; etal. (November 2001), "Lunar Lava Tubes Radiation Safety Analysis", Bulletin of the American Astronomical Society, 33: 1037, Bibcode:2001DPS....33.1003D
↑ Walden, Bryce E.; etal. (January 1998), "Utility of Lava Tubes on Other Worlds", Workshop on Using In Situ resources for Construction of Planetary Outposts, p.16, Bibcode:1998uisr.work...16W
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