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Featured Image - 09/09/2008
Lunar Lava Tubes: Home Sweet Home?

In southeastern Mare Serenitatis, not far from the Apollo 17 landing site in the Taurus-Littrow Valley , lie features thought to be lunar lava tubes. These features are found in a variety of lunar mare environments. Scientists first suggested drained lunar lava tubes as possible human habitats in 1962, and since then the tubes have remained at the center of both geological debate and the future of a sustained human presence on the Moon.

Apollo Metric image (frame ID AS15-M-1115) Southeastern Mare Serenitatis. The arrow points to the Apollo 17 landing site. The box contains location of lava tubes.

 
Figure 1. Southeastern Mare Serenitatis. The arrow points to the Apollo 17 landing site. The box contains the location of lava tubes. (Apollo Image AS15-M-1115 [NASA/JSC/Arizona State University])

Apollo Metric image (frame ID AS15-M-1116) Close-up view of potential human habitats in Mare Serenitatis.

 
Figure 2. Close-up view of possible lava tubes in Mare Serenitatis. This particular lava tube complex contains 5 distinct sections. The largest is approximately 0.88 km long, 0.5 km wide, and has an estimated roof thickness of at least 40 meters.(Apollo Image AS15-M-1116 [NASA/JSC/Arizona State University])

Geologists suspect that lunar lava tubes form similarly to terrestrial lava tubes. However, lunar lava tubes are much larger, probably due to the lower gravity and the lack of an atmosphere. Studies of lava tubes on Earth show that most are hollow. If lunar lava tubes form in a similar way, then they too are most likely hollow. The lunar tubes would be an ideal location for a lunar base because they a) require little construction and enable a habitat to be placed inside with a minimal amount of building, b) provide a natural environmental control (insulating and temperature stability), and c) provide protection from natural hazards (i.e., cosmic rays, meteorites, and micrometeorite impacts, impact crater ejecta). A lava tube may form when an active basaltic lava flow develops a continuous crust. For instance, an open lava channel may form a crust of hardened rock that extends from the sides and, over time, meets in the middle, forming a roof. Alternatively, a roof may form when pieces of lower density crustal material break off and float to the top of a lava flow, providing a frame for a roof. The exact mechanism for lunar lava tube formation remains enigmatic. Even if a lava tube develops a roof, it still has lava running through it. There is a possibility that the cooling lava would solidify inside the tube and block it. However, on Earth most lava tubes do not "plug up." As the rate of lava flowing from the source diminishes over time, the level of liquid in the tube drops, leaving an empty space between the top of the flow and the roof of the tube. Further lava flows may have come along and filled in the tubes, but it is unlikely that this happened in all of the lunar lava tubes.

Future lunar missions, especially the high-resolution (0.5 m/pixel) from the Lunar Reconnaissance Orbiter Camera will clarify the geological mechanism behind lunar lava tube formation and identify locations with hollow lava tubes that will help humans survive on the Moon for extended periods of time.

References: C.R. Coombs and B.R. Hawke (1992), "A Search for Intact Lava Tubes on the Moon: Possible Lunar Base Habitats." 2nd Conference on Lunar Bases and Space Activities (LPI): 219-229.

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