Occupying a cave has many advantages for future inhabitants of the Moon. Being underground, people are protected from the constant rain of micrometeorites, galactic cosmic rays (high energy nuclei that can penetrate any material and might cause genetic mutations) and coronal mass ejections from the Sun—the occasional eruption of large streams of highly charged material that can fry both equipment and people left unshielded from these solar “storms.” In addition, caves offer a near-constant thermal environment. This allows people to dwell in benign conditions that do not undergo the temperature extremes on the lunar surface, where it may be as hot as 100°C during the local noon and as cold as -150°C just before dawn. A lunar day consists of two weeks of sunlight followed by two weeks of darkness, but at the poles the sun circles around on the horizon rather than rising and setting—a more benign environment with near constant (-50°C) temperatures.
The existence of caves on the Moon was only theoretical until two recent orbital missions (Kaguya and the Lunar Reconnaissance Orbiter, or LRO) imaged pits on the surface (above) that appear similar to “skylights” in terrestrial lava caves—small portions of the tube roofs that have collapsed, exposing interior voids and allowing access to an underground cave. On Earth, lava tubes are often found in basaltic (i.e., lavas rich in iron and magnesium) volcanic complexes, either as part of a large shield volcano (as in Hawaii) or within some basaltic plateaus in continental interiors, such as the Snake River Plain of Idaho. Massive amounts of lava, in the form of a thick flow, erupt from a vent that then cools from the outside, gradually solidifying inward and restricting the flow of molten lava to the center of a hot interior. This hot interior “tube” can transport lava great distances from the vent, allowing the flow to cover many thousands of square kilometers. When the eruption stops, the hot lava flowing in the tube interior can drain out, leaving a void space—an underground cave.
Despite this relatively simple scenario, complications often arise. Volcanoes are not quiet, peaceful places. Earthquakes, sometimes of enormous magnitude, are common during active volcanic cycles and may occur before, during and after eruptions. These seismic events can cause the collapse of newly formed tubes, with roof fragments creating a cascade of loose rubble, falling into and filling the drained tube. Sometimes the lava does not fully drain but freezes in place, leaving the “tube” filled with a thick plug of solidified rock. It’s even possible for later lava flows to cover and fill pre-existing tubes, obliterating the caves that do survive the various fates described above. Thus, although lava caves are not uncommon in basaltic terrains, due to difficulties in their creation, they are not a reliable consequence of an eruption.
After the first Apollo missions to the Moon, it was confirmed that the dark maria are made of basaltic lava, a rock type similar in composition to basalt on Earth, raising the possibility that the same processes operating on the Moon would produce the same kinds of landforms, including lava tubes and caves. Of course, the unique environment of the Moon (low gravity, no atmosphere, lack of water) might create differences of scale and type. Sinuous valleys in the lunar maria had been observed from orbit (below) and it was hypothesized that these features might be lava channels and tubes. The Apollo 15 mission in 1971 was sent to Hadley Rille, a large, sinuous channel at the base of the Apennine Mountains on the Moon, and basalts were collected from the rim of that feature. This mission did not clearly establish that the rille was a lava tube, although images taken and data acquired during that mission are consistent with that idea. Hadley Rille is over 300 meters deep at the Apollo 15 landing site—an enormous chasm. Parts of the rille north of the site appear to be roofed over, although we cannot say if any parts of this rille consist of caves.
A newly published scientific paper asserts that empty lava tubes on the Moon (giant caves with free-standing roofs on the Moon) may have enormous dimensions—hundreds of meters across, large enough to accommodate whole villages inside. The new paper suggests that lunar rilles, despite their large size, are able to support roofs of significant dimensions. Modeling of the stress induced by the low lunar gravity suggests that a lava cave with a roof only 2 meters thick might be stable over a kilometer-wide tube. Thicker roofs would be stable for tubes as large as 5 km across, thus permitting very large habitats to be established within them (below). Such a volume is large enough to accommodate a city-sized settlement.
As mentioned, habitats set up in lava tubes have many advantages, including radiation protection and a benign thermal environment. In practice, the pressurized habitat would be placed in the cave and access roads would be built to go to and from the facility. In some cases, it may even be possible to seal the cave walls with some type of material (e.g., plastic) and then pressurize the entire interior volume. This approach would provide significant amounts of space for many residents and would only be employed in advanced stages of lunar habitation.
We have found tube “skylights” on the Moon (which may be entrances to lava caves) and calculations show such features might not only exist, but that they could be quite large and commodious as well. However, we still are not certain that large void tubes exist on the Moon. And there are several reasons to be skeptical. As mentioned above, even though tubes and channels are common in basaltic terrains, lava caves are not. In addition to all of the factors that destroy void tubes on Earth, on the Moon, impact craters are constantly forming that generate seismic disturbances and throw ejecta across the lunar surface. Both factors could contribute to the degradation and destruction of cave spaces.
But lava tubes are found only in the maria, which are mostly concentrated near the equator and at low latitudes. One of the biggest drawbacks to cave living on the Moon is that we don’t have any near the poles. At the poles there is near-constant sunlight, along with deposits of water ice—valuable resources essential for human habitation. Yet enjoying the advantages of underground living doesn’t require caves. It is possible to place a habitat at the bottom of a deep crater, lay out an airlock, access tunnels, electrical cables and cooling lines, and then backfill (cover over) the crater with lunar regolith (soil) using a bulldozer. This simple construction technique provides all the thermal and protection advantages of cave dwelling, without restricting the outpost location to a less than optimum locality.
The advantages of cave life seem so attractive that every time a new lunar lava tube discovery is made, there is a call to use them to live on the Moon or other planets (most recently shown in the current “Mars” television mini-series). But people live where they can make their livelihoods and on the Moon, that “pay dirt” is at the poles, in the form of water and electrical power. The Willie Sutton principle still prevails.