Building a base on the Moon in certainly going to be challenging. In fact, living anywhere other than the surface of the Earth is extremely challenging – the rest of our solar system is an unlivable hellscape. The thin biosphere clinging to the surface of Earth is the only place that humans can comfortably live (and even not everywhere there). But we will be returning to the Moon again with the upcoming Artemis mission, and this time NASA plans on staying, not just leaving behind “flags and footprints”. There are no current plans for a permanent moon base or settlement, but the work of Artemis will continue to lay the groundwork for eventually future bases.

The surface of the Moon is hostile to life in several respects. First, there is almost no atmosphere. Humans can survive in a hard vacuum for about 90 seconds, falling unconscious after 15-20. Second, the surface of the Moon is exposed to radiation (solar wind and cosmic rays) and micrometeors. There are filtered out on Earth by its thick atmosphere and magnetic field, neither of which protect the Moon. Finally, the temperature on the surface of the Moon varies from one extreme to the other – during the day surface temperatures reach 260 degrees Fahrenheit (126° Celsius), while nighttime temps can drop to -280 F (-173 C).

But these harsh conditions do not necessarily exist everywhere on the Moon. The hard vacuum, yes, there is simply no air on the Moon, which has too little gravity to hold onto a significant atmosphere. But there may be protection from the second two features: exposure to radiation and variable temperatures – in lunar pits, caves, and lava tubes.

Lava tubes exist on every rocky world in the solar system. They are channels through which molten lava flowed and eventually solidified, leaving behind solid tubes in the rock. You can explore lava tubes on Earth, such as in Hawaii. The size of lava tubes tends to correlate with the gravity of the world on which they occur, and the Moon’s light gravity (16.6% Earth’s gravity, or 0.166 g) allows for giant lava tubes. Recent evidence suggests they can be  1,600 to 3,000 feet (500 to 900 m) in diameter. This is large enough to contain a city. The big advantage of building a permanent base or settlement inside a lava tube is that the rocky covering will protect it from radiation and micrometeors.

A recent study now adds another feature of lava tubes that makes them an attractive location for a future base – they have a stable and comfortable temperature of about 17°C (or 63° F). That is a surprisingly comfortable temperature, what the scientists are calling “sweater weather”. How does this work?

First, they made this discovery by analyzing data from the Diviner Lunar Radiometer onboard the Lunar Reconnaissance Orbiter, which has over 11 years of data. The Diviner instrument can measure the temperature of the lunar surface, which gets very hot when the sun is shining on it, then cools off rapidly at night. The lunar day is one month, so the surface has daylight for about 15 days then no sunlight for 15 days. The researchers examined data for the Mare Tranquillitatis and Mare Ingenii pits on the Moon, finding that at night they were about 100 K warmer than the lunar surface.

A pit is simply that, a depression in the surface. There are lots of pits on the Moon, mostly caused by meteor impacts. But 16 of them appear to be different, and likely result from a surface collapse into a lava tube below. These pits have overhangs, which suggests they connect to a cave or lava tube. These overhangs appear to be critical to the warm temperature. The researchers did computer simulations based on the data from Diviner. During the day the overhangs shade part of the pit from direct sunlight, and therefore keep it from heating up. At night the overhang prevents pit from quickly radiating away its heat, keeping them from cooling down. The result is that the part of the pits shaded by the overhang are in black body equilibrium, with a constant temperature of 17°C, with less than 1°C variation throughout a 30 day lunar cycle.

Further, if these pits with overhangs do connect to a lava tube, the entire length of the lava tube should also be at this stable temperature. How perfect is that? A base built inside such a lava tube would be protected from radiation and already have a relatively comfortable and stable temperature. Just add a little air and you have a livable environment. It is an open question whether or not any of the lava tubes are airtight enough by themselves to hold air. Tiny cracks from prior impacts may render them leaky. But it is not impossible that one or more of them may be airtight. Then all we would need to do is cap two ends of the tube with airlocks and in between we can have a livable space. If all lava tubes are too leaky for this, then in the worst-case scenario we need to build air-tight habitats inside the lava tubes. We would probably want to do this anyway, as a fail safe. It’s also conceivable that the inner surface of a lava tube could be sprayed with some kind of sealant, hopefully made from local materials, to plug any cracks. This is all highly speculative, but plausible.

At this point it seems highly likely that future settlements on the Moon will be build inside lava tubes. Building a base on the surface with sufficient protection from even just cosmic rays would likely be prohibitive (but not impossible). What, then would such a future settlement look like? Again, we are getting into highly speculative territory here, but let’s have fun. The pits themselves could be excavated and made into a landing pad for lunar landers. A dome that can open and close (like in the movie, 2001) could be built to protect any ships on the landing pad. A the bottom of the pit are airlocks, perhaps going in both directions leading away from the pit into different sections of the lava tube. Inside the lava tube is a city of networked habitants. Perhaps the lava tube, or maybe only sections of the lava tube, are sealed and pressurized themselves. Some sections will contain hydroponic farms. The entire tube may be illuminated with artificial light.

Energy for the lunar city comes through cables that snake down the pit, leading from solar arrays on the surface. Of course, these solar arrays only collect energy 15 days out of 30, so there would need to be massive battery storage. But also, there would likely be several small modular nuclear reactors, positioned farther down in the tube, away from occupied sections. These would provide steady reliable power. Far enough in the future these would likely be replaced with fusion reactors.

Inside the lava tube the environment would be very comfortable, except for two things. First, there is no outside. However, the lava tubes are large enough that a park could be created, with enough light to give the illusion of being outside on Earth. However, there is one aspect of life inside a lunar lava tube that there is likely no solution for (maybe not ever, but at least not for the foreseeable future), and that is the low gravity. At 0.166 g lunar life would be very different from Earth, and those growing and living on the Moon may not be able to even visit the Earth with its 1 g. There may be mitigating strategies, but it will be difficult to compensate for living day to day in low gravity.  But there may also be fun aspects to living in low gravity. One thing is for sure – it will be different.