Astronomers have discovered multiple “pits” on the surface of the moon – these look superficially like craters, but on closer inspection are actually vertical pits. There has been considerable speculation that these pits might be cave openings. Now, an analysis of data from the Lunar Reconnaissance Orbiter from 2010 reveals that at least one of these pits is in fact a cave opening.

The pit is located in the Mare Tranquillitatis, near the site of the Apollo 11 landing (the Sea of Tranquility) and is therefore called the Mare Tranquillitatis Pit (MTP). It is 100 meters across and 130-170 meters deep. The study used radar data at a downward angle which was able to image the sides of the bottom of the pit, showing that there is a possible conduit there for an underground cave system. This conduit is at least tens of meters long, but could be much larger.

While this is exciting, it’s not surprising. One hypothesis is that these lunar pits are “skylights” of underground cave systems, carved out by lava tubes when the Moon was more geologically active. If true, then it’s possible that they are extensive, and can also be quite large. Why is this so exciting?

There are two main reasons NASA and others are interested in lunar caves. One is geological – such cave systems might be billions of years old, and therefore can preserve lunar rocks by protecting them from the radiation and micro meteors that pummel the lunar surface. When we send astronauts back to the Moon (or even just highly capable robots) they could explore these caves and are likely to make some interesting discoveries about the Moon.

But the second application is the most intriguing. I have deliberately buried the lede here, partly because I suspect most readers know where this story is going. Such lunar caves could be ideal locations for future lunar bases. This is for the same reason they are good locations to do some geological investigations on the Moon – the caves are protected. This is something that science fiction shows give very short shrift to, and for this reason perhaps is greatly underappreciated by the public. Space is a very dangerous place, and not just because it’s largely a cold vacuum. Space is full of radiation, and stellar systems are full of fast-moving debris.

The Moon has no significant atmosphere or magnetic field to protect the surface, so it is bombarded with galactic cosmic rays. These are high energy particles that diffuse the galaxy, mostly from supernova and other high energy events. They are comprised of 87% protons, 12% helium nuclei, and 1% other heavier nuclei, and represent about 75% of the radiation hitting the lunar surface. From Smithsonian:

According to NASA, the standard radiation dose for a person on Earth is about 0.0036 Sv/ year (0.36 rad). The Apollo astronauts received an average radiation dose on the skin of 0.38 rad — equivalent to two head CT scans. Overall, Apollo 14 received the skin dose of 1.14 rad, which was the highest. All this during missions not longer than 12 days.

How long could an astronaut survive on a surface lunar base? We don’t really know. The longer they are there, the more radiation they are exposed to, the more DNA damage they get, and the higher their risk for cancer. But cosmic rays are not the only risk. There are also solar storms. The solar wind is another form of constant radiation – plasma from the solar surface, including electrons, protons, and alpha particles (2 protons and 2 neutrons). Since these are mostly charged particles, a magnetic field could protect from them, but again, the Moon does not have one. Also – there are solar storms and coronal mass ejections. If these hit the Moon they would be much more dangerous to astronauts, even potentially fatal.

There are also micro meteors, and larger meteors, that hit the lunar surface. On the Earth, such particles would burn up in the atmosphere, but on the Moon they just hit the surface. These could potentially cause massive damage to any lunar surface station.

All of these hazards can be avoided, however, if a lunar base were built under ground. You could also build a base with a 2-3 foot thick concrete wall, which would block a lot of radiation and micrometeors, but that would not be as good as just building the base underground. This is where lunar lava tubes, pits, and cave systems come it.

The MTP, for example, might be an excellent site for a lunar base. That far under ground there would essentially be no radiation, and you would be protected from all but the largest meteors. In the permanent shade of the caves there could also be volatiles in the lunar regolith, including water. Tubes could even theoretically be sealed off and pressurized, creating a large living area. There are challenges, however. One is simply getting down through the deep pit into the cave. An astronaut could climb down on a rope, or be lowered, but that is not very safe or convenient. Ideally a large elevated can be installed from the bottom of the pit to the surface. That kind of infrastructure will take time to install on the Moon, however.

The second challenge is that the walls of such caves, protected from weathering effects on the surface, could be very sharp. Space suits might not fair well in such an environment. Imagine walking through a cave of glass where one tear in your suit means death. This is a solvable issue, but is one more engineering problem that will need to be tackled. Robots, for example, could be equipped with machinery that will polish down the surfaces of the cave, making them smooth and safe. Of course, this will be after scientists get all the data they can. Or at least there will need to be some sections of the cave left pristine for scientific investigation, while other sections are smoothed down for habitation.

All this is still a bit speculative. We need to get probes down into these caves to see what the actually conditions are. But if they are truly billions of years old, which seems likely, then they are also likely highly stable. There are estimates, however, that some of these lunar caves may be huge, hundreds of meters across – large enough to build a small city in. The potential benefits of these locations are too huge to ignore. If we want long term or permanent occupation of the Moon, lunar caves are the likely option.