Giant lava tubes possible on the Moon

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New analysis of the lunar geology combined with gravity data from GRAIL now suggests that the Moon could harbor lava tubes several miles wide.

David Blair, a graduate student in Purdue’s Department of Earth, Atmospheric and Planetary Sciences, led the study that examined whether empty lava tubes more than 1 kilometer wide could remain structurally stable on the moon. “We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the moon,” Blair said. “This wouldn’t be possible on Earth, but gravity is much lower on the moon and lunar rock doesn’t have to withstand the same weathering and erosion. In theory, huge lava tubes – big enough to easily house a city – could be structurally sound on the moon.”

You can read their paper here. If this is so, then the possibility of huge colonies on the Moon increases significantly, as it will be much easier to build these colonies inside these giant lava tubes.



  • Tom Billings

    More data!

    The next set of orbiters around the Moon should be a constellation of ground-penetrating radars that map the lunar underground to a depth of 6 kilometers. I pick that number because Dr. Brown’s Apollo 17 Apollo Lunar Sounder was stated by him to have got returns from 6 kilometers under the lunar surface. Since that was with a very long wavelength, to obtain good penetration, and the instrument was a small one in the CSM module of the Apollo spacecraft, it could do nothing to construct images. The obvious solution for getting images of lunar lava tubes’ dimensions and positions is a synthetic aperture ground penetrating radar (GPR) made from a constellation of orbiting receivers, and a high power transmitting module co-orbiting with them. The constellation would be spread over several kilometers, providing a very large aperture. It would map the upper 6 kilometers of the lunar surface.
    Meanwhile, further analysis of the Grail data should be useful.

    Such a GPR will give us a better data set on the Moon than we have on Earth’s upper 6 kilometers. If the enthusiasm for lunar settlements keeps growing, there may even be a market in lunar settlement companies that would purchase the data needed to find a site they want. The dryness of the Moon makes empty lava tubes easier to detect than those on Earth. If they are filled with ice, even better, and that should show up in the data as well.

    The lunar base simulation at the NSS in Second Life island continues building lava tubes for our concept of how to go about building a lunar settlement on the Moon. We have not been so bold as a 5,000m cave diameter, but have stuck to the 120m diameter tube we can fit into one island as our largest, along with some 40m tubes above it. The idea of simulating a cave of 5,000m diameter in Second Life is more daunting, both from the aspect of plausibly sealing the cave, even with metals extracted from surface regolith, and from the sheer amount of building involved for a small team here at Oregon L5 Society. That’s a *lot* of area! As bad for a simulation, it would require a string of 20 islands to simply span the diameter of the cave, because an island’s size is set at 256m. Then if it’s only twice as long as it is wide, we have to multiply those 20 islands by 40. That 800 island requirement is possibly large enough to justify an investor setting up an entire virtual world just for lunar base simulations.

    For now, though, this paper gives us warm fuzzies to last months and months!

  • Max

    Lava tubes? I I suppose anything is possible but one needs proof or evidence to make such a claim. Where are all of the volcanoes, lava flows, and other evidence of any volcanic activity that would create such large underground lava tubes? In the low gravity of the moon, viscous lava would create volcano Miles high and obvious. ( The black Galilean sea on the moon were found to be made of obsidian, scorch marks of melted rock)

  • Tom Billings

    As to proof of lava tubes, that was mentioned in the article. We have pics of many skylight openings, quite similar to what we see in lava tubes here on Earth. The company called Astrobotics, a competitor in the Google Lunar X-Prize competition, is building a robot they hope can roll into one of these caves, from a ramp where the skylight’s side has broken down. This is in Lacus Mortis, about 35 degrees North of the lunar Equator.

    The vast majority of samples brought back by the Apollo missions from the Lunar Mare were basalt, similar to what we see from volcanoes here on Earth. These Mare are seen today as vast lava fields emplaced over 3 billion years ago, by lava that welled up after huge impacts. There are several places where similar up-wellings have taken place here on Earth. They are called “traps, as in”the Deccan Traps, the “Siberian Traps”, and in Eastern Oregon, the smaller “Oregon Traps”. The sinuous rilles of the Moon are what we call collapse trenches in lava fields here on Earth. Hadley Rille is the largest such sinuous rille on the Moon, IIRC.

    The Oregon Traps have lava tubes, though much smaller, and the shield volcanoes in the area have produced so many lava tubes that it is not unheard of for someone digging a basement to find the excavation collapsing into a lava tube under the position of the house. Some have made wine cellars out of them.

    There *is* some volcanic activity reported in the center of the crater “Aristarcus”, which may be a remnant of older times. More evidence of remains of conventional volcanoes in other places on the Moon is yet to be found. However, the majority of lunar lava tube discoveries have been made in the Lunar Mare, where the pressure of a solidified lava layer on the still liquid lava underneath created pressure. That pressure caused still-liquid lava to burst forth around the perimeter of the Mare Basin in fountains. This lava then ran downhill on the shallow slope pointing towards the depressed center of the Mare. The impact apparently brought up quite liquid and runny lava, of the sort we call Pahoehoe. It was named that in Hawaii for the “ropy” texture it often solidifies in. Not too surprisingly, these flows tended to create lava tubes on the perimeter of the Mare, winding their way in the general direction of its center.

    As usually happens with lava tubes, this flow’s top crusted over because it radiated heat into Space. The lava underneath remained liquid, and ran out of the tube thus created.

  • wodun

    I don’t think people would find living on the Moon to be all that pleasant because of the physiological effects.

  • mpthompson

    Could be great for the elderly because the low gravity would increase mobility and decrease injuries.

  • Tom Billings

    Not only can elderly mobility be improved, but other things may. We are becoming better at understanding how the body reacts to freefall, but not to partial gravity, …yet. In total, we can adapt to the Moon. For those who intend to go and stay, that will be enough. For those who will consider themselves expats who will return, we will need to study further how the body’s muscles can keep from deteriorating. The myostatin pathways that deconstruct unused muscle tissues have a number of spots where they can be interfered with.

  • wodun

    I guess if they like to skip everywhere and don’t mind the constipation. There are some others I am forgetting.

  • pzatchok

    Just because they are mathematically possible does not mean they are probable or practical.

    I would not trust such a large natural structure when smaller more stable options are available.

    You build underground to keep away from stuff falling on you. Why increase the risk by trying to use the largest possible natural structures?

    I would also not try to build anything underground out of metal processed on the moon.
    I would first try to use as much lunar material as possible to make a concrete type of material. Spray that around the inside of a smaller cave to seal it and make a living and working space that way.

  • Tom Billings

    “Just because they are mathematically possible does not mean they are probable or practical.”

    We are seeing things, skylights, in the middle of long winding depressions in the ground, that can be explained by very few things other than lava tubes.

    “I would not trust such a large natural structure when smaller more stable options are available.”

    These structures have stood for 3 billion+ years. We find on Earth that, since all basalt cracks as it cools, all of the breakdown talus boulders we see will drop within 2 weeks of emplacement by a lava flow. To get an idea of how strong and stable lava tubes are, contemplate Ape Cave, on Mt. St. Helens. There, the largest explosive volcanic eruption in centuries in the US shook the tube, on the slope of the mountain, with Earth tremors being felt hundreds of miles away. Yet, the total debris fall in Aape cave suspected, though not confirmed as such, is a single fist-sized rock that was not remembered as being on the floor before by Park Rangers.

    Any talus will have been dropped soon after emplacement, or in the millions of years after emplacement, when smaller, but still large impactors were striking the surface and shaking things drastically. The structure has already been through its “shakedown cruise”.

    “You build underground to keep away from stuff falling on you. Why increase the risk by trying to use the largest possible natural structures?”

    These large structures have several excellent characteristics for Phase 3 settlements. The worst thing to fall on any lunar settlement would be galactic cosmic ray particles, (GCRs). The many meters of rock between the tube and the surface are specifically what is most useful about it. The second useful attribute is the volume to surface ratio. Since all basalt cracks as it cools, a settlement in which you want to have open living space *must* be sealed, and that takes *lots* of material, even for smaller tubes. You get the most volume for the least sealant effort with larger tubes. Lowering capital costs/settler for *any* settlement is important.

    Third in importance is the first psychological advantage to settlers. The simple fact of long sight lines in a 5,000m diameter tube will allow sufficient variety of greenery along a person’s view that their need for this in their environment will be fulfilled. Forth is that those long sight lines also allow us to believably extend the greenery out into the farther distance by projecting a scene of desirable terrain onto the wall interior of the lava tube sealant using laser projectors. With 2,500m of distance from a viewing hill or ridge line constructed down the center of tube, the perspective will be indistinguishable from the real thing. From the far wall, even more so. Lastly, but far from least, such a large tube floor would allow even greater space for watercourses, carved into the floor before sealing, for viewing, swimming, and for maintaining a comfortable vapor pressure of H2O.

    “I would also not try to build anything underground out of metal processed on the moon.”

    Indeed, we believe at Oregon L5’s research team that interior structures for the Phase 2 community of builders will most likely be built from waste materials left from the extraction of Oxygen from regolith, and its later Carbonyl processing to extract Iron and Nickel. These processes will leave behind a material high in both Silica and Alumina, which are useful in making high-strength “S-glass” fibers. Those can be used with lower melting point mixes for “glass-in-glass” composites, a concept tested as early as the late 1970s by Dr. Goldsworthy. They can be used for both commercial and housing structures, even inside the vacuum-dominated volumes that a Phase 2 community would occupy.

    In the Phase 3 settlement they can be used as well for cantilevered “shelves” up the sides of the lava tube to support houses overlooking the settlement’s green common areas and commercial areas on the floor of the tube. Those could be spaced along the tube in smaller “shelf neighborhoods”.

    The superiority of metal as sealant comes in its toughness and its ability to be a self-forging micro-particle direct from the Carbonyl processing, that can be projected by electrostatic accelerators at the walls and floors of lava tubes. These can seal the cracks with a centimeter-thick layer of Iron. Over that is laid at most a millimeter-thick layer of Nickel to discourage rusting from contact with the water-laden atmosphere humans feel comfortable in. Lastly will be a layer of Alumina or Titania powder to provide the reflective surface which the laser projectors will use.

    “I would first try to use as much lunar material as possible to make a concrete type of material. Spray that around the inside of a smaller cave to seal it and make a living and working space that way.”

    Until sufficient large amounts of mined lunar water or sulfur are definitely found for making lunacrete, we have been skeptic of its use. This is also true because making such layers tough would require nearly as much steel reinforcement as would be used in the sealant layers of a metal-sealed lava tube. Certainly this is true in early Phase 2 communities, which can probably be constructed as pressurized glass-in-glass structures in vacuum, since the population will be hard-core pioneers, building the larger Phase 3 settlements.

  • LocalFluff

    Concerning stability, the lunar lava tubes might be death traps. The lava tubes on Earth today are very stable, because the unstable ones have crashed down long ago. On the Moon nothing much happens. So when humans enter a lava tube they might cause it to collapse. Load, temperature, vibrations, excavations could do bad things to a lunar lava tube. But for sure, some part of some lava tube is very stable and a very useful starting point for colony. Life seems to thrive subsurface, away from space.

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