More cave images from the Moon

James Fincannon of NASA has forwarded me two additional pictures of the same cave on the Moon, taken recently at different times by the camera on Lunar Reconnaissance Orbiter and made available by the Goddard Space Flight Center and Arizona State University.

two images of the same pit

These images clearly show that the skylight looks down into a much larger space, with the underground room belling out from the skylight in all directions. This can be seen by how the angle of sunlight hitting the floor of the cave changes over time. Below is a very crude cartoon I have drawn to illustrate what I think we are seeing in the image on the left. The dashed lines indicate unseen walls whose precise location is not yet determined.

cartoon

James also forwarded me this link, showing even more images of additional lava tube skylights on the Moon.

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15 comments

  • Dave West

    So the moon really IS made of swiss cheese! Seriously, what is the thinking on what caused the skylight. Result of impact? What else COULD cause it?

    • Tom Billings

      Dave, on Earth, skylights are usually formed during or shortly after emplacement of lavatube caves, when a spot in the cave roof is too thin to support its own weight. Indeed, the sinuous rilles of the Moon are almost certainly lavatubes that have collapsed because their width is so great that the ceiling cannot be supported. Because of lower gravity, the largest diameter of a lavatube cave on the Moon has been calculated as about 380m, given strength equal to that here on Earth. However, there are some indications of far larger tubes, that may result from the dryness of the Moon. Highly silicious solids are weakened by the presence of water, whose molecules wiggle microcracks in the material wider over time. So, there exists a possibility of lavatubes as large as 1050m, which we have surface indicators for that do not, yet, include skylights.

      It is possible that an impactor could cause a skylight with a hit in just the right place, and just large enough to punch through, but not large enough to collapse long lengths of the tube. I’d expect a lot more of them from cracked basalt simply falling down because it’s too thin to hold its weight over the diameter of the tube.

  • Kelly Starks

    I wonder how brittle those roofs and crater edges are?

    Wont be very usefull to settle in, if these pristine caves are a house of cards over your head.

    • Tom Billings

      Kelly, these lavatube caves have probably existed for about 3+ Billion years. On Earth, most ceiling collapse happens in the first few weeks after emplacement of the tube, since all basalt cracks as it cools from lava. At ORL5 we expect the ceilings to be stable enough to hang inflatable base modules from them, and later the modules built from lunar resources.

  • Bob Hammond

    Tom, try as I might, I cannot see anything in either photo but features which stand up above the surface. The left photo is casting a shadow as if it’s a short volcanic neck sticking up….to my eyes. Guess it’s my eyes.

    • Jim

      Bob, the left one is tricky – it is like one of those optical illusions. Both photos are of the same area, offset slightly, with the sun coming from different angles. In the right image, the sun is coming from the right casting highlights on the smaller crater;s right walls and leaving the left walls in shadow.

      In the image on the left, the sun is coming from the right, below the center line. What appears to be a large, brightly-lit mesa is actually a disk of sunlight shining through a hole illuminating the bottom of a underground pit.

  • Kelly Starks

    > Kelly, these lavatube caves have probably existed for about 3+ Billion years. ==

    The moons geologically dead. Even if the roof edges are as brittle as glass, nothing touched them, no wind rain, etc. So how can you be sure the backwash from a nearby landing, moisture from leaky camp. hab module.

    • Actually the moon is amazingly active. There was an article this week on one of the major space sites (sorry – no time to find the link) that describes 4 different kinds of ‘moon quakes’ one of which would register as high as 5.5 and last over 10 minutes! That’s some serious rocking and rolling…

  • Tom Billings

    Tom said:

    “Kelly, these lavatube caves have probably existed for about 3+ Billion years. ==

    Kelly said:

    The moons geologically dead. Even if the roof edges are as brittle as glass, nothing touched them, no wind rain, etc. So how can you be sure the backwash from a nearby landing, moisture from leaky camp. hab module.”

    Actually, no, the Moon shows signs of some geological activity, even internal activity. That is one of the categories of probable Lunar Transient Events that are looked for by an astronomy network of observers around the world. Even if those were not present there is a continuing widespread contribution to lunar geology. That is the continuing bombardment by micrometeorites, which have rounded off many surface features, and the continuing slow rain of larger meteoric bodies. No single impactor has much chance to *hit* a tube, but the seismic waves from the larger ones would definitely stress the basalt, if only temporarily.

    Basalt *is* a highly silicious material, but it is not glass, because it contains high amounts of Iron oxides and Aluminum oxides. Basalt *does* crack as it cools, and that is why we can get skylights in the first place. The stress breaks a particular area, localized by the cracks, and they can shorten the effective thickness of a lavatube roof if they line up vertically in one place in the tube. Without the cracks, the collapse would be all or nothing for certain, at all times, instead of just a lot of the times, which results in the presence of the Moon’s sinuous rilles.

    However, once the roof of the lavatube is emplaced, and all the places that gravity can pull down have fallen in the first few weeks, one of the things these cracks can do is damp out just the sort of shocks you seem to be concerned about, because they let the rock move just a little, without needing to break further. As to leaky hab modules wetting lavatube surfaces, that would require very large amounts of water, far more than settlers in even a large city should allow of the precious stuff to escape their recycling systems.

    • Joe

      Also, isn’t water unable to be liquid in a low/no atmospheric pressure environment? It would either sublimate to a gas before it hits the surface, or condense into a solid as hard as rock if in the shade. But, as you say, human habitations wouldn’t even let enough water escape to get to the point where steam or ice would form in the lavatubes.

      However, one question that I am wondering about, what if a habitation were to be built such that the ceiling of the lavatube itself holds in air pressure, and any skylights are covered with a material dome to hold in the air. What would be the effect of the added stress of air pressure from the inside of the lavatube?

  • Tom Billings

    Joe asked:

    “However, one question that I am wondering about, what if a habitation were to be built such that the ceiling of the lavatube itself holds in air pressure, and any skylights are covered with a material dome to hold in the air. What would be the effect of the added stress of air pressure from the inside of the lavatube?”

    That would depend on the thickness of the roof. On the Moon we expect lavatubes of hundreds of meters in diameter. Here on Earth, our ORL5 team has seen a ratio of about 1/5th the diameter for a stable roof, up to the approximately 25-30 meters max diameter here. If that holds on the Moon, then the seeming 350m diameter tube the Marius Hills Hole lets into would have a roof about 70m thick. 70m of basalt will not even notice 14.7 psi, even with a seal on the lavatube pushing continuously over the whole surface.

    Seal it with Iron from Lunar LOX production wastes, and you can avoid bringing large amounts of sealant from Earth.

  • It is a good subject – but tricky to check. Lab test subjects are two times as highly-priced to continue as the death, however they are a more suitable style nearer to our body customer happiness size. Also, is really a situation, since you need to systematically want to be an adequate amount of an appropriate way of rat.

  • David

    To answer some of the posters worries, these tunnels are battle tested in the extreme so to speak. For billions of years enormous impacts have hit them and still they do not fall. The odds are overwhelming that 1) they are extremely strong 2) the straw that broke the camel’s back is not likely barring a strike near by. Moon quakes would be small potatoes by comparison to impact strikes, especially a 5.o magnitude (at what depth? most quakes are unbelievably super deep by Earth standards, as the moon has a nearly solid body with the Earth being viscious only a few miles down and not prone to buildup of stresses.)

    Any optimal use of the cave would pressurize the enclosure. Also of purpose is to use the mass for protection against solar storms which are of periodic lethal levels topside and need many feet worth of lunar material to fully block.

    The pressurized part need be sealed by heat and miminal air pressure, which is not as difficult as sounds since rock is a great insulator and buildup could take months via IR light tube. Actually, this would be done several times since each would have condensing cracks. Surprising to the uninitiated, this proceedure is much easier than building structures topside. Not only is the overburden for radiation taken care of, so is the pressure problem.

    Remember that 14.7 pound of pressure on every square inch on Earth is around a ton per square foot. If there were no atmosphere to give steady back pressure, Earth homes would need that ton plus hefty safety factor engineered into the building codes. Moon and Space habitated structures need the same. Native moon materials are rough but cheap, Earth based strong enough but expensive. Caves are ready made except the skylights. The goof balls who for decades have spouted domes being really the best answer have a lot of engineering to do with hand wavium.

    And a pressurized cave would even have less, not more, chance of falling down, as a relatively modest back pressure would be in force (unless not thick in a few locations, then the danger would be greater in the form of outward pressure. But generally most all places would have collapsed the tube already over the milennia.)

    A cave system could need only a few feet of entrance blocking plus light tube(s), minor construction for sealing, filling with atmosphere and water to make a huge several square miles of area location stretching many miles in length and width of football fields. Truly a great advantage at little effort, would you not agree? This is why skylights have been awaited for many years and are most welcome with the new discoveries on the Moon/Mars (next Mercury, Vesta, Ceres, and ice moons?).

  • Tim

    “And a pressurized cave would even have less, not more, chance of falling down, as a relatively modest back pressure would be in force (unless not thick in a few locations, then the danger would be greater in the form of outward pressure. But generally most all places would have collapsed the tube already over the milennia.)”

    Just a guess, but not necessarily so. As long as the pressure is more than some parts of the overburden, like near the entrances for light tube and natural skylight, fractures might well be easier due to the native rock. Many a lunar rille has already collapsed over the eons, like the Hadley Rille of Apollo 15. Those that survive may well have more fractures one way than the other, due to meteor strikes. Yes, the great tendency is downward, and lunar soil is extremely compact lower than 4 inches, but the strikes just to the side of the tube that form a crater larger than 10 meters are what concern me.

    Also, if there is a leak, it could relatively quickly weaken the structure above, since the pressures are great, sort of like that which happens in a dam break in the toe or other locations. With larger chunks blasted off the supports of the outward force have a degenerative curve, in part due to something roughly similar to fracking, as the rough parts get stuck in the passageway and pressures can build up near the surface.

    Water weakening is, though, the dominant factor on Earth, just as posters mention. This is especially true on the Hawaiian islands where most of the mass is moved by solution in water rather than sediment transported by the same or aeolian transfer. This so weakens the rock that huge landslides, literally as large as a super caldera of Yellowstone, has more than once slipped away. The cliffs on the North Shore of Oahu is confirmed case of this, with huge remnants well over a hundred miles northward littering the ocean floor.

    But there is some weakening after 3 billion years from the meteor strikes.

    Until certain, there might be added safety factors until complete surveys are done. As with hydraulic dams, the chance of failure is mainly at the time of initial filling (St. Francis Dam) or soon afterwards. That being said, the 14 psi is only about the same as 16 feet water depth if remembered correctly (that or 32 feet), which is like leeves instead of actual dams. Also, with sprinkler systems and possible emergency air dumps, nearly pure 02 or nearly so is fine, and only 1.4 psi still grows a lot of crops. The Apollo 1 fire happened in 14 psi pure Oxygen, without emergency vacuum option, by the way.

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