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Basically a planet with a global network of caves filled with gas with occasional openings to the airless surface.

The openings would look like underwater brine pools, ranging in diameter from a few millimeters to dozens of kilometers. The deepest the caves go is 12 kilometers deep (from surface)

Goo Lagoon IRL

Solar wind strips any gas that exits the openings away from the surface, leaving only a barely detectable thin atmosphere above ground. The atmosphere underground is mostly methane, CO2, and water vapor with some O2 and other gasses. The cloudy surface is caused by organic compounds suspended in the air reacting with light and scattering it, a similar thing happens on Saturn's moon Titan.

The planet may harbor simple microbial and colonial life subsisting on what light comes into the cave system from openings, chemical reactions and eating other organisms.

So, is this a feasible concept for a planet?

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  • $\begingroup$ Slightly off-topic, but could such caves support themselves at a depths of 12 km? $\endgroup$ Jan 6, 2023 at 17:52
  • $\begingroup$ It can be if you want it to be, since it's not such a far-fetched idea to turn off readers. $\endgroup$
    – RonJohn
    Jan 6, 2023 at 18:30
  • $\begingroup$ What you're describing seems similar to the setting from Cat-Women of the Moon (1953). My read of the answers is that this isn't doable, but that didn't stop an earlier author from doing it. You can, too! $\endgroup$
    – Tom
    Jan 7, 2023 at 3:29

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Think of household plumbing, and the sink/toilette trap.

Basically, it does what you want, it keeps the sewer odors from coming back into your house. That is exactly what you want, but on the other side of the trap. You want the atmosphere to stay inside the cave/sewer from escaping into the outer world. So if you manipulate your sinkholes into traps, with the cave curving back up above the water level in the sinkhole (like a toilette) then yes what you want is possible.

Whatever goes between the cave system and the outside world has to navigate through this fluid barrier in the trap.

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  • $\begingroup$ I was going to add a comment along the lines of "no, not without some sort of air locks", you've provided some sort of air locks .. hard to see how this can be entirely natural though? .. it will require some sort of maintenance .. 🤔 instinctive animals could suffice (like beavers and their damns or termites etc) if that's what the OP wants to go with, or even some sort of plant perhaps .. you will need these throughout all the underworld tunnel network as cutoffs rather than just the entrances or one breach or failure destroys it all $\endgroup$
    – Pelinore
    Jan 5, 2023 at 23:38
  • $\begingroup$ I might suggest that the geology of the planet crust could be very folded, rippled, and convoluted, but devoid of plate tectonics which create rifts up to the surface. Natural erosion following serpentine veins of soft material between harder rock. This would produce natural traps. The spelunkers that were trapped in the cave in Asia, where the water rose and trapped them deep inside, would be an example of how it might happen. On a global scale, if the geology of this region were universal on the land, it becomes plausible. Add a rising water level overall. Melting ice cover? $\endgroup$ Jan 6, 2023 at 0:12
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    $\begingroup$ @ Pelinore A close reading of the question indicates the OP does not require ALL caves to contain an atmosphere, so if the atmosphere seeps in from the surrounding rock, it is plausible that these cave systems could come and go. $\endgroup$ Jan 6, 2023 at 0:33
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    $\begingroup$ Yeah but the way I see it over any evolutionary time-frame none of them will have any atmosphere if they don't have something maintaining them, without some maintenance from the life occupying them they'll all be dead airless space in relatively short order as these things go .. all I'm saying is if you can throw in the subterranean bromeliads of Mars and their symbiotic communities of tadpoles, mosquito larva and frogs you'll have boosted my perceived plausibility quotient 😁 .. 🤔 a little bit 🤗 $\endgroup$
    – Pelinore
    Jan 6, 2023 at 0:39
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    $\begingroup$ @Pelinore: I agree that some regeneration is necessary. Even without dramatic events, gases dissolve in liquids, and gases from liquids exposed to the void will leak into that void. This means the airlocks are not actually airtight here, and thus to maintain pressure in the caves, new gases need to be formed somehow. $\endgroup$ Jan 6, 2023 at 8:55
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Unlike the surface of an ocean or the boundary of a brine pool, the planetary boundary between an atmosphere and the vacuum above it is a very, very gradual transition.

For Earth, the highest breathable altitude is 6 km; but the "Kármán line" (the definition of where "outer space" starts) is at 100km.

So, if by "airless" you mean the same air pressure as Earth's Kármán line, you're going to have dozens of vertical km of unbreathable cave systems between the breathable cave and the surface.

If you can accept a more flexible definition of "airless", then you can reduce this distance.

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You just described Mercury

Mercury has a thin oxygen-sodium-hydrogen atmosphere so sparse it is almost a vacuum. Just as you describe, solar wind strips away gases on the surface, meaning Mercury is shrinking fast.

Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids.

Unlike the planet you described, Mercury has virtually no atmosphere to scatter the light. Because of this, temperatures range in the extreme, from boiling hot by day to frigid at night. The sky is black, possibly because Mercury is smeared with carbon. (Just a thought: if the planet is "airless" and has "no atmosphere," as you say, how do you justify your scattering light? You cite Titan as an example, but Titan has a rich nitrogen-methane atmosphere which react to produce organic molecules.)

Despite having no atmosphere, Mercury may have underground lava tubes which could serve as a future human settlement

Research indicates that Mercury possesses hollows, holes on its surfaces that indicate potential lava tubes underground. These tunnels, formed by molten lava flowing from a volcano, could be emptied out to form a network of caves. The concept of an underground cave settlement on Mercury is the topic of many headlines.

Building Subterranean Cities on Mercury "Not So Crazy," Scientist Says

NASA has toyed with the idea of taking advantage of frozen tunnels on the moon as a prime location to build a moonbase. Discovery's Ray Villard explains: the lava tubes would allow for ant farm-like colonies of humans living underground... Recent satellite data reveals that Mercury too might have some of those lava tubes

How do we colonize Mercury?

Colonies built inside stable lava tubes would be naturally shielded to cosmic and solar radiation, extremes in temperature, and could be pressurized to create breathable atmospheres [italics mine]. In addition, at this depth, Mercury experiences far less in the way of temperature variations and would be warm enough to be habitable.

Although these articles go more in depth, the general consensus? Mercury has no atmosphere, but has water sources and potentially lava tubes, which could be used to create a breathable atmosphere underground. The only part of your planet I'd cast doubt on is its organic-compound, light-scattering "airless" atmosphere.

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  • $\begingroup$ The light scattering happens at the boundary layer between the Surface "atmosphere" and the cave's atmosphere $\endgroup$ Jan 6, 2023 at 13:18
  • $\begingroup$ @redfrogcrab that could work if there is enough air escaping the caves’ atmosphere. $\endgroup$ Jan 6, 2023 at 16:15
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Ice caves?

If you take a 'water world' for example a moon like Europa and postulate a thick ice crust many kilometers deep covering its surface its possible that a combination of gravitational stresses and internal thermal dynamics could cause fissures, cracks and caves to form in the ice crust. As heated water from the depths of the planet rises and it could be forced through cracks in the bottom of the ice layer and would melt more ice in the process forming ice domes and channel systems. If/when the flow of hot water and gases ceases or shifted location the underside breaches would re-freeze and could well leave hollows in the crust filled with water and any gasses mixed in with that water.

Add in gravitational stresses caused by the moons primary and you would perhaps get 'ice quakes' and water geysers that would cause more fractures letting liquid water under pressure escape both upwards onto the surface and downwards back into the world ocean. Once vented empty caverns and channels would be left behind. After that how long any one system of caves would last depends on how dynamic the whole process of 'ice tectonics' was. A highly dynamic system might see constant turnover and only short lived cave systems. A slower, less frequent process (perhaps even seasonal?) might see some systems last much longer.

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