I have a terrestrial planet that is similar to Mars in many ways, a desert-like surface, thin atmosphere, and inhospitable to large flora and fauna as we know it.

This planet has ceased tectonic activity yet still has a weak active core producing geothermal power. The planet also has a wide array of interconnected caverns, tunnels, and underground pathways that are spread across the planet but don't have any openings to the planet's surface.

The caverns were largely created through excavation by organisms (they could create tunnels through rock) that inhabited the planet before going extinct, and the holes have just been covered up over time by chance.

Is it possible that this cave system could hold an atmosphere similar in pressure to that of Earth's, for a window of 100 to 200 million years?

  • $\begingroup$ How did the network of caverns form? Why isn't it connected to the atmosphere? $\endgroup$
    – Gimelist
    Jan 12, 2019 at 4:48
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    $\begingroup$ No tectonic activity but producing geothermal power is kinda contradictory. A weak active core cannot reach up to the surface. $\endgroup$
    – L.Dutch
    Jan 12, 2019 at 4:50
  • $\begingroup$ The caverns were largely created through excavation by organisms (they could create tunnels through rock) that inhabited the planet before going extinct, and the holes have just been covered up over time by chance. @Gimelist $\endgroup$
    – Thalassan
    Jan 12, 2019 at 4:53
  • $\begingroup$ I haven't found any resources that explicitly state a planet couldn't maintain production of geothermal heat yet still have no tectonic activity. This planet is more or less a product of luck. @L.Dutch $\endgroup$
    – Thalassan
    Jan 12, 2019 at 4:55
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    $\begingroup$ @elemtilas, how do you call this a duplicate? There is a pit, here is a closed cave systems. $\endgroup$
    – L.Dutch
    Jan 12, 2019 at 5:36

4 Answers 4


Based on what you state, it looks like the cave system is somehow sealed from the outside by sedimentary rock.

Sedimentary rock is usually an aggregate of granular materials, with quite some porosity in it. It takes a metamorphic transformation (bring the rock to high pressure and temperatures for long times due to tectonic activity) to remove the porosity, and your planet, per your statement, doesn't have tectonic activity.

This leads us to the conclusion that we have only porous rocks separating a high pressure environment from a low pressure one.

It is straightforward to conclude that the pressure differential cannot be held for 200 million years.


While I agree with L.Dutch's answer it is possible that the caves could hold an atmosphere in a short to medium term (in geological terms) - so long as they had something feeding pressure into them.

If there were geothermal vents or other similar processes powered by the weak remaining core then they could be venting gasses into the tunnels. If those tunnels were deep enough then the loss of pressure out would be slow enough to sustain some level of atmospheric pressure for an indeterminate amount of time.

The time would depend on how long the input of new gas can last for and the rate of escape, both of which are basically impossible to estimate but which you can set at whatever level seems reasonable to you.

In the long term this isn't stable, the pressure will slowly drop and vents will stop producing new gasses (they also won't be very breathable gasses!) but it could be stable for a long time.

Hundreds of millions is a stretch, but a significant period of time is certainly plausible.


Building on the previous answers. I would think that venting from geo-thermal vents would fill the caverns full of carbon dioxide and/or hydrogen sulfide, possibly along with simple hydrocarbons. To provide a level of oxygen you could have the cavern surfaces covered with massive colonies of mold, fungus, and/or lichen that would use chemosynthesis to sequester carbon and release free oxygen. A byproduct of the colonial growth might include depositing layers of biological material on the surfaces of the caverns and tunnels that would act as a sealing barrier against pressure loss. The layers might resemble coral considering a method of of carbon sequestering using the calcium carbonate that would percolate down through the strata, or be more like a biological pitch with little mineral constituents.

The sulfur in the gas along with the ancient food source and/or biological leavings of the creatures that created the tunnel system could provide the ingredients to perpetuate a sealed subterranean environment for a considerable span. I don't know about millions of years.

Over the many millennia that you propose, the biological eco-system that created the environment you describe may have no doubt changed, possibly many times much like the environments on the surface.


If the caverns are deep enough? Easily.

Earth's atmosphere is only around sixty miles thick. If the extinct species that carved the tunnels liked to dig deep, and the planet is indeed tectonically dead, I see no fundamental reason why there couldn't be caverns so deep that one atmosphere's worth of... well, atmosphere could pool up at the bottom. The caverns don't even need to be sealed from the surface.

If your planet is less massive than Earth, then these caverns would need to go down quite a bit further than sixty miles. I don't know how to calculate how deep they'd need to be exactly.

You probably don't need to worry too much about the air in these caves escaping to the surface, getting blown off into space, and needing to be replenished. Mars, after all, has an atmosphere that it has managed to hold on to since its formation. Mars's atmosphere is quite thin, of course, but the atmosphere at your planet's surface doesn't need to be any thicker. I'm not sure how Mars replenishes its atmosphere, or how quickly it needs to, but your planet can simply do the same thing.


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