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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?

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  • $\begingroup$ How did the network of caverns form? Why isn't it connected to the atmosphere? $\endgroup$ – Gimelist Jan 12 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 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 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 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 at 5:36
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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.

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