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I am making an exoplanet for a game and was wondering whether a planet without an atmosphere (or at least with a very weak atmosphere) could support caves that do have an atmosphere, or whether the air would escape through all the cracks immediately.

The caves are connected to the surface with some small holes. They also have liquid water and black smokers.

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    $\begingroup$ @JanDoggen But it is ontopic also here, and it was posted here. $\endgroup$ – peterh - Reinstate Monica Dec 12 '17 at 9:38
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    $\begingroup$ It's an interesting question, I wouldn't mind seeing it answered as there's likely some science. $\endgroup$ – GdD Dec 12 '17 at 9:39
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    $\begingroup$ Nothing wrong with a well formulated question about Planetary Science on this site if this is where the OP chooses to ask it! Somewhat related on this site ; Have there been any determinations of the water pressure in Europa's ocean just below the ice? and also in Astronomy SE Enceladus; why use the words “geysers”, “jets”, and “plumes” interchangeably? and also this answer; astronomy.stackexchange.com/a/18525/7982 $\endgroup$ – uhoh Dec 12 '17 at 10:22
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    $\begingroup$ OK well it looks like your question is going to be closed none-the-less. If you would like to learn about the physics of sub-surface liquid water, I'd recommend you edit this question or just ask a new one, but before you do, do some reading on your own first, and then describe what you have learned so far and what specific questions you need help with. Good luck! $\endgroup$ – uhoh Dec 12 '17 at 13:10
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    $\begingroup$ I have nominated this for reopening because it is about atmospheric dynamics on another world - planetary science, which is on topic. $\endgroup$ – kim holder Dec 14 '17 at 15:56
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Yes, in fact it can happen. It happens on Earth to some extent. The term is a "Gas Pocket", and is most commonly associated with oil and natural gas deposits. There are other times of gasses that are sometimes stored as well under the Earth.

If there is a connection with the outside world in the form of a small hole, then the likelihood of there still being any gas in it is much lower. The only way this would work is if the gas was very heavy. Again, this happens to some extend on Earth, most notably in Radon being concentrated in some basements. This likely wouldn't lead to a thick atmosphere by any means, but could produce something.

The only way that it could sustain long term an atmosphere is if it was completely sealed up, or if there was a source generating the atmosphere on a regular basis. There are a few things that could cause this to happen. It could come from outgassing due to heat, from some kind of biological process, or possibly from some subterranean vents, which is really just an extension of the outgassing problem. It could also be caused by some kind of chemical reaction.

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    $\begingroup$ The "source generating" is not far-fetched at all. Many geological processes involve outgassing. Oil deposits constantly outgas lots of methane. Plenty of mineral waters are naturally carbonated and exposed to pressure drop (e.g. entering a cavern system) readily release CO2. Magma contains many dissolved gases, that also escape when temperature drops below their solubility levels. Finding actual breathable atmosphere like N2+O2 would be more tricky but not impossible. $\endgroup$ – SF. Dec 12 '17 at 15:42
  • $\begingroup$ But the deposits of natural oil and gas would not exist on an Earth with no atmosphere. Just like coal deposits. All these deposits have biological origin. Only deposits of pure helium or radon have no biological origin. $\endgroup$ – Uwe Dec 12 '17 at 18:30
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    $\begingroup$ The question states there are small holes, so gas pockets are not an answer. Can you add some science to explain why "heavy" gas makes it work if there are holes? If there were holes, wouldn't you need to propose a source to replenish the gas in order to make your answer more than unsupported speculation? The science is already available at the links I've left under the question. This is a very low quality stackexchange answer, and is indistinguishable from an opinion. It's just a series of unsupported statements, some of which would in fact be difficult to support with science $\endgroup$ – uhoh Dec 12 '17 at 18:46
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    $\begingroup$ @Uwe: don't take continued presence of atmosphere for granted. Deposits could have been created while the planet still had an atmosphere... $\endgroup$ – SF. Dec 13 '17 at 4:01
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    $\begingroup$ @uhoh It's only a thought exercise, but it seems to me that molecules that are heavy enough will linger long enough on a planet with enough gravity and cold to count as a significant atmosphere. If the paths required to escape are long, and the area over which escape can occur is a small fraction of a surface, the odds of a molecule completing that journey would be small. The question is the formula to calculate the linger time and density of an atmosphere made of x, at y temperature, in z gravity field, with, uh, k fraction of area for escape, and escape paths of d distance. $\endgroup$ – kim holder Dec 14 '17 at 16:15

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