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Could a moon or planet with otherwise no atmosphere maintain gas pockets in sufficiently deep craters, and could it be habitable by humans without pressure or space suits?

I found this similar question Would oxygen pool in sink holes if the upper atmosphere was helium, hydrogen and methane? but it focusses on a planet with a full atmosphere, I want to know if it would be realistic for a planet without enough gas to maintain a breathable atmosphere beyond depressions to exist.

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  • $\begingroup$ Welcome to Worldbuilding, Professor_Hoover! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon - Reinstate Monica Jul 30 '18 at 0:08
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    $\begingroup$ Consider that Earth has a breathable atmosphere only in depressions (unless you're a Sherpa, of course :-)). It's just that the depressions cover most of the surface. $\endgroup$ – jamesqf Jul 30 '18 at 17:34
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    $\begingroup$ Perhaps of interest - there are worlds in which all the breathable air exists inside of canyons in the C.S. Lewis's "Out of the Silent Planet" and a few of Larry Niven's "Known Space" books (the relevant planet in these was called Canyon). $\endgroup$ – Qami Jul 30 '18 at 19:25
  • $\begingroup$ @Qami beat me to it. Canyon (at the time the Kzin planet Warhead) was hit by a high-power disintegrator cannon, tearing a hole into the crust of the planet about the size of the Baja peninsula. The atmosphere rushed into the new trench, becoming dense enough to become breathable, even forming an sea at the bottom. The sides of the canyon became a human city, after they took over the planet. $\endgroup$ – VBartilucci Jul 30 '18 at 19:50
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Yes, it could (or at least reasonably enough for a story).

For life to develop or be habitable by colonists, the gasses would have to be continually be replenished from some source because it will eventually dissipate for the same reason that our atmosphere continuously leaks into space.

https://en.wikipedia.org/wiki/Atmospheric_escape

One classical thermal escape mechanism is Jeans escape.[2] In a quantity of gas, the average velocity of any one molecule is measured by the gas's temperature, but the velocities of individual molecules change as they collide with one another, gaining and losing kinetic energy. The variation in kinetic energy among the molecules is described by the Maxwell distribution. The kinetic energy and mass of a molecule determine its velocity by $E_{kin}=$$1 \over 2 $$\cdot mv^2$

Individual molecules in the high tail of the distribution may reach escape velocity, at a level in the atmosphere where the mean free path is comparable to the scale height, and leave the atmosphere.

The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature, and the less likely it is that any of them reach escape velocity.

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    $\begingroup$ If you actually read that article you linked, you will find the paragraph saying that $O_2$ is too heavy to be lost in that process (on an Earth sized and weighed planet, I assume). $\endgroup$ – Nobody Jul 30 '18 at 8:34
  • $\begingroup$ @Nobody I was thinking more of moons. $\endgroup$ – RonJohn Jul 30 '18 at 8:39
  • $\begingroup$ Then, sure, you are right in that case (I assume). $\endgroup$ – Nobody Jul 30 '18 at 8:42
  • $\begingroup$ Even without leaking to the space, such as being inside a cave or a dome, the oxygen will react with nearly everything to form compounds, either by combustion (Hydrogen, Carbon) or by oxidation (pretty much everything). Free oxygen doesn't last. $\endgroup$ – Rekesoft Jul 30 '18 at 9:40
  • $\begingroup$ @Rekesoft Not if you just assume the oxygen has been there for a long time, then all those reactions would have already happened in the past. $\endgroup$ – Nobody Jul 30 '18 at 10:50
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Kind of, if it doesn't need to be a crater

Here is a very long article talking about helium production: https://www.chemistryworld.com/news/scientists-unearth-one-of-worlds-largest-helium-gas-deposits/1010122.article

The interesting tidbit: after the helium is produced (by radioactive decay; beta radiation is basically a helium nucleus expelled at high speed), in the right spots it gets trapped because it can't penetrate the rock layers above it. Whereas when helium gets into the atmosphere, it's so light it just drifts up through the atmosphere and leaks into outer space.

If your settlement does not need to have an open roof, then this will work just fine. Craters typically have denser material from whatever impact created them. It could easily trap enough gas to hold a pocket of livable atmosphere.

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If you look at Mars, you will find that the atmosphere in the deeper craters is different to that on the high surface. There's no need to ask if it could exist - because it does.

https://en.wikipedia.org/wiki/Atmosphere_of_Mars

"It ranges from a low of 30 pascals (0.0044 psi; 0.30 mbar) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi; 11.55 mbar) in the depths of Hellas Planitia"

The only question left to ask is "could this atmosphere sustain life" - and while the answer for Mars is no; there's no reason why, if the chemical composition of it was different, it wouldn't.

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  • $\begingroup$ In the depths, that is ~1% of Earth's atmospheric pressure, correct? $\endgroup$ – Yakk Jul 30 '18 at 14:27
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    $\begingroup$ @Yakk correct ... just dig a deeper hole; and add some more air and you can get to the point where the "surface" is lower pressure than Everest, and at the bottom of the hole is the same pressure as Earth - just like earth if you consider the top of Everest to be at height "0ft" - (everything else is just a hole - Mars just has more obvious "holes" than we do.) $\endgroup$ – UKMonkey Jul 30 '18 at 16:07
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Yes, it is possible. Consider that at the bottom of a valley one is closer to the center of mass of the planet, thus gravity can be relatively stronger, and this could reflect on the local atmospheric pressure.

gravity vs distance from center

On Earth we don't have valleys deep enough to experience dramatic difference, but we have such feature on Mars: Valles Marineris.

Up to 7 km deep, the pressure at its bottom is about 0.168 psi, while the average atmospheric pressure on Mars is 0.087 psi. About double, as you see.

Still not high enough to take a walk in T-shirt, but if the atmosphere on Mars would be more dense, it would be first spot to achieve habitable conditions.

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    $\begingroup$ Closer to the center of mass means stronger gravity only outside of the celestial body. Below average surface level shell theorem kicks in, and the deeper you go, the weaker gravity is. Thus, this answer is built on assumption that's iffy at best, and simply false if craters are not the most common feature. $\endgroup$ – Mołot Jul 30 '18 at 8:14
  • $\begingroup$ On Earth the lack of valleys isn't the problem, according to your graph. For some reason gravity has its peak 2500km below the surface (surprises me that Earth's density curve is that irregular). $\endgroup$ – Nobody Jul 30 '18 at 8:23
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    $\begingroup$ @Mołot Shell theorem assumes uniform density. The graph in the answer is real. $\endgroup$ – Nobody Jul 30 '18 at 10:47
  • $\begingroup$ @Mołot, indeed that chart is based on actual measurement, not on theoretical considerations. $\endgroup$ – L.Dutch - Reinstate Monica Jul 30 '18 at 10:52
  • $\begingroup$ Dutch, this answer also works for the linked question. Which I think preceded your answer here by some months. Why don't you repost this answer there? worldbuilding.stackexchange.com/questions/104982/… $\endgroup$ – Willk Nov 7 '18 at 16:47
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It is absolutely possible for a body to have breathable atmospheres in some places but not others. Earth already has that actually. Look at Mount Everest, you cannot survive at the peak for an extended period of time without supplemental oxygen. Now imagine if Earth's atmosphere was 1/10th as dense as it really is. You would then have that same low pressure 1/10th of the way up Everest. This would mean places as low as Denver, Colorado would not have enough air to breathe. Push it further, and you'll get isolated low areas with breathable atmospheres with barren wastelands between them. Places like Death Valley, New Mexico, or the Great Rift Valley in Africa would be breathable.

One problem is that if the air really is so thin the only breathable areas are deep in valleys or craters is that it will slowly bleed off into space. Every atmosphere, even our own, loses atoms to space. As the sun heats the atmosphere, some atoms gain enough energy to reach escape velocity. Some of these are stopped by running into other atoms, but some escape. With so much less atmosphere above these craters, this proportion will probably be a lot higher. The craters will not have a breathable atmosphere for long, at least not on geologic timescales.

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Factor in life. A natural evolved or genetically engineered amoeba like entity (ALE) which has adapted to living on the edge of a pocket of gas in colonies. The ALE secretes an airtight liquid which hardens into an airtight shell at the edge of the colony. As the colony expands, it expands the shell, sealing a larger and larger pocket. Inside or under this pocket more complex life prospers.

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Yes and no.

As the question is asked in the title the answer is yes--you can perfectly well have a body whose atmosphere is only dense enough at the lowest altitudes.

As the question is asked in the body, however, the answer is no. You simply need too much distance between the airless surface and the habitable pocket--more distance than the strength of the rocks of which your planet is made. Any pocket deep enough to hold a breathable atmosphere collapses.

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