Say there is a planet in the range of minineptune/super earth, with a large rocky surface and hydrogen and helium in its atmosphere, which rises upwards, while oxygen remains closer to the ground. Could this result in a calm, life supporting surface with gas giant-type storms far above?
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1$\begingroup$ Where is the oxygen coming from? $\endgroup$– KilisiCommented Aug 27 at 0:44
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1$\begingroup$ I understand from the title what your intent is, but as currently written there are two distinct questions in the body of the text, which is not permitted. Suggest that you rephrase the first to "...result in a calm, life-supporting surface..." and remove the last sentence. $\endgroup$– KerrAvon2055Commented Aug 27 at 0:57
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$\begingroup$ In addition to @KerrAvon2055's issue, which I agree with, a mini-neptune is the high-end of the super earth mass scale, so which are we dealing with? Also, have you done some research? Are there any planets in our own solar system that have violent upper-atmosphere storms that leave the ground calm? $\endgroup$– JBHCommented Aug 27 at 2:08
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3 Answers
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MiniNeptune - No
The primary reason of inability for gas giants to have life on their surface is sheer pressure. At mere 12 kilometers down here on Earth the pressure (granted it's rock, not gas) is so high that the rock starts to flow like a very viscous liquid. These 12 km of rock would translate into some 200 km of gas, since its density grows IIRC exponentionally with depth, together with total mass. Since that pressure causes any structures to change, lifeforms would be unable to retain integrity for any reasonable period of time, thus will never be able to form in the first place.
Super-Earth - Maybe
If your super-Earth is big enough to hold hydrogen in its gravity well, then it would catch more atmosphere over time, increasing its mass, thus turning into a gas giant. And a gas giant would eventually stop being a planet that hosts life, at least for the reason above. But if it could only capture helium, then such a runaway process would not "run away" fast enough to cause it to turn into a gas giant. The atmospheric pressure would certainly be greater than on Earth, but not too great to cause all the light to not reach its surface, allowing climate and energy gradients to exist and potentially allow life to emerge.
PS: we do not exactly know whether Earth's atmosphere was denser than today while life started to emerge on its surface (in its waters), so it's possible that the answer is a yes, however the word "calm" would not apply for certain, as the winds' momentum would rise linearly with the atmospheric pressure, and energy with the square of it, causing proportionally harder storms than what we endure here.
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You can only have a hydrogen-helium layer with gas giant storms above a oxygenated atmosphere if vertical atmospheric mixing is effective zero. This means you have to have an atmosphere deep enough that all insolation is fully absorbed in the upper layers. It also requires that the planet have very little geological activity, large scale volcanism would otherwise induce atmospheric mixing. This has certain implications for supporting life, not least because it means that sunlight is not reaching the surface of the planet, at all. The surface being pitch black is not necessarily a barrier to life as we know it, there are Chemotrophic organisms that exist on Earth without sunlight as a direct or indirect energy source. However chemotrophy tends to be low energy and in a world with low geological activity it may not be a longterm option given the low rate of mineral turnover.
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You haven't specified if you want a realistic or more fantastical answer. My answer is in regards to a Mini-Neptune, not a Super Earth, so keep that in mind.
The realistic answer is you wouldn't have a 'surface' as you think of it. If the planet is large enough to accumulate hydrogen/helium, then there is no point within its mantle at which matter suddenly 'becomes solid'. True, oxygen is denser, and will clump further towards the core, however, we have these things called molecules which are combinations of elements that may weigh more, like water, and this may shift the balance.
Mini-Neptune is a term that is somewhat used in tandem with Gas Dwarf. However, it emphasises a smaller ice giant more so than a gas giant (which is mostly only hydrogen and helium). This means your upper atmosphere will be hydrogen/helium, quickly followed underneath by a large majority of ice or water (or ammonia, or a myriad of other chemicals, but lets stick to Earth analogues for now).
From there, your 'solid surface' would instead be more like layers of an ocean, with a helium/hydrogen surface, mixed water, followed by denser or more pressurised ices. Think like the abyssal plain or ocean trenches. Now, all of this would be pitch black, no light at all would reach past the hydrogen layer, but you still could technically have life... maybe... through chemosynthetic reactions: deep ocean monsters and tube worms and whatnot. There is the temperature to think about, all that pressure is going to cause an awful lot of heat, and your planet doesn't have a tectonic 'crust' per se to isolate volcanism, it's possible your 'ocean depths' might function more like a transition from thin water, to thick water, to watery magma, to actual magma. But again, there's nothing technically stopping you from developing a workaround, just a lot of improbability.
The fantastical answer is you can do what you want. Put a dome around it or something, there's an air pocket, or maybe the entire ecosystem is in the stomach of a large fish, it don't matter. You can make up what you want, the rule of cool reigns supreme in science fiction.
