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Would you not be able to float on Neptune without being crushed?

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dblake:
I got into an argument with my brother and was told to ask an expert. I am only a BS biochemistry and am still learning as well.
I looked up the surface of the planet. There was some dispute, but I arrived at something less then ~ -200 degrees Celsius.
I understood that the vacuum of space creates a gradient of pressures down to the liquid layer.
With this understanding. For reference, at pressure = ~1 bar, hydrogen would liquify at night if the temperature dips below ~ 250 Celsius. With this knowledge, I suggested that water would liquify under the ~11 m/s at a temperature and pressure in the ball park of Earth. This means that rather then being crushed by the atmosphere, you would float on a cold cold liquid of various gasses at a lower pressure then earth rather then sink into a body crushing atmosphere simply because of how cold the surface temperature is. What do the experts have to say about this?

Borek:
This is so chaotic I have serious problems following what you wrote.

No idea what you mean by


--- Quote from: dblake on December 20, 2020, 09:22:12 PM ---With this knowledge, I suggested that water would liquify under the ~11 m/s at a temperature and pressure in the ball park of Earth
--- End quote ---

Why 11 m/s? And how is water related to the situation at -200°C?

Whether you are crushed or not doesn't depend on whether you are in the gas or in the liquid, but on the pressure.

When it comes to floating - it all depends on the density of the liquid. Somehow I doubt liquefied Neptune atmosphere would be dense enough to support you.

dblake:
Sorry for the typo and lack of clarity! That is 11m/(s^2) for Neptune's gravity approximation. This value was just to point out that the pressure difference due to gravity could not be much different then 9.80665 m/(s^2) or earths gravity. Things tend to be denser at higher gravity levels. Example is Jupiter with a much different gravity level changing variables a bit too much... For Neptune, I think mainly temperature does matter because it determines at what pressure the atmosphere turns into an liquid. For example, why would there be any gas vapor in the atmosphere at -200 Celsius if the pressure was at 1 bar? As you descend from the vacuum of space, the pressure increases and increases until you reach 1 bar. I am saying on the way down into the increasing pressure of the atmosphere, gasses would liquify at far smaller atmospheric pressure then Earth due to the low surface temperature. Before you descend into 1 bar atmospheric pressure, you would land in an ocean. As long as we are at 1 bar, we should remain less dense the the liquid form of any gas that is cooled into liquid form assuming the gasses we breath and our cells are still relatively warm. Perhaps, if the gasses in the lung are also at -200 Celsius thought, we would sink.

Borek:

--- Quote from: dblake on December 21, 2020, 05:00:36 PM ---Oh no typo! That is 11m/(s^2) for Neptune's gravity approximation. This value was just to point out that the pressure difference due to gravity could not be much different then 9.80665 m/(s^2) or earths gravity. Things tend to be denser at higher gravity levels.
--- End quote ---

Do they? Atmospheric pressure at the Earth surface is around 1 bar, at the bottom of the Marianas trench around 11 kbar, despite the g being (almost) the same. So there is definitely something wrong with this line of thinking.


--- Quote ---Example is Jupiter with a much different gravity level changing variables a bit too much...
--- End quote ---

There is definitely a layer in the Jupiters atmosphere where the pressure is identical to that on the Earth surface.


--- Quote ---For Neptune, I think mainly temperature does matter because it determines at what pressure the atmosphere turns into an liquid. For example, why would there be any gas vapor in the atmosphere at -200 Celsius if the pressure was at 1 bar?
--- End quote ---

Both P and T matter. Besides, there is always a gas in the equilibrium with the other phase, be it liquid or solid. That is, unless we are above critical temp, but this is at much higher temperatures than the ones we are talking about here.


--- Quote ---As you descend from the vacuum of space, the pressure increases and increases until you reach 1 bar. I am saying on the way down into the increasing pressure of the atmosphere, gasses would liquify at far smaller atmospheric pressure then Earth due to the low surface temperature. Before you descend into 1 bar atmospheric pressure, you would land in an ocean.
--- End quote ---

That's more or less correct, but just because there is a liquid doesn't mean you will float in it. Liquid water has a relatively high density compared with gases typical in the atmospheres of the gas giants.

dblake:
Ah yes. So if we assume that a person has a space suit with warm gases, or a boat, he will float on top the colder liquid surface at a pressure of less then 1 bar. However, without a space suit, the gas in the lungs of the astronaut would liquify and he, being more densely composed, would sink frozen to the layer where he would find a buoyancy equilibrium and be crushed by the pressure.

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