<p>Where is &quot;on&quot; Neptune? Neptune has a deep gas atmosphere, and no solid surface.  Instead, you can ask if water would be a solid, liquid or gas in Neptune's atmosphere.  I've got below a graph that shows the temperature and pressure profile of Neptune's atmosphere.  And I've overlaid a graph showing the state of water at different temperatures and pressures.</p>
<p><a href="https://i.sstatic.net/p6nNR.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/p6nNR.png" alt="enter image description here" /></a></p>
<p>You can see that in the atmosphere, water would be solid. In the stratosphre and troposphere, pressures are between <span class="math-container">$10^{-6}$</span> bar and <span class="math-container">$10^2$</span> bar and temperatures vary from about 150 K at the top of the stratosphere, reaching a minimum of about 50 K at about 0.1 bar and then increasing deeper within the planet. However all this line is within the blue region of the graph, in which water is solid.</p>
<p>You can imagine that at the edge of space, in the thermosphere, the pressure may be low enough for water to exist as a gas, but this is in space not on Neptune.</p>
<p>Deeper within Neptune, at very high pressures, liquid water may exist, or water in exotic forms of ice, with a different crystalline structure from normal water ice (and a different freezing point)</p>
<p>Excuse the backwards writing, and the alignment of the two graphs was done by eye, so may be slightly off.  But I hope not by enough to change the qualitative answer that Neptune's atmosphere is cold and pressurised enough for water to exist as a solid.</p>
<p>It's impossible to say how quickly water would freeze as it would depend on the size, shape, and initial temperature of the water. A raindrop would freeze quickly, a swimming pool would freeze more slowly, a boiling ocean even slower.</p>
<p><a href="https://www.researchgate.net/figure/Temperature-profile-of-Neptunes-atmosphere-derived-from_fig3_253391065" rel="nofollow noreferrer">source of temp pressure profile</a></p>
<p>source of water phase diagram: wikipdia <a href="https://commons.wikimedia.org/w/index.php?curid=34865054" rel="nofollow noreferrer">Cmglee - Own work, CC BY-SA 3.0</a></p>

CC BY-SA 4.0
2023-10-22T19:39:53.913Z
You're right. In Neptune's atmosphere is a much better phraseology. Secondly, thank you very much for the response! I have a couple of follow up questions, if you don't mind? I receive conflicting answers about this, but in a nasa article, I read that there is a liquid ocean that is hot but doesn't boil due to pressure. Could you explain how this makes sense?
PhysicsNoob
CC BY-SA 4.0
2023-10-22T19:47:10.403Z
Imagine following that line down and to the right. At some point it would cross into the green "Liquid" region.  But at such high pressures the distinction between "liquid" and "gas" disappear, and you have water as a "supercritical fluid"  These can't boil because the gas and liquid states merge at very high pressures. Above 225 bar, there is no such thing as "water vapour".
James K
CC BY-SA 4.0
2023-10-22T19:55:35.713Z
So in the lowest and second lowest regions of Neptunes atmosphere, water is a super critical liquid and gaseous. But any higher than that, the water would be solid? Forgive my extreme ignorance about this subject, but how could water be gaseous anywhere on Neptune given the freezing temperatures there? Also, how is the super critical water even considered hot to begin with?
PhysicsNoob
CC BY-SA 4.0
2023-10-22T19:59:56.073Z
In the atmosphere water would be solid. Deeper within Neptune pressure dominates and water may be a fluid (not a gas) It is hot in the interior of planets, just like on Earth, the interior is hot rocks, and molten metal.
James K
CC BY-SA 4.0
2023-10-22T20:02:48.513Z
understood. I appreciate the replies! I have one last couple of questions. First, how quickly would water freeze in Neptune's atmosphere? Secondly, how would you explain to somebody that the pressures of Neptune wouldn't cause water to BOIL (as somebody claimed to me today)?
PhysicsNoob
CC BY-SA 4.0
2023-10-22T20:08:13.337Z
How quickly It would depend on the size and shape of the mass of water. the heat needs to be transferred out of the water so a raindrop would freeze quickly and a swimming pool would freeze more slowly.  And 2. I would show them this graph.
James K
CC BY-SA 4.0
2023-10-23T16:11:07.953Z
Kudos to make a quick overlay of the pressure profile with the phase diagramme of water!
planetmaker
CC BY-SA 4.0
2023-10-22T19:35:43.690Z
2023-10-22T20:10:12.497Z
2023-10-22T20:10:12.497Z
James K
<p>Someone told me that water would boil on Neptune because of the pressure. But this seems suspect to me. Given a temperature of -353 Fahrenheit, I'd expect water to freeze pretty much instantly. So would water freeze on Neptune, or boil? How long would it take for either to occur?</p>

CC BY-SA 4.0
2023-10-22T22:05:56.553Z
It's not clear why your "someone" believes the high pressure would cause boiling. Usually, low pressure promotes evaporation / boiling, and high pressure promotes condensation and freezing. (OTOH, water is a bit unusual in that it expands when it freezes, so pressure *can* cause ice to melt).
PM 2Ring
CC BY-SA 4.0
2023-10-22T22:13:03.577Z
The deep interiors of the planets are hot, and at high pressure, but the heat isn't caused by the pressure, per se. The heat was released during the formation of the planet, by gravitational potential energy being converted to kinetic energy.
PM 2Ring
CC BY-SA 4.0
2023-10-23T02:52:26.387Z
I don't know where this boiling comes from. At such temperature, you would expect matter to freeze.
Nilay Ghosh
<astrophysics>
Would water freeze on Neptune?
PhysicsNoob
# Would water freeze on Neptune?

(Asked by: username_0 on 2023-10-22)

Someone told me that water would boil on Neptune because of the pressure. But this seems suspect to me. Given a temperature of -353 Fahrenheit, I'd expect water to freeze pretty much instantly. So would water freeze on Neptune, or boil? How long would it take for either to occur?

**Tags:** astrophysics

> **Comments:**
> **username_1**: It's not clear why your "someone" believes the high pressure would cause boiling. Usually, low pressure promotes evaporation / boiling, and high pressure promotes condensation and freezing. (OTOH, water is a bit unusual in that it expands when it freezes, so pressure *can* cause ice to melt).  
> **username_1**: The deep interiors of the planets are hot, and at high pressure, but the heat isn't caused by the pressure, per se. The heat was released during the formation of the planet, by gravitational potential energy being converted to kinetic energy.  
> **username_2**: I don't know where this boiling comes from. At such temperature, you would expect matter to freeze.  

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## 1 answers

### Answer by username_3 on 2023-10-22. Score: 3

Where is "on" Neptune? Neptune has a deep gas atmosphere, and no solid surface. Instead, you can ask if water would be a solid, liquid or gas in Neptune's atmosphere. I've got below a graph that shows the temperature and pressure profile of Neptune's atmosphere. And I've overlaid a graph showing the state of water at different temperatures and pressures.

[![enter image description here](https://i.sstatic.net/p6nNR.png)](https://i.sstatic.net/p6nNR.png)

You can see that in the atmosphere, water would be solid. In the stratosphre and troposphere, pressures are between $10^{-6}$ bar and $10^2$ bar and temperatures vary from about 150 K at the top of the stratosphere, reaching a minimum of about 50 K at about 0.1 bar and then increasing deeper within the planet. However all this line is within the blue region of the graph, in which water is solid.

You can imagine that at the edge of space, in the thermosphere, the pressure may be low enough for water to exist as a gas, but this is in space not on Neptune.

Deeper within Neptune, at very high pressures, liquid water may exist, or water in exotic forms of ice, with a different crystalline structure from normal water ice (and a different freezing point)

Excuse the backwards writing, and the alignment of the two graphs was done by eye, so may be slightly off. But I hope not by enough to change the qualitative answer that Neptune's atmosphere is cold and pressurised enough for water to exist as a solid.

It's impossible to say how quickly water would freeze as it would depend on the size, shape, and initial temperature of the water. A raindrop would freeze quickly, a swimming pool would freeze more slowly, a boiling ocean even slower.

[source of temp pressure profile](https://www.researchgate.net/figure/Temperature-profile-of-Neptunes-atmosphere-derived-from_fig3_253391065)

source of water phase diagram: wikipdia [Cmglee - Own work, CC BY-SA 3.0](https://commons.wikimedia.org/w/index.php?curid=34865054)

> **Comments:**
> **username_0**: You're right. In Neptune's atmosphere is a much better phraseology. Secondly, thank you very much for the response! I have a couple of follow up questions, if you don't mind? I receive conflicting answers about this, but in a nasa article, I read that there is a liquid ocean that is hot but doesn't boil due to pressure. Could you explain how this makes sense?  
> **username_3**: Imagine following that line down and to the right. At some point it would cross into the green "Liquid" region.  But at such high pressures the distinction between "liquid" and "gas" disappear, and you have water as a "supercritical fluid"  These can't boil because the gas and liquid states merge at very high pressures. Above 225 bar, there is no such thing as "water vapour".  
> **username_0**: So in the lowest and second lowest regions of Neptunes atmosphere, water is a super critical liquid and gaseous. But any higher than that, the water would be solid? Forgive my extreme ignorance about this subject, but how could water be gaseous anywhere on Neptune given the freezing temperatures there? Also, how is the super critical water even considered hot to begin with?  
> **username_3**: In the atmosphere water would be solid. Deeper within Neptune pressure dominates and water may be a fluid (not a gas) It is hot in the interior of planets, just like on Earth, the interior is hot rocks, and molten metal.  
> **username_0**: understood. I appreciate the replies! I have one last couple of questions. First, how quickly would water freeze in Neptune's atmosphere? Secondly, how would you explain to somebody that the pressures of Neptune wouldn't cause water to BOIL (as somebody claimed to me today)?  
> **username_3**: How quickly It would depend on the size and shape of the mass of water. the heat needs to be transferred out of the water so a raindrop would freeze quickly and a swimming pool would freeze more slowly.  And 2. I would show them this graph.  
> **username_4**: Kudos to make a quick overlay of the pressure profile with the phase diagramme of water!  

---

