<p>Hardly. Maybe. Probably. It depends.</p>
<p>Liquid water <em>at atmospheric pressure</em> is densest at 4°C - but that is a function of pressure and temperature.</p>
<p>The phase of a material (solid, liquid, gas, critical) depends on both, pressure and temperature - and so does the density which depends on both, linearly within one phase, discontinuous at phase boundaries. The <a href="https://ergodic.ugr.es/termo/lecciones/water1.html" rel="nofollow noreferrer">phase diagram of water</a> which describes the phase you find for any combination of temperature and pressure is surprisingly complex for water and thus one of the least well-established ones.</p>
<p>Maybe one can answer your question already with a clear 'yes', if you consider a sub-surface ocean an acceptable answer. Enceladus' interior is somewhat heated by tidal interaction with Saturn. From Cassini observations we know that under about a 30km thick ice sheet on Saturn's moon <a href="https://en.wikipedia.org/wiki/Enceladus" rel="nofollow noreferrer">Enceladus</a> we have an <a href="https://www.jpl.nasa.gov/news/nasa-space-assets-detect-ocean-inside-saturn-moon" rel="nofollow noreferrer">ocean of liquid saline water</a> - a source for the geyser activity on that moon.</p>

CC BY-SA 4.0
2021-05-30T12:56:21.300Z
Thanks for the clarification. Of course, sub-surface oceans are acceptable to me, but as you wrote yourself, the interior of Enceladus may be liquid because of tidal interaction and not pressure. Does pressure play even the slightest role here? I don't know.
AlgebraicsAnonymous
CC BY-SA 4.0
2021-05-30T12:59:56.167Z
Also, the phase diagram seems to forbid liquid water at less than -30° Celsius even at very high pressures. So the exoplanet could only be a bit outside the habitable zone. The question now becomes: How much mass is necessary to create a pressure of 10^8 Pascal?
AlgebraicsAnonymous
CC BY-SA 4.0
2021-05-30T14:05:09.403Z
Apparently, according to https://arxiv.org/ftp/arxiv/papers/1005/1005.2440.pdf there may be such a pressure at the bottom of a 10km deep ocean. Hence I consider the question answered. Thank you very much indeed!
AlgebraicsAnonymous
CC BY-SA 4.0
2021-05-31T22:59:33.690Z
No, hang on, can't the temperature itself depend on the pressure?
AlgebraicsAnonymous
CC BY-SA 4.0
2021-06-02T05:22:36.887Z
Sometimes. It depends on what (if any) thermophysical processes play a role. https://en.wikipedia.org/wiki/Equation_of_state Yet not in the way you hint here. Pressure itself never causes temperature change. You can change temperature by a pressure change in an e.g. adiabatic process in gases - which means volume also changes.
planetmaker
CC BY-SA 4.0
2021-06-03T11:01:16.553Z
I just found this one: https://pubmed.ncbi.nlm.nih.gov/30923121/
AlgebraicsAnonymous
CC BY-SA 4.0
2021-06-03T12:26:43.693Z
Note: They find this high-pressure liquid by *heating isobaric*, thus at constant pressure they heat their high-pressure ice. IMHO this confirms that there's still a lot of discoveries to be made with water and its phase diagramme and its properties.
planetmaker
CC BY-SA 4.0
2021-05-29T14:11:58.627Z
2021-05-30T20:49:35.890Z
2021-05-30T20:49:35.890Z
planetmaker
<p>Let's say there is an exoplanet orbiting its host star well outside its habitable zone. Suppose further that it has a lot of water, being perhaps comparable to earth when it comes to the volume ratio of water compared to the remainder of the planet.</p>
<p>Now water at its densest is 4° Celsius warm. My question is: Could the exoplanet's weight force the water into a liquid state by squeezing it to the temperature of lowest density?</p>
<p>My intuition would say no, because after all, temperature is nothing but molecular movement, and every kind of accelerated movement is supposed to lower the total energy of a system, which seems to be absurd.</p>
<p>Also, this seems to be an explanation of the so-called &quot;faint young sun paradox&quot;, which is well-known in geology.</p>
<p>If the answer differs from that, I would be pleased to read an explanation of how my reasoning was flawed.</p>

CC BY-SA 4.0
2021-05-29T14:55:43.570Z
This might also fit well on Physics SE.
WarpPrime
CC BY-SA 4.0
2021-05-30T11:20:58.803Z
I think this can at best be interpreted here or at Physics SE or Chemistry SE. Personally I fail to understand what is being asked and the various lines of reasoning. This said, the answer below is certainly OK as for the answer to whatever is being asked must be the phase diagram of water.
Alchimista
CC BY-SA 4.0
2021-05-30T12:57:01.203Z
@Alchimista: Perhaps you could clarify what you don't understand.
AlgebraicsAnonymous
CC BY-SA 4.0
2021-05-30T20:51:36.703Z
The close vote is **unnecessary**, as the OP has provided enough details to answer the question.
WarpPrime
CC BY-SA 4.0
2021-05-31T09:13:07.483Z
I did not vote for closing (I cannot and it wasn't my aim). Just asked for clarification, if possible.
Alchimista
CC BY-SA 4.0
2021-05-31T09:26:48.223Z
@AlgebraicsAnonymousc water at is densest is...".That depends on P. "every kind of...." totally obscure. The title is already less than clear. Still, has I said, once cleaned, the answer it is certainly in the phase diagram of water. In addition, the P would depends non only on the mass & size of the planet, but also on the eight of the water columns (though you mention water comparable to what is here on Earth).
Alchimista
<exoplanet><temperature><water>
Could a heavy exoplanet squeeze water into being liquid and 4° warm?
AlgebraicsAnonymous
# Could a heavy exoplanet squeeze water into being liquid and 4° warm?

(Asked by: username_0 on 2021-05-29)

Let's say there is an exoplanet orbiting its host star well outside its habitable zone. Suppose further that it has a lot of water, being perhaps comparable to earth when it comes to the volume ratio of water compared to the remainder of the planet.

Now water at its densest is 4° Celsius warm. My question is: Could the exoplanet's weight force the water into a liquid state by squeezing it to the temperature of lowest density?

My intuition would say no, because after all, temperature is nothing but molecular movement, and every kind of accelerated movement is supposed to lower the total energy of a system, which seems to be absurd.

Also, this seems to be an explanation of the so-called "faint young sun paradox", which is well-known in geology.

If the answer differs from that, I would be pleased to read an explanation of how my reasoning was flawed.

**Tags:** exoplanet, temperature, water

> **Comments:**
> **username_1**: This might also fit well on Physics SE.  
> **username_2**: I think this can at best be interpreted here or at Physics SE or Chemistry SE. Personally I fail to understand what is being asked and the various lines of reasoning. This said, the answer below is certainly OK as for the answer to whatever is being asked must be the phase diagram of water.  
> **username_0**: @username_2: Perhaps you could clarify what you don't understand.  
> **username_1**: The close vote is **unnecessary**, as the OP has provided enough details to answer the question.  
> **username_2**: I did not vote for closing (I cannot and it wasn't my aim). Just asked for clarification, if possible.  
> **username_2**: @AlgebraicsAnonymousc water at is densest is...".That depends on P. "every kind of...." totally obscure. The title is already less than clear. Still, has I said, once cleaned, the answer it is certainly in the phase diagram of water. In addition, the P would depends non only on the mass & size of the planet, but also on the eight of the water columns (though you mention water comparable to what is here on Earth).  

---

## 1 answers

### Answer by username_3 on 2021-05-29. Score: 2

Hardly. Maybe. Probably. It depends.

Liquid water *at atmospheric pressure* is densest at 4°C - but that is a function of pressure and temperature.

The phase of a material (solid, liquid, gas, critical) depends on both, pressure and temperature - and so does the density which depends on both, linearly within one phase, discontinuous at phase boundaries. The [phase diagram of water](https://ergodic.ugr.es/termo/lecciones/water1.html) which describes the phase you find for any combination of temperature and pressure is surprisingly complex for water and thus one of the least well-established ones.

Maybe one can answer your question already with a clear 'yes', if you consider a sub-surface ocean an acceptable answer. Enceladus' interior is somewhat heated by tidal interaction with Saturn. From Cassini observations we know that under about a 30km thick ice sheet on Saturn's moon [Enceladus](https://en.wikipedia.org/wiki/Enceladus) we have an [ocean of liquid saline water](https://www.jpl.nasa.gov/news/nasa-space-assets-detect-ocean-inside-saturn-moon) - a source for the geyser activity on that moon.

> **Comments:**
> **username_0**: Thanks for the clarification. Of course, sub-surface oceans are acceptable to me, but as you wrote yourself, the interior of Enceladus may be liquid because of tidal interaction and not pressure. Does pressure play even the slightest role here? I don't know.  
> **username_0**: Also, the phase diagram seems to forbid liquid water at less than -30° Celsius even at very high pressures. So the exoplanet could only be a bit outside the habitable zone. The question now becomes: How much mass is necessary to create a pressure of 10^8 Pascal?  
> **username_0**: Apparently, according to https://arxiv.org/ftp/arxiv/papers/1005/1005.2440.pdf there may be such a pressure at the bottom of a 10km deep ocean. Hence I consider the question answered. Thank you very much indeed!  
> **username_0**: No, hang on, can't the temperature itself depend on the pressure?  
> **username_3**: Sometimes. It depends on what (if any) thermophysical processes play a role. https://en.wikipedia.org/wiki/Equation_of_state Yet not in the way you hint here. Pressure itself never causes temperature change. You can change temperature by a pressure change in an e.g. adiabatic process in gases - which means volume also changes.  
> **username_0**: I just found this one: https://pubmed.ncbi.nlm.nih.gov/30923121/  
> **username_3**: Note: They find this high-pressure liquid by *heating isobaric*, thus at constant pressure they heat their high-pressure ice. IMHO this confirms that there's still a lot of discoveries to be made with water and its phase diagramme and its properties.  

---

