<p>I'll start with a typical phase diagram, which appears below.</p>
<p><a href="https://i.sstatic.net/2fkrWtGM.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2fkrWtGM.png" alt="Typical phase diagram" /></a></p>
<p>Note that there are three solid curves on the graph:</p>
<ul>
<li>A red curve that starts at the origin and ends at the triple point. This is the sublimation curve.</li>
<li>A green curve that starts at the triple point. This is the melting curve.</li>
<li>A blue curve that also starts at the triple points but ends at the critical point. Theis is the evaporation curve.</li>
<li>As a side note, water is an atypical substance. The dashed green curve that slopes to the left with increasing pressure rather than curving to the right is the melting curve for water.</li>
</ul>
<p>Note that a substance does not have a liquid phase at pressures below the triple point pressure. Per the <a href="https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&amp;Mask=4" rel="nofollow noreferrer">NIST Chemistry WebBook</a>, the triple point for nitrogen (<span class="math-container">$\text{N}_2$</span>) is a temperature of 63.14K (-210.0 C) and a pressure of 0.1253 bar (or 12530 pascals).</p>
<p>Conditions on Pluto are very cold and a very thin atmosphere. It's the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting. Note that estimates of the temperature vary. (<a href="https://www.nationalgeographic.com/science/article/dwarf-planets-pluto-ceres" rel="nofollow noreferrer">National Geographic</a> interesting claims an average temperature just below the triple point temeperature.) That's irrelevant because nitrogen exists as a solid or as a gas at pressures very far below the triple point pressure. It cannot melt at such low pressures. Instead, it transfers directly from solid to gas via sublimation, and from gas to solid via deposition.</p>

CC BY-SA 4.0
2024-07-28T13:53:08.713Z
I always get confused with phase diagrams. If the triple point is at 63 K and the temperature of Pluto is 40 K , why exactly is it "the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting" and not the temperature? I don't think the solid-liquid boundary for nitrogen goes below 60 K at any pressure at least below 100 MPa - it looks pretty vertical, then curves to the right. ([random example](https://www.researchgate.net/figure/The-temperature-pressure-phase-diagram-for-nitrogen_fig1_315888614))
uhoh
CC BY-SA 4.0
2024-07-28T14:52:17.490Z
@uhoh Some comments regarding your comment. (1) I  could not find a good reference for the temperature of Pluto at perihelion. They seem to be all over the map. Some are at 65 K, others closer to your value of 40 K. (2) The temperature is irrelevant so long as it is lowish and so long as the pressure is extremely low (which it is). Nitrogen will either be a solid or a gas at a pressure of 1 pascal.
David Hammen
CC BY-SA 4.0
2024-07-28T15:45:24.457Z
OK so "It's the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting" is true *as long as it's below the melting point*. Sounds like a pretty important caveat.
uhoh
CC BY-SA 4.0
2024-07-28T18:58:13.770Z
@uhoh If the pressure is below the triple point pressure, there is no melting point.
John Doty
CC BY-SA 4.0
2024-07-29T00:00:48.247Z
@JohnDoty I think I see, my problem is more to do with the lopsided word "melting point" which applies to what happens along a horizontal line in the plot (constant pressure variation in temperature) There are cases where a vertical line (constant temperature, varying pressure) could also result in a "melting point" my brain wants to treat the two mathematical scenarios the same - below **some** temperature there is no melting point. However that temperature might not necessarily e be the triple point because the solid-liquid boundary can have a reentrant shape.
uhoh
CC BY-SA 4.0
2024-07-29T00:15:22.003Z
@JohnDoty so as long as I force myself to think the way normal people do ("melting point" is a point along only a horizontal (constant pressure) line) then I can put this to rest in my head. I see that for example in the [phase diagram for water](https://ergodic.ugr.es/termo/lecciones/water1.html) even for our familiar [ice 1h](https://en.wikipedia.org/wiki/Phases_of_ice#Known_phases) somewhere between 100 and 1000 atmospheres the "melting point" curves to the left. There is no melting point below say 240 or 250 K, but that point is *below* the triple point.
uhoh
CC BY-SA 4.0
2024-07-29T00:32:29.757Z
@JohnDoty [Are there materials with an isobaric melting point below the triple point pressure or NO isothermal melting points below the triple point temperature?](https://chemistry.stackexchange.com/q/184043/16035)
uhoh
CC BY-SA 4.0
2024-07-28T13:15:44.447Z
2024-07-28T13:20:51.447Z
2024-07-28T13:20:51.447Z
David Hammen
<p>Based on <a href="https://astronomy.stackexchange.com/a/58159/35206">this answer</a>, it is concluded that Pluto never receives sufficient amount of sunlight to melt nitrogen ice. Yet, sublimation pits is observed across Sputnik Planitia. From <a href="https://slate.com/technology/2015/10/pluto-fields-of-sublimation-pits.html" rel="nofollow noreferrer">here</a>:</p>
<blockquote>
<p><a href="https://i.sstatic.net/2qGuTmM6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2qGuTmM6.png" alt="enter image description here" /></a></p>
<p>These are likely to be sublimation pits, depressions in the surface when ice turned directly into a gas from sunlight. They probably start very small and grow over time as the Sun shines down on them.</p>
</blockquote>
<p>There is some explanation on <em>how</em> sublimation happens in below article:</p>
<ol>
<li>Morison, A., Labrosse, S. &amp; Choblet, G. Sublimation-driven convection in Sputnik Planitia on Pluto. Nature 600, 419–423 (<strong>2021</strong>). <a href="https://doi.org/10.1038/s41586-021-04095-w" rel="nofollow noreferrer">https://doi.org/10.1038/s41586-021-04095-w</a></li>
<li><a href="https://arstechnica.com/science/2021/12/a-sublime-landscape-new-model-explains-plutos-lumpy-plains/" rel="nofollow noreferrer">https://arstechnica.com/science/2021/12/a-sublime-landscape-new-model-explains-plutos-lumpy-plains/</a></li>
</ol>
<p>But I don't understand <em>why</em> sublimation occur in the first place but not melting.</p>

CC BY-SA 4.0
2024-07-27T10:13:09.477Z
You might want to look up the definition of sublimation.
John Doty
CC BY-SA 4.0
2024-07-27T10:16:03.370Z
I know what sublimation means. If Pluto never receives sufficient sunlight for melting to occur, why does sublimation occur? @JohnDoty
Nilay Ghosh
CC BY-SA 4.0
2024-07-27T11:20:03.047Z
Sublimation generally occurs without melting. If preceded by melting it is called "evaporation".
John Doty
CC BY-SA 4.0
2024-07-27T13:35:52.573Z
Sublimation of ice happens quite readily here on Earth. Sublimation is a direct transition from solid to vapor. All materials have an equilibrium vapor pressure. For the elements you can find expressions at https://en.m.wikipedia.org/wiki/Vapor_pressures_of_the_elements_(data_page) and calculate them at your temperature of interest. Even if the vapor pressure is very low, given enough time macroscopic things can happen.
Jon Custer
CC BY-SA 4.0
2024-07-27T13:52:08.743Z
From the comment under the question you  have linked its answer: *During its orbit, pluto does come close enough to the sun to melt/partially melt some of that ice...*
Snack Exchange
CC BY-SA 4.0
2024-07-27T14:00:33.837Z
@SnackExchange so if melting does occur, how much it melts and how much it sublimates? What is the defining factor?
Nilay Ghosh
CC BY-SA 4.0
2024-07-27T21:48:25.320Z
*Interesting question!*  [Space.com](https://www.space.com/18563-pluto-temperature.html) says "When NASA's New Horizons mission buzzed the dwarf planet in July 2015, it revealed a thin atmosphere made up of nitrogen, methane and carbon monoxide. But the atmosphere most likely changes as Pluto orbits the sun. **When the distance to Pluto from the sun is at a minimum, temperatures are warm enough for the ices to sublimate directly into gas, creating a thin atmosphere.** As Pluto moves away from the sun, the gases re-freeze and the planet's atmosphere disappears from view."
uhoh
CC BY-SA 4.0
2024-07-27T21:54:33.700Z
A proper answer should cite a proper source which references a vapor pressure curve for nitrogen, which seems to rise about a factor of 10 for ever 5 K at around 40 K https://i.sstatic.net/E42U7uGZ.png
uhoh
CC BY-SA 4.0
2024-07-27T23:23:35.963Z
The pressure at Pluto’s surface is much too low for N$_{2}$ to exist in a liquid state. So it’s like CO$_{2}$ on the earth’s surface, which can exist as a solid (dry ice), if the temperature is low enough, or as a gas, but not as a liquid. (Dry ice exposed to room temperature sublimates to a gas; it cannot melt.)
Peter Erwin
CC BY-SA 4.0
2024-07-27T23:48:32.153Z
@PeterErwin or like H₂O in a freezer - why unused ice cubes shrink and eventually disappear.
uhoh
CC BY-SA 4.0
2024-07-28T15:20:44.437Z
@uhoh Even better, a well-sealed bag of raw meat in a freezer. Modern freezers tend to have dehumidifiers so as to prevent a big block of snow-like ice forming around the edges of the freezer block. That well-sealed hunk of meat will eventually become freezer burned and covered with snow and ice. The ice cubes disappear because while there is sublimation, there is no deposition back on to the ice cubes.
David Hammen
<pluto><heat>
If Pluto never comes close enough to the sun to melt the nitrogen ice, why does sublimation occurs?
Nilay Ghosh
# If Pluto never comes close enough to the sun to melt the nitrogen ice, why does sublimation occurs?

(Asked by: username_0 on 2024-07-27)

Based on [this answer](https://astronomy.stackexchange.com/a/58159/35206), it is concluded that Pluto never receives sufficient amount of sunlight to melt nitrogen ice. Yet, sublimation pits is observed across Sputnik Planitia. From [here](https://slate.com/technology/2015/10/pluto-fields-of-sublimation-pits.html):

> [![enter image description here](https://i.sstatic.net/2qGuTmM6.png)](https://i.sstatic.net/2qGuTmM6.png)
>
> These are likely to be sublimation pits, depressions in the surface when ice turned directly into a gas from sunlight. They probably start very small and grow over time as the Sun shines down on them.

There is some explanation on *how* sublimation happens in below article:

1. Morison, A., Labrosse, S. & Choblet, G. Sublimation-driven convection in Sputnik Planitia on Pluto. Nature 600, 419–423 (**2021**). <https://doi.org/10.1038/s41586-021-04095-w>
2. <https://arstechnica.com/science/2021/12/a-sublime-landscape-new-model-explains-plutos-lumpy-plains/>

But I don't understand *why* sublimation occur in the first place but not melting.

**Tags:** pluto, heat
> **Comments:**
> **username_1**: You might want to look up the definition of sublimation.  
> **username_0**: I know what sublimation means. If Pluto never receives sufficient sunlight for melting to occur, why does sublimation occur? @JohnDoty  
> **username_1**: Sublimation generally occurs without melting. If preceded by melting it is called "evaporation".  
> **username_2**: Sublimation of ice happens quite readily here on Earth. Sublimation is a direct transition from solid to vapor. All materials have an equilibrium vapor pressure. For the elements you can find expressions at https://en.m.wikipedia.org/wiki/Vapor_pressures_of_the_elements_(data_page) and calculate them at your temperature of interest. Even if the vapor pressure is very low, given enough time macroscopic things can happen.  
> **username_3**: From the comment under the question you  have linked its answer: *During its orbit, pluto does come close enough to the sun to melt/partially melt some of that ice...*  
> **username_0**: @SnackExchange so if melting does occur, how much it melts and how much it sublimates? What is the defining factor?  
> **username_4**: *Interesting question!*  [Space.com](https://www.space.com/18563-pluto-temperature.html) says "When NASA's New Horizons mission buzzed the dwarf planet in July 2015, it revealed a thin atmosphere made up of nitrogen, methane and carbon monoxide. But the atmosphere most likely changes as Pluto orbits the sun. **When the distance to Pluto from the sun is at a minimum, temperatures are warm enough for the ices to sublimate directly into gas, creating a thin atmosphere.** As Pluto moves away from the sun, the gases re-freeze and the planet's atmosphere disappears from view."  
> **username_4**: A proper answer should cite a proper source which references a vapor pressure curve for nitrogen, which seems to rise about a factor of 10 for ever 5 K at around 40 K https://i.sstatic.net/E42U7uGZ.png  
> **username_5**: The pressure at Pluto’s surface is much too low for N$_{2}$ to exist in a liquid state. So it’s like CO$_{2}$ on the earth’s surface, which can exist as a solid (dry ice), if the temperature is low enough, or as a gas, but not as a liquid. (Dry ice exposed to room temperature sublimates to a gas; it cannot melt.)  
> **username_4**: @PeterErwin or like H₂O in a freezer - why unused ice cubes shrink and eventually disappear.  
> **username_6**: @uhoh Even better, a well-sealed bag of raw meat in a freezer. Modern freezers tend to have dehumidifiers so as to prevent a big block of snow-like ice forming around the edges of the freezer block. That well-sealed hunk of meat will eventually become freezer burned and covered with snow and ice. The ice cubes disappear because while there is sublimation, there is no deposition back on to the ice cubes.  

---

## 1 answers

### Answer by username_6 on 2024-07-28. Score: 5 | Accepted answer

I'll start with a typical phase diagram, which appears below.

[![Typical phase diagram](https://i.sstatic.net/2fkrWtGM.png)](https://i.sstatic.net/2fkrWtGM.png)

Note that there are three solid curves on the graph:

* A red curve that starts at the origin and ends at the triple point. This is the sublimation curve.
* A green curve that starts at the triple point. This is the melting curve.
* A blue curve that also starts at the triple points but ends at the critical point. Theis is the evaporation curve.
* As a side note, water is an atypical substance. The dashed green curve that slopes to the left with increasing pressure rather than curving to the right is the melting curve for water.

Note that a substance does not have a liquid phase at pressures below the triple point pressure. Per the [NIST Chemistry WebBook](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&Mask=4), the triple point for nitrogen ($\text{N}\_2$) is a temperature of 63.14K (-210.0 C) and a pressure of 0.1253 bar (or 12530 pascals).

Conditions on Pluto are very cold and a very thin atmosphere. It's the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting. Note that estimates of the temperature vary. ([National Geographic](https://www.nationalgeographic.com/science/article/dwarf-planets-pluto-ceres) interesting claims an average temperature just below the triple point temeperature.) That's irrelevant because nitrogen exists as a solid or as a gas at pressures very far below the triple point pressure. It cannot melt at such low pressures. Instead, it transfers directly from solid to gas via sublimation, and from gas to solid via deposition.

> **Comments:**
> **username_4**: I always get confused with phase diagrams. If the triple point is at 63 K and the temperature of Pluto is 40 K , why exactly is it "the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting" and not the temperature? I don't think the solid-liquid boundary for nitrogen goes below 60 K at any pressure at least below 100 MPa - it looks pretty vertical, then curves to the right. ([random example](https://www.researchgate.net/figure/The-temperature-pressure-phase-diagram-for-nitrogen_fig1_315888614))  
> **username_6**: @uhoh Some comments regarding your comment. (1) I  could not find a good reference for the temperature of Pluto at perihelion. They seem to be all over the map. Some are at 65 K, others closer to your value of 40 K. (2) The temperature is irrelevant so long as it is lowish and so long as the pressure is extremely low (which it is). Nitrogen will either be a solid or a gas at a pressure of 1 pascal.  
> **username_4**: OK so "It's the thinness of the atmosphere, about 1 pascal, that prevents nitrogen from melting" is true *as long as it's below the melting point*. Sounds like a pretty important caveat.  
> **username_1**: @uhoh If the pressure is below the triple point pressure, there is no melting point.  
> **username_4**: @JohnDoty I think I see, my problem is more to do with the lopsided word "melting point" which applies to what happens along a horizontal line in the plot (constant pressure variation in temperature) There are cases where a vertical line (constant temperature, varying pressure) could also result in a "melting point" my brain wants to treat the two mathematical scenarios the same - below **some** temperature there is no melting point. However that temperature might not necessarily e be the triple point because the solid-liquid boundary can have a reentrant shape.  
> **username_4**: @JohnDoty so as long as I force myself to think the way normal people do ("melting point" is a point along only a horizontal (constant pressure) line) then I can put this to rest in my head. I see that for example in the [phase diagram for water](https://ergodic.ugr.es/termo/lecciones/water1.html) even for our familiar [ice 1h](https://en.wikipedia.org/wiki/Phases_of_ice#Known_phases) somewhere between 100 and 1000 atmospheres the "melting point" curves to the left. There is no melting point below say 240 or 250 K, but that point is *below* the triple point.  
> **username_4**: @JohnDoty [Are there materials with an isobaric melting point below the triple point pressure or NO isothermal melting points below the triple point temperature?](https://chemistry.stackexchange.com/q/184043/16035)  

---

