<p>Below table shows the list of energy sources for Pluto:
<a href="https://i.sstatic.net/826uRsET.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/826uRsET.png" alt="Energy source" /></a></p>
<p>As you can see, the energy from radioactive decay comfortably exceeds the required heat to melt Pluto's entire ice inventory. However, <em>Rubochon and Nimmo</em> investigated this and put forward a differentiated model of Pluto with an ammonia-free ice shell and modeled the thermal evolution for different ice basal viscosities. If the viscosities were low enough (&lt; 2×10<sup>6</sup> Pa s), the ice shell would undergo convection and remove heat rapidly enough and the temperature to remain below melting point. Other models were also suggested like Hussmann-Desch-Hammond model which says that presence of ammonia which acts as anti-freeze.</p>
<p>But <a href="https://www.nature.com/articles/s41467-022-29056-3" rel="nofollow noreferrer">cryovolcanism is noted on Pluto</a> which indicates that Pluto might have subsurface ocean but that is hypothesized to have come from internal source heat and it doesn't contribute to surface ice melting. Do note that the ocean is not a free-flowing liquid but it is highly viscous and very cold.</p>
<p><strong>Source</strong>: <em>The Pluto System After New Horizons</em> by S. Alan Stern, Jeffrey M. Moore, William M. Grundy, Leslie A. Young, Richard P. Binzel, University of Arizona Press, 2021</p>

CC BY-SA 4.0
2024-07-26T03:25:17.643Z
2024-07-26T03:25:17.643Z
Nilay Ghosh
<p>The smooth plain you're referring to is known as <a href="https://en.wikipedia.org/wiki/Sputnik_Planitia" rel="noreferrer">Sputnik Planitia</a>, the region is comprised of mainly nitrogen ice convection cells, which have a much lower freezing and melting point than water. From what I've found, Pluto never comes close enough to the sun to melt that nitrogen ice, and it remains in a solid state.</p>
<p>The process that refreshes the surface is caused when the relatively soft nitrogen ice is heated by the interior (either through radioactive decay or tidal heating from the moon Charon), causing it to become slightly less dense and rise in the center, and the colder and older nitrogen ice on the surface cools slightly and becomes denser and sinks back down in a similar process to what's going on in Earth's mantle. It of course happens very slowly (7 cm per year) though quick in geologic terms. From what I have found there are no large and smooth <em>planitiae</em> that are comprised of water ice, however the bordering <em>montes</em> are comprised of mainly water ice.</p>
<p>Pluto does likely contain a subsurface ocean of liquid water from either tidal heating or radioactive decay, though.</p>

CC BY-SA 4.0
2024-07-23T20:40:13.657Z
On Pluto, water-ice is a rock and nitrogen is a very thick fluid.
Darth Pseudonym
CC BY-SA 4.0
2024-07-23T22:13:11.077Z
Do you have any references for these claims?
planetmaker
CC BY-SA 4.0
2024-07-23T22:50:33.967Z
The pressure of Pluto's atmosphere is too low to permit liquid nitrogen. However, the nitrogen ice can *sublimate* directly to gaseous form (and in fact Pluto's atmosphere is mostly nitrogen). So the nitrogen ice does not remain permanently in a solid state; instead, there is cycling between nitrogen sublimating into the atmosphere and atmospheric nitrogen condensing into frost/ice.
Peter Erwin
CC BY-SA 4.0
2024-07-24T14:50:28.507Z
For details on the tidal heating: [The tidal–thermal evolution of the Pluto–Charon system](https://www.sciencedirect.com/science/article/pii/S001910352100508X), April 2022. It is said that _Tides, despite intense for a short time, have not affected the thermal history much ... Our results show that because of relatively fast circularization and synchronization of the orbits of Pluto and Charon, tidal heating is only important during the early stages of evolution (<1 Myr)_
David Tonhofer
CC BY-SA 4.0
2024-07-24T14:52:41.777Z
And: _In all the studied cases, oceans on Pluto are always predicted to remain liquid to the present, ranging in thickness from 40 km to 150-km, whereas oceans on Charon, while in-place for approximately 4 Gyr, have solidified._
David Tonhofer
CC BY-SA 4.0
2024-07-24T15:30:50.353Z
And even _without_ internal heating, it'd've still flowed into a sphere.  Imagine a ball made of glaciers...
Vikki
CC BY-SA 4.0
2024-07-25T07:26:50.810Z
"and the colder and older nitrogen ice on the surface cools slightly and becomes less dense and sinks back down" - I assume you mean "more dense" in this one?
Syndic
CC BY-SA 4.0
2024-07-25T21:30:48.263Z
@Syndic ah yes, let me fix that.
ArchiveOfStars
CC BY-SA 4.0
2024-07-27T10:11:01.163Z
I have asked [If Pluto never comes close enough to the sun to melt the nitrogen ice, why does sublimation occurs?](https://astronomy.stackexchange.com/questions/58180/if-pluto-never-comes-close-enough-to-the-sun-to-melt-that-nitrogen-ice-how-does)
Nilay Ghosh
CC BY-SA 4.0
2024-07-23T19:56:37.980Z
2024-07-25T21:30:22.997Z
2024-07-25T21:30:22.997Z
ArchiveOfStars
<p>To melt ice, Pluto's surface must have gotten above the freezing point of water. On Pluto, it's always night and the sun is just a bright star. So how could this possibly be possible?</p>
<p>Because there are few craters, the heat event had to have been relatively recent, not during the planet's creation.</p>
<ul>
<li><p>Charon is close, so is Pluto subject to tidal stretching like IO, despite Charon's low mass?</p>
</li>
<li><p>Its orbit is eccentric, but is it eccentric enough to melt ice every few hundred years?</p>
</li>
<li><p>Heat from Kuiper belt asteroids slamming into it? Earth is still cooling down from that.</p>
</li>
<li><p>Heat from the radiator of the nuclear batteries on New Horizons as it flew by? Voyager's hot radiator scorched one side of Iapetus something terrible.</p>
</li>
</ul>

<pluto>
Pluto has smooth ice plains, almost like lunar maria. Where did the heat come from to melt all that ice?
Miss Understands
# Pluto has smooth ice plains, almost like lunar maria. Where did the heat come from to melt all that ice?

(Asked by: username_0 on 2024-07-23)

To melt ice, Pluto's surface must have gotten above the freezing point of water. On Pluto, it's always night and the sun is just a bright star. So how could this possibly be possible?

Because there are few craters, the heat event had to have been relatively recent, not during the planet's creation.

* Charon is close, so is Pluto subject to tidal stretching like IO, despite Charon's low mass?
* Its orbit is eccentric, but is it eccentric enough to melt ice every few hundred years?
* Heat from Kuiper belt asteroids slamming into it? Earth is still cooling down from that.
* Heat from the radiator of the nuclear batteries on New Horizons as it flew by? Voyager's hot radiator scorched one side of Iapetus something terrible.

**Tags:** pluto

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## 2 answers

### Answer by username_2 on 2024-07-23. Score: 25 | Accepted answer

The smooth plain you're referring to is known as [Sputnik Planitia](https://en.wikipedia.org/wiki/Sputnik_Planitia), the region is comprised of mainly nitrogen ice convection cells, which have a much lower freezing and melting point than water. From what I've found, Pluto never comes close enough to the sun to melt that nitrogen ice, and it remains in a solid state.

The process that refreshes the surface is caused when the relatively soft nitrogen ice is heated by the interior (either through radioactive decay or tidal heating from the moon Charon), causing it to become slightly less dense and rise in the center, and the colder and older nitrogen ice on the surface cools slightly and becomes denser and sinks back down in a similar process to what's going on in Earth's mantle. It of course happens very slowly (7 cm per year) though quick in geologic terms. From what I have found there are no large and smooth *planitiae* that are comprised of water ice, however the bordering *montes* are comprised of mainly water ice.

Pluto does likely contain a subsurface ocean of liquid water from either tidal heating or radioactive decay, though.

> **Comments:**
> **username_3**: On Pluto, water-ice is a rock and nitrogen is a very thick fluid.  
> **username_4**: Do you have any references for these claims?  
> **username_5**: The pressure of Pluto's atmosphere is too low to permit liquid nitrogen. However, the nitrogen ice can *sublimate* directly to gaseous form (and in fact Pluto's atmosphere is mostly nitrogen). So the nitrogen ice does not remain permanently in a solid state; instead, there is cycling between nitrogen sublimating into the atmosphere and atmospheric nitrogen condensing into frost/ice.  
> **username_6**: For details on the tidal heating: [The tidal–thermal evolution of the Pluto–Charon system](https://www.sciencedirect.com/science/article/pii/S001910352100508X), April 2022. It is said that _Tides, despite intense for a short time, have not affected the thermal history much ... Our results show that because of relatively fast circularization and synchronization of the orbits of Pluto and Charon, tidal heating is only important during the early stages of evolution (<1 Myr)_  
> **username_6**: And: _In all the studied cases, oceans on Pluto are always predicted to remain liquid to the present, ranging in thickness from 40 km to 150-km, whereas oceans on Charon, while in-place for approximately 4 Gyr, have solidified._  
> **username_7**: And even _without_ internal heating, it'd've still flowed into a sphere.  Imagine a ball made of glaciers...  
> **username_8**: "and the colder and older nitrogen ice on the surface cools slightly and becomes less dense and sinks back down" - I assume you mean "more dense" in this one?  
> **username_2**: @Syndic ah yes, let me fix that.  
> **username_1**: I have asked [If Pluto never comes close enough to the sun to melt the nitrogen ice, why does sublimation occurs?](https://astronomy.stackexchange.com/questions/58180/if-pluto-never-comes-close-enough-to-the-sun-to-melt-that-nitrogen-ice-how-does)  

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### Answer by username_1 on 2024-07-26. Score: 3

Below table shows the list of energy sources for Pluto:
[![Energy source](https://i.sstatic.net/826uRsET.png)](https://i.sstatic.net/826uRsET.png)

As you can see, the energy from radioactive decay comfortably exceeds the required heat to melt Pluto's entire ice inventory. However, *Rubochon and Nimmo* investigated this and put forward a differentiated model of Pluto with an ammonia-free ice shell and modeled the thermal evolution for different ice basal viscosities. If the viscosities were low enough (< 2×106 Pa s), the ice shell would undergo convection and remove heat rapidly enough and the temperature to remain below melting point. Other models were also suggested like Hussmann-Desch-Hammond model which says that presence of ammonia which acts as anti-freeze.

But [cryovolcanism is noted on Pluto](https://www.nature.com/articles/s41467-022-29056-3) which indicates that Pluto might have subsurface ocean but that is hypothesized to have come from internal source heat and it doesn't contribute to surface ice melting. Do note that the ocean is not a free-flowing liquid but it is highly viscous and very cold.

**Source**: *The Pluto System After New Horizons* by S. Alan Stern, Jeffrey M. Moore, William M. Grundy, Leslie A. Young, Richard P. Binzel, University of Arizona Press, 2021


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