<blockquote>
  <p>Gerald quoted Wikipedia: "At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium".</p>
</blockquote>

<p>Interesting to read that what we see of Neptune's atmosphere is only the top part of the atmosphere. Thus, Neptune's always stated atmospheric composition of 80% hydrogen and 19% helium would only apply to this very outer layer.</p>

<blockquote>
  <p>www.Quora.com: "In their diatomic forms, which is how we commonly encounter them, an oxygen molecule weighs 32 and a hydrogen molecule weighs 2. So oxygen is 16 times heavier than hydrogen". </p>
</blockquote>

<p>Since oxygen, which we breath, is one of the heavier gases, there could exist quite a lot of it near Neptune's surface.</p>

CC BY-SA 4.0
2018-09-14T18:25:01.677Z
Unlikely for there to be free oxygen. There would be enough random lightening to ignite it at some point and then there wouldn't be free oxygen.
zeta-band
CC BY-SA 4.0
2018-09-14T00:07:41.983Z
2018-09-14T00:07:41.983Z
Constantthin
<p>Transmutation of elements from one to another had been understood since 1901 as a result of natural radioactive decay, but when Rutherford projected alpha particles from alpha decay into air, he discovered this produced a new type of radiation which proved to be hydrogen nuclei (Rutherford named these protons). Further experimentation showed the protons to be coming from the nitrogen component of air, and the reaction was deduced to be a transmutation of nitrogen into oxygen in the reaction
14N + α → 17O + p 
This was the first-discovered nuclear reaction. We could use this reaction to terraform Neptune to our liking.</p>

CC BY-SA 3.0
2017-10-27T11:39:55.310Z
That's a cool example, but very energy intensive.  The trick with Neptune is stripping it of it's hydrogen but it's got a lot of hydrogen, an Earth's mass perhaps more.
userLTK
CC BY-SA 3.0
2017-10-27T16:38:56.927Z
It'd take a higher level civilization to pull that off. But I like it.
Wayfaring Stranger
CC BY-SA 3.0
2017-10-28T22:29:04.163Z
This doesn't seem to be an answer to the question.
James K
CC BY-SA 3.0
2017-10-27T09:43:48.050Z
2017-10-27T09:43:48.050Z
Matthew Ellingsen
<p>"At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium" (<a href="http://en.wikipedia.org/wiki/Neptune" rel="nofollow">Wikipedia</a>). No significant abundancy of free oxygen to react with.</p>

<p>A source of oxygen could easily made burn on Neptune, like a source of hydrogen on Earth.
Or take a sample of Neptun's atmosphere. It would easily burn in Earth's atmosphere.</p>

<p><a href="http://en.wikipedia.org/wiki/Oxyhydrogen" rel="nofollow">Hydrogen oxygen combustion</a> is sufficiently <a href="http://en.wikipedia.org/wiki/Exothermic_reaction" rel="nofollow">exothermic</a> (483.6 kJ/mol of O2) to sustain burning even at the low temperatures on Neptune, provided the atmospheric pressure is high enough: With a <a href="http://en.wikipedia.org/wiki/Gas_constant" rel="nofollow">gas constant</a> of about 8.3 J / mol K, the gas can easily be heated up to the point of combustion by the energy released by the combustion.</p>

<p>Assuming oxygen would be provided, just sufficiently to burn up hydrogen and trace hydrocarbons:
After the explosion and condensation Neptune would get a huge ocean of water, later freezing to water ice, with an atmosphere of helium, and some less abundant gases like CO2, nitrogen, and argon, traces of water vapor. Traces of hydrogen and methane could survive the presence of oxygen. Hence some oxygen would remain in the atmosphere. Most of the CO2 would be resolved in the water ice.</p>

<p>The average density of Neptune would increase, it would shrink a bit, the core would be compressed and heated by <a href="http://en.wikipedia.org/wiki/Adiabatic_process" rel="nofollow">adiabatic compression</a>.</p>

<p>Before combustion Neptune's gravity is strong enough to hold an atmosphere of hydrogen, the easiest gas to escape. Hence the denser planet after combustion wouldn't improve its capability to keep an atmosphere significantly: The <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html" rel="nofollow">escape velocity of Neptune</a> is 23.5 km/s, its surface <a href="http://en.wikipedia.org/wiki/Temperature" rel="nofollow">temperature</a> is below 100K. With <a href="http://en.wikipedia.org/wiki/Boltzmann%27s_constant" rel="nofollow">Boltzmann constant</a> $k=1.38\cdot 10^{-23}J/K$ and $E=3kT/2$ we get $$E=3\cdot 1.38\cdot 10^{-23}J/K\mbox{ or }4.14\cdot 10^{-21}J\mbox{ at }100K$$ for an average $H_2$ molecule with a mass of about $3.35\cdot 10^{-27}kg$. Hence for Neptun's escape velocity the kinetic energy of a hydrogen molecule is $$E=0.5\ mv^2=0.5\cdot 3.35\cdot 10^{-27}kg \cdot (23.5\cdot 10^3 m/s)^2 = 9.25\cdot 10^{-19} J.$$ Hence the energy needed to escape is more than 200-times the kinetic energy of the hydrogen molecules; no way to escape, 2000K or more would be needed to let escape some hydrogen over time (to get below a factor of about ten between the two energies). Photolyic dissociated atomic hydrogen would still need more than 1000K to escape.</p>

<p>The probability to catch asteroids or comets would be reduced due to the reduced radius, if the total mass is adjusted to the mass before combustion, since asteroids would miss instead of hit the planet.</p>

CC BY-SA 3.0
2017-01-15T22:31:14.727Z
We should do that.   A planet on fire would look very cool.
userLTK
CC BY-SA 3.0
2017-10-27T15:38:26.177Z
I expect there is oxygen on Neptune, but it's all locked up in frozen water and carbon dioxide; not the sort of stuff you'll see in the upper atmosphere. Looks like verifying lightning on Neptune is not a high priority:http://onlinelibrary.wiley.com/doi/10.1029/GL016i008p00937/abstract but its temp of 30,000 kelvins or so ought be high enough to trigger oxidation reactions even way out on the dim edges of the solar system.
Wayfaring Stranger
CC BY-SA 3.0
2014-02-04T20:59:06.463Z
So, if oxygen was found on neptune, sufficiently to burn all of the other gases up, what would it turn into (the planet)??
Yoda
CC BY-SA 3.0
2014-02-04T21:19:48.130Z
Added the case to the answer.
Gerald
CC BY-SA 3.0
2014-02-04T21:25:42.947Z
Excellent, thank you very much.. Another matter I hope you could add to your answer; Would neptune start now to collect gases again as the gravity now would be significantly larger at the surface and this new planet would act as it's core?
Yoda
CC BY-SA 3.0
2014-02-04T22:34:47.113Z
It wouldn't. Calculated in detail why.
Gerald
CC BY-SA 3.0
2014-02-04T22:40:35.503Z
I don't understand "why" about what I'm afraid. But I guess you are asking about the gravity at the surface. I have assumed that because as the planet shrinks and become more dense, the surface will be more closer to the center of gravity. Ofcourse I assume the mass wouldn't change, so overall gravity would remain the same. If that's not the question, please clarify.
Yoda
CC BY-SA 3.0
2014-02-04T22:46:02.373Z
That's not meant as a question. I meant, that I calculated the reason.
Gerald
CC BY-SA 3.0
2014-02-04T22:49:09.670Z
That was very useful, I agree with you. Thank you very much :D
Yoda
CC BY-SA 3.0
2014-02-04T16:01:02.887Z
2014-02-04T22:41:39.757Z
2014-02-04T22:41:39.757Z
Gerald
<p>Since Neptune/Uranus have high percentages of methane, wouldn't it be highly likely that there will be combustion (triggered by the lightning storms or any other factor) if oxygen was abundant? </p>

<p>Will the hydrogen combust as well?</p>

<p><a href="https://i.sstatic.net/gi3qc.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/gi3qc.jpg" alt="Neptune"></a></p>

CC BY-SA 3.0
2014-02-04T11:04:36.207Z
I think we need a good temperature level. Iced gasoline will not burn too.
Magno C
<planet><exoplanet><neptune><gas-giants>
If oxygen was abundant in Neptune, would there be combustion?
Yoda
# If oxygen was abundant in Neptune, would there be combustion?

(Asked by: username_0 on 2014-02-04)

Since Neptune/Uranus have high percentages of methane, wouldn't it be highly likely that there will be combustion (triggered by the lightning storms or any other factor) if oxygen was abundant?

Will the hydrogen combust as well?

[![Neptune](https://i.sstatic.net/gi3qc.jpg)](https://i.sstatic.net/gi3qc.jpg)

**Tags:** planet, exoplanet, neptune, gas-giants

> **Comments:**
> **username_1**: I think we need a good temperature level. Iced gasoline will not burn too.  

---

## 3 answers

### Answer by username_8 on 2014-02-04. Score: 6 | Accepted answer

"At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium" ([Wikipedia](http://en.wikipedia.org/wiki/Neptune)). No significant abundancy of free oxygen to react with.

A source of oxygen could easily made burn on Neptune, like a source of hydrogen on Earth.
Or take a sample of Neptun's atmosphere. It would easily burn in Earth's atmosphere.

[Hydrogen oxygen combustion](http://en.wikipedia.org/wiki/Oxyhydrogen) is sufficiently [exothermic](http://en.wikipedia.org/wiki/Exothermic_reaction) (483.6 kJ/mol of O2) to sustain burning even at the low temperatures on Neptune, provided the atmospheric pressure is high enough: With a [gas constant](http://en.wikipedia.org/wiki/Gas_constant) of about 8.3 J / mol K, the gas can easily be heated up to the point of combustion by the energy released by the combustion.

Assuming oxygen would be provided, just sufficiently to burn up hydrogen and trace hydrocarbons:
After the explosion and condensation Neptune would get a huge ocean of water, later freezing to water ice, with an atmosphere of helium, and some less abundant gases like CO2, nitrogen, and argon, traces of water vapor. Traces of hydrogen and methane could survive the presence of oxygen. Hence some oxygen would remain in the atmosphere. Most of the CO2 would be resolved in the water ice.

The average density of Neptune would increase, it would shrink a bit, the core would be compressed and heated by [adiabatic compression](http://en.wikipedia.org/wiki/Adiabatic_process).

Before combustion Neptune's gravity is strong enough to hold an atmosphere of hydrogen, the easiest gas to escape. Hence the denser planet after combustion wouldn't improve its capability to keep an atmosphere significantly: The [escape velocity of Neptune](http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html) is 23.5 km/s, its surface [temperature](http://en.wikipedia.org/wiki/Temperature) is below 100K. With [Boltzmann constant](http://en.wikipedia.org/wiki/Boltzmann%27s_constant) $k=1.38\cdot 10^{-23}J/K$ and $E=3kT/2$ we get $$E=3\cdot 1.38\cdot 10^{-23}J/K\mbox{ or }4.14\cdot 10^{-21}J\mbox{ at }100K$$ for an average $H\_2$ molecule with a mass of about $3.35\cdot 10^{-27}kg$. Hence for Neptun's escape velocity the kinetic energy of a hydrogen molecule is $$E=0.5\ mv^2=0.5\cdot 3.35\cdot 10^{-27}kg \cdot (23.5\cdot 10^3 m/s)^2 = 9.25\cdot 10^{-19} J.$$ Hence the energy needed to escape is more than 200-times the kinetic energy of the hydrogen molecules; no way to escape, 2000K or more would be needed to let escape some hydrogen over time (to get below a factor of about ten between the two energies). Photolyic dissociated atomic hydrogen would still need more than 1000K to escape.

The probability to catch asteroids or comets would be reduced due to the reduced radius, if the total mass is adjusted to the mass before combustion, since asteroids would miss instead of hit the planet.

> **Comments:**
> **username_0**: So, if oxygen was found on neptune, sufficiently to burn all of the other gases up, what would it turn into (the planet)??  
> **username_8**: Added the case to the answer.  
> **username_0**: Excellent, thank you very much.. Another matter I hope you could add to your answer; Would neptune start now to collect gases again as the gravity now would be significantly larger at the surface and this new planet would act as it's core?  
> **username_8**: It wouldn't. Calculated in detail why.  
> **username_0**: I don't understand "why" about what I'm afraid. But I guess you are asking about the gravity at the surface. I have assumed that because as the planet shrinks and become more dense, the surface will be more closer to the center of gravity. Ofcourse I assume the mass wouldn't change, so overall gravity would remain the same. If that's not the question, please clarify.  
> **username_8**: That's not meant as a question. I meant, that I calculated the reason.  
> **username_0**: That was very useful, I agree with you. Thank you very much :D  
> **username_5**: We should do that.   A planet on fire would look very cool.  
> **username_6**: I expect there is oxygen on Neptune, but it's all locked up in frozen water and carbon dioxide; not the sort of stuff you'll see in the upper atmosphere. Looks like verifying lightning on Neptune is not a high priority:http://onlinelibrary.wiley.com/doi/10.1029/GL016i008p00937/abstract but its temp of 30,000 kelvins or so ought be high enough to trigger oxidation reactions even way out on the dim edges of the solar system.  

---

### Answer by username_2 on 2018-09-14. Score: 1

> username_8 quoted Wikipedia: "At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium".

Interesting to read that what we see of Neptune's atmosphere is only the top part of the atmosphere. Thus, Neptune's always stated atmospheric composition of 80% hydrogen and 19% helium would only apply to this very outer layer.

> www.Quora.com: "In their diatomic forms, which is how we commonly encounter them, an oxygen molecule weighs 32 and a hydrogen molecule weighs 2. So oxygen is 16 times heavier than hydrogen".

Since oxygen, which we breath, is one of the heavier gases, there could exist quite a lot of it near Neptune's surface.

> **Comments:**
> **username_3**: Unlikely for there to be free oxygen. There would be enough random lightening to ignite it at some point and then there wouldn't be free oxygen.  

---

### Answer by username_4 on 2017-10-27. Score: 0

Transmutation of elements from one to another had been understood since 1901 as a result of natural radioactive decay, but when Rutherford projected alpha particles from alpha decay into air, he discovered this produced a new type of radiation which proved to be hydrogen nuclei (Rutherford named these protons). Further experimentation showed the protons to be coming from the nitrogen component of air, and the reaction was deduced to be a transmutation of nitrogen into oxygen in the reaction
14N + α → 17O + p 
This was the first-discovered nuclear reaction. We could use this reaction to terraform Neptune to our liking.

> **Comments:**
> **username_5**: That's a cool example, but very energy intensive.  The trick with Neptune is stripping it of it's hydrogen but it's got a lot of hydrogen, an Earth's mass perhaps more.  
> **username_6**: It'd take a higher level civilization to pull that off. But I like it.  
> **username_7**: This doesn't seem to be an answer to the question.  

---

