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?


  • $\begingroup$ I think we need a good temperature level. Iced gasoline will not burn too. $\endgroup$ – Magno C Feb 4 '14 at 11:04

"At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium" (Wikipedia). 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 is sufficiently exothermic (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 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.

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 is 23.5 km/s, its surface temperature is below 100K. With Boltzmann 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.

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  • $\begingroup$ So, if oxygen was found on neptune, sufficiently to burn all of the other gases up, what would it turn into (the planet)?? $\endgroup$ – Yoda Feb 4 '14 at 20:59
  • $\begingroup$ Added the case to the answer. $\endgroup$ – Gerald Feb 4 '14 at 21:19
  • $\begingroup$ 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? $\endgroup$ – Yoda Feb 4 '14 at 21:25
  • $\begingroup$ It wouldn't. Calculated in detail why. $\endgroup$ – Gerald Feb 4 '14 at 22:34
  • $\begingroup$ 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. $\endgroup$ – Yoda Feb 4 '14 at 22:40

Gerald 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.

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  • $\begingroup$ 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. $\endgroup$ – zeta-band Sep 14 '18 at 18:25

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.

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  • $\begingroup$ 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. $\endgroup$ – userLTK Oct 27 '17 at 11:39
  • $\begingroup$ It'd take a higher level civilization to pull that off. But I like it. $\endgroup$ – Wayfaring Stranger Oct 27 '17 at 16:38
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    $\begingroup$ This doesn't seem to be an answer to the question. $\endgroup$ – James K Oct 28 '17 at 22:29

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