Neptune's Internal Heat Source

Unlike its sister planet Uranus,Neptune has a substantial internal heat source which gives rise to a dynamic atmosphere & weather.

As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets.

Have plausible theories been proposed for the source of the heat necessary to keep Neptune's ocean liquid?

Could there be sufficient methane in the atmosphere to produce a significant greenhouse effect?

• It's information I've acquired over the last 20 years & I cant remember what the sources were. It's the same with all the other information I have in my head,not only about astronomy.However,I assure you it's accurate. You only have to look at Neptune & compare it to Uranus to see it has a much more active atmosphere in spite of being further away from the sun. Jun 5 '19 at 8:10
• The most important fact you've omitted is that even with all the talk about water and methane in U&N's atmospheres, those atmospheres are still $\rm H_2$ and $\rm He$ dominated. I don't see where a water ocean should come into play there. Jun 6 '19 at 3:35
• I think the issue is not whether there is an ocean - there undoubtedly is. What is still in doubt is whether this ocean is frozen or not. I don't think we have any reliable estimate of surface temperature. The 'surface temperature' usually quoted is the temperature of the cloud tops. Jun 6 '19 at 5:45
• You did not answer to my comment: Why should there be an ocean in a gas mixture dominated by hydrogen and helium? Jun 6 '19 at 6:13
• No it is the most important 'detail': Water under high pressure (provided by the dominant $\rm H_2$&$\rm He$ atmosphere) & temperature becomes supercritical. en.wikipedia.org/wiki/Phase_diagram#/media/… Supercritical states don't form droplets (see the Venusian atmosphere, dominated by supercritical $\rm CO_2$, but no $\rm CO_2$-ocean) and therefore no rain, no ocean. Your question about the heat source is valid, I'm merely critisizing that for no reason at all you throw in an imaginary gas giant ocean. Jun 6 '19 at 7:42

Neptune and Uranus have estimated core temperatures of 7273 K and 5255 K respectively. Uranus and Neptune however likely do not have liquid water oceans. The combination of pressure and temperature on both planet results in unsuitable conditions for liquid water oceans. In fact for liquid water oceans to form on Neptune, it's predicted it would have to cool down.

Paper on this at: Cornell University

(Irrelevant, but fun) Diamonds oceans might exist. Uranus and Neptune are made up of 10% carbon some of which is methane. Methane when exposed to the conditions available in Uranus and Neptune can form diamond. Diamond can melt and form liquid diamond under again, the conditions on Neptune and Uranus. Therefore there could potentially be diamond oceans on Neptune and Uranus.

Paper on this at; Nature Materials

Two plausible theories considered for Neptune source of heat is that as you said, the methane concentration in the atmosphere is high enough to retain the formation heat after the formation of the planet. The other one is that the tidal forces generated by Triton's retrograde degrading elliptical orbit, but the paper on it concludes that it doesn't explain the difference between Neptune and Uranus.

Another thing to consider is that Uranus might be cooler than it should be rather than Neptune being warmer. This could be a result of an Earth sized indirect collision earlier in its lifetime. The collision causes this decrease in emitted temperature because it deposited a significant amount of material around the core which made heat transfer from the core to the outer parts of the planet much slower. Note that if this collision were to occur, it is what would've caused the unusual tilt of 97.7°.

Paper on this collision at: The American Astronomical Society

To the second question, the methane in the atmosphere of Neptune, as covered before, can produce a significant greenhouse effect.

Neptune emits 2.7 times more energy than it receives from the Sun. This access energy powers the atmosphere to produce the storms that are not seen on its twin planet Uranus. The source of internal energy cannot be due solely to leftover energy from formation (i.e. Jupiter) since Neptune is smaller and would have radiated away the energy long ago. Nor is it due to an unusual chemical change, such as the helium rain for Saturn. Rather, it appears that Neptune is more efficient at trapping leftover formation heat due to the fact that methane is highly abundant in Neptune's atmosphere, and methane is an excellent insulator of heat (i.e. the greenhouse effect). Neptune has a sub-zero type greenhouse effect that is trapping formation heat that should have been radiated billions of years ago like Uranus.

This extract is from: University of Oregon

In regards to why Uranus atmospheric methane doesn't appear to have as much of an effect as on Neptune, it could be that either Neptune simply has more methane in it's atmosphere or that a combination of factors, some of which mentioned above causes the difference. Beyond that I do not know.

Paper on Tritons Tidal Heating: University of Texas

• In view of the fact that Uranus is reputed to be the coldest of the planets,I don't see how it could have a liquid ocean,even though the H2O has some ammonia & methane in it. Your assertion that some of it might be diamond is also puzzling. Some of Earth's ocean is also diamond (Dianond Coast,S.Africa),but such a minute proportion of total volume that it's not worth mentioning..The methane of Uranus doesn't seem very effective at raising temperature,& I would expect a gigantic collision to raise the temperature rather than lower it. Jun 5 '19 at 14:36