If Neptune was a bit cooler then it would have a deep ocean full of dissolved hydrogen, instead of what’s probably a super-critical atmosphere of dense “vapour”. According to modelling, by Sloane Wiktorowicz and Andrew Ingersoll, Voyager’s gravity data for Neptune rules out an ocean, which would be denser at a shallower depth than an all vapour atmosphere. Yet there’s a chance that Voyager tracking wasn’t quite as precise as certainty in this matter requires, so Neptune might yet be an Ocean Planet.
Ocean planets are a fairly new idea, based on advanced modelling of the Equations of State of silicates, ice and iron/nickel. Basically the planets are heavy enough to fully differentiate according to the density of their most common elements and compounds – volatiles on top, silicates in-between and dense iron-nickel alloy in the Core. The most common volatile is, of course, water, followed by carbon dioxide (after methane decomposes) which dissolves and precipitates into a layer within the water. Thing with water is that it forms a huge range of ices. We know ice as amorphous ice and crystalline ice, but under sufficient pressure liquid water will form (mostly) Ice VII, which can remain solid up to quite high temperatures (its triple point at 22,000 atmospheres is at 85 degrees C.) While thousands of atmospheres of pressure isn’t achieved anywhere in Earth’s oceans an Ocean Planet is half ocean, which means Ice VII (and higher pressure phases of ice) forms a thick, dense mantle surrounding the inner silicate mantle and the core.
What results is a liquid ocean about 100 km deep, a layer of carbonate, then Ice VII beneath that for thousands of kilometres. The atmosphere can be H/He, but if the planet is small enough it might not capture a lot of primordial gas, and instead it may out-gas the standard chondritic mix recently determined by geochemical analysis – methane, ammonia, nitrogen and hydrogen. Both the methane and ammonia will eventually be oxidised by reacting with oxygen produced by photolysis of the water, making more water, nitrogen and carbon dioxide. On a smaller world an aerosol of longer hydrocarbons would form a haze protecting the ammonia and methane somewhat, but a larger world’s atmosphere would wash it out before it could accumulate.
Would Life find a foothold on such a world? You might think that so much water meant an idyllic environment for life, but in reality oceans are vast deserts with a very thin veneer of life. Most of the action happens in shallow seas and lakes, something lacking on an Ocean Planet. Yet I can’t shake the thought that methanogenic life might not cause a different outcome for carbon on such a world, turning it into eventually a slick of hydrocarbons, instead of deeply buried and dissolved carbonates. But I very much doubt animal life and an oxygen biosphere.
As PhysOrg News has noted Ocean Planets may be detectable very soon.