Ice Giants – Uranus, Neptune
Even though these mantles are often referred to as icy, their interiors are very hot. The internal core temperatures of Uranus are thought to reach 5000 K (4726.85°C) and possibly a bit higher for Neptune. Core pressures of the planets are approximately double that of the Earth 7-8 Mbar (~700-800 GPa). Under these conditions the mantles are very hot dense liquids. Why then are they referred to as ices?
Look back to the phase diagram we discussed before. At high pressures and temperatures the boundary between the ice and liquid is nearly horizontal. The core pressures of 7-8 Mbar solidly within the ice-only field and even the mantles are icy mixtures. The take-away from this is that under extreme conditions substances are forced into atomic arrangements unlike anything we experience on Earth.
The icy mantles of the ice giants exhibit unusual properties and electrically are very conductive. Both Uranus and Neptune have a layer of superionic water. Despite its name, this is not a new health drink, but a theoretical phase of water under extreme heat and pressure which has properties of both a solid and a liquid.
[As an aside and not related to water, but odd things also happen to the methane. If you are interested in reading about a possible “diamond crystals that rain downwards like hailstones”, or "an ocean of liquid diamond, with floating solid 'diamond-bergs'" follow the links. Given the locations, the terrestrial diamond market is safe for the foreseeable future.]
One question remains. If there is water vapour in the atmosphere and a strange hot, liquid water ice mixture in the mantle, is there a layer of liquid water somewhere in between? This question has been addressed in a paper entitled “Liquid water oceans in ice giants”. The conclusion is that at present Neptune only has a 15% chance of a liquid ocean. That chance will increase when the Sun becomes a cool white dwarf as it ages. So, let’s check back in a few billion years.
