Because, inside, the pressure is enormous and the temperatures reach thousands of degrees. There, the water could take on a very strange form: superionic ice.
On Earth, we usually associate ice with cold, but the state of a substance also depends on pressure. When both conditions are extreme, molecules can rearrange themselves in ways impossible in our daily lives. For this reason, scientists investigate what happens to water at great depths on giant planets, where it would not necessarily behave as it does on the Earth’s surface.
On Neptune, each layer supports the weight of the material above it. The further we go into the planet, the greater that burden becomes. Thus, to know what state the water would be in at a depth, we need to know both the temperature and the pressure it would endure there.
It’s not ice like the one in our freezer. Its oxygen atoms remain ordered, forming a solid structure, while the hydrogen nuclei move through it as if they were a liquid.
At this stage, the word “ice” does not mean that everything is still. The oxygen network maintains some organization, but protons can circulate between its positions. That combination of solid structure and internal mobility is what distinguishes the superionic state. In addition, the movement of charged particles allows the transport of electric current.
To imagine it, let’s think of a grid drawn on the ground. The intersections mark the positions of the oxygen network, while marbles rolling between them represent the protons. The drawing retains its shape even if the marbles change places. Thus the order of the network and the movement of charges can coexist.
To study it, scientists compressed water between two diamonds and heated it with lasers. Thus, they observed a new arrangement of oxygen atoms, organized in hexagonal layers.
One way to visualize these layers is to place coins on a table. If we squeeze them in alternating rows, each coin inside is surrounded by six others. We can imagine a second layer whose coins rest in the gaps of the first. This stacking helps to understand the type of order they found in the sample.
Diamonds allow gigantic pressures to be exerted on tiny samples. To examine its structure, techniques are used to reveal how the atoms are arranged. The results show that, under certain conditions, superionic ice can present a geometry different from those previously observed. It does not imply that the entire interior of Neptune is made up of that same variety. Planetary models must also consider other substances, mixtures and conditions that vary with depth.
The next step is to compare the experimental conditions with those predicted by the models. Even if they match, it remains to be estimated how much material would be in that phase and whether it would occupy a narrow strip or a broader portion of the planet.
This material can conduct electricity and could help explain Neptune’s peculiar magnetic field. But we haven’t seen that ice directly inside the planet yet.
REFERENCES
Alexis Forestier, Gunnar Weck, Sandra Ninet, Gaston Garbarino, Mohamed Mezouar, Frédéric Datchi and Paul Loubeyre. “Observation of Hexagonal Close-Packed Water Ice at Conditions in Ice Giant Planetary Interiors.” Physical Review LettersSeptember 9, 2026.
