There is an ice hotter than a forge

At the pressures inside a giant planet, water forms a solid oxygen lattice with liquid-like hydrogen flowing through it. Experiments have made this superionic ice at thousands of kelvin.

Compress water hard enough and the oxygen atoms lock into a crystal while the hydrogens remain mobile, carrying charge. That is superionic ice. Laser-driven shocks have produced it and recorded its structure with nanosecond X-ray diffraction, at pressures of millions of atmospheres and temperatures of thousands of kelvin — hotter than lava, hotter than the melting point of iron at the surface, and solid only because the pressure will not let the oxygens leave their lattice. It absorbs visible light. It would not look like an ice cube.

Uranus and Neptune are the reason anyone cares. Their mantles are water, ammonia, and methane at these pressures, and a superionic layer would conduct electricity and could help explain their odd magnetic fields. That layer has not been sampled. It has been made, briefly, in a laboratory that can assemble the pressure and then must get out of the way of its own debris. The phase is real. The planet’s inventory of it is a reasoned guess.