Ordinary ice is a crystal of water molecules: every hydrogen (proton) is bonded to one oxygen and the whole lattice is rigid. Squeeze water to tens of gigapascals and heat it to a few thousand kelvin — the pressure and temperature found a few thousand km below the cloud tops of Uranus and Neptune — and it enters a strange intermediate phase: superionic ice. The oxygen atoms stay locked in a crystal lattice (it is still technically a solid), but the protons become mobile, hopping between interstitial sites and diffusing through the fixed oxygen sublattice almost like ions in a liquid electrolyte.
Illustrative phase boundaries used here (schematic, not a precise EOS):
Melting curve: T_melt(P) ≈ 273 + 6·P [K, P in GPa]
Superionic onset: T_super(P) ≈ 1200 + 6·(P−20) [only for P ≥ 20 GPa]
Full ionic fluid: T_fluid(P) ≈ 4500 + 3·(P−50)
Proton diffusion (Arrhenius-style): D ≈ D0 · mobility(P,T)
Conductivity (order of magnitude): σ ≈ mobility(P,T) · 200 S/m
- Pressure / Temperature sliders — move the state point across the phase diagram; the lattice redraws live as protons transition from vibrating-in-place, to hopping between sites, to fully free.
- Planetary presets — jump straight to the pressure–temperature conditions in Uranus's and Neptune's ice mantles (where laboratory shock-compression experiments, e.g. Millot et al. 2018–2019, first confirmed superionic ice), a cold comet surface for contrast, and the mantle-core boundary where ice finally becomes a fully ionic fluid.
- Show O–H lattice bonds — toggles the thin bond lines that make the ordered-solid structure easier to read.
Why it matters: a shell of superionic ice is highly electrically conductive while still being mechanically rigid enough to support convection differently than a liquid core would. Models suggest this thin conducting shell — rather than a deep metallic core, as in Earth or Jupiter — generates the strangely lopsided, strongly tilted and off-centre magnetic fields Voyager 2 measured at Uranus (1986) and Neptune (1989), instead of the clean dipole fields seen at most other planets.