Deep inside Neptune and Uranus, water is nothing like the substance that fills an ice cube tray. Under the colossal pressures and temperatures of those planets' interiors, it enters a bizarre state known as superionic ice — a phase in which oxygen atoms lock into a rigid solid grid while hydrogen nuclei drift through that grid like a liquid. Because those mobile hydrogen nuclei can carry charge, the material conducts electricity, a property that may be tied to the strange magnetic fields wrapped around both ice giants.
Researchers have long known this hot, conducting ice should exist. What they did not know was exactly which crystal structures it adopts under planetary conditions. Since no probe can dig a sample out of Neptune and haul it back to Earth, a team led by Alexis Forestier at the CEA, France's Alternative Energies and Atomic Energy Commission, did the next best thing: they rebuilt those extreme conditions inside a laboratory, and watched what the ice did.
Recreating an ice giant's interior in miniature
The team's experimental approach relied on a device called a diamond anvil cell, which traps a microscopic sample between two ultra-hard diamond tips and then squeezes them together. This setup allows researchers to generate pressures that exist nowhere naturally on Earth's surface. In this case, the scientists pushed their ice samples to as much as 230 gigapascals — more than two million times the pressure of Earth's atmosphere at sea level.
Pressure alone was only half the recipe. To mimic the searing heat of a planetary interior, the researchers fired lasers at the trapped ice, raising its temperature above 1,800 Kelvin. At that combination of crushing force and extreme heat, the sample entered the superionic regime.
To determine what was happening inside the tiny cell, the team turned to X-ray diffraction at the European Synchrotron Radiation Facility. This technique reveals how atoms arrange themselves by measuring how X-rays scatter off the sample, effectively giving scientists a picture of the crystal lattice without ever touching it. The results, published in the peer-reviewed journal Physical Review Letters, showed something the researchers had not previously pinned down.
A material with a split personality
Superionic ice is remarkable because it behaves like two states of matter at once. Its oxygen atoms occupy fixed positions, forming a crystalline skeleton that stays put. Meanwhile, the hydrogen nuclei — the protons that would normally be bonded into water molecules — become mobile, migrating through the oxygen framework much as ions move through a liquid electrolyte.
That division of labor is what makes the phase so unusual. The solid oxygen lattice gives the material a definite structure and shape, while the flowing hydrogen supplies electrical conductivity. In a planetary interior, a layer of such material could behave very differently from ordinary water ice, altering how heat moves, how material deforms over time, and how electrical currents circulate.
Until now, though, scientists lacked a clear answer about the geometry of that oxygen skeleton. Different crystal arrangements — cubic, hexagonal, and others — imply different physical properties, so identifying the correct one matters for any model of what happens inside an ice giant.
Hexagonal packing takes over at extreme pressure
The synchrotron measurements delivered a definitive answer for the conditions tested. Above roughly 200 gigapascals and 1,800 Kelvin, the oxygen atoms settled into a hexagonal close-packed arrangement, abbreviated hcp. In that pressure-temperature window, this hexagonal form became the dominant phase of superionic ice, displacing the face-centered cubic, or fcc, structure that had been expected to prevail.
The authors were explicit about the significance of the finding, writing that they report "the unambiguous observation of a novel H2O ice phase adopting an hcp oxygen sublattice." In other words, this is not a subtle refinement of an existing picture but the identification of a distinct arrangement of water's oxygen atoms under conditions relevant to real planets.
The discovery emerged from two separate experimental runs, and the team mapped their measurements onto a pressure-temperature diagram showing exactly where each phase appears. That kind of boundary map is what allows other researchers to predict which version of ice should be stable at a given depth inside a planet.
Why the crystal structure may matter for magnetic fields
The practical payoff of this work lies in planetary science. Neptune and Uranus are both classified as ice giants, and both possess magnetic fields that are far more complex than Earth's. Their fields are not neatly aligned with the planets' rotation axes, and the magnetic environment around them is chaotic compared with the tidy dipole that shields our own planet.
One leading explanation involves the deep interior layers where superionic ice is thought to exist. Because the material conducts electricity, moving currents within it could generate magnetic fields. The precise crystal structure of the ice affects how easily current flows and in which directions, so pinning down whether the oxygen lattice is hexagonal or cubic changes the inputs to those models.
By establishing that hcp ice dominates above 200 gigapascals and 1,800 Kelvin, the CEA-led team has given theorists a firmer foundation for simulating the interiors of these distant worlds — and for explaining why their magnetic fields behave the way they do.
Questions that remain open
The experiment captures a specific slice of pressure and temperature space, generated over short laboratory timescales in samples far smaller than a planet. Bridging that gap to full planetary interiors requires modeling, and the team's pressure-temperature diagram is best understood as a guide for that modeling rather than a complete description of Neptune or Uranus.
Still, the result sharpens a picture that had been fuzzy. Scientists now have direct evidence of which crystal structure superionic water prefers under ice-giant conditions, and they have a clear target for the next round of experiments and simulations.
Key takeaways
- A team led by Alexis Forestier at the CEA in France recreated ice-giant interior conditions in the lab using a diamond anvil cell.
- Ice was heated above 1,800 Kelvin and subjected to pressures up to 230 gigapascals, more than two million times Earth's atmospheric pressure.
- X-ray diffraction at the European Synchrotron Radiation Facility showed oxygen atoms forming a hexagonal close-packed lattice.
- Above roughly 200 gigapascals and 1,800 Kelvin, this hexagonal phase became dominant, replacing the face-centered cubic phase.
- The findings, published in Physical Review Letters, could help explain the unusual magnetic fields and deep interior layers of Neptune and Uranus.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
