Professor Dominik Kraus of the University of Rostock is leading an international research effort that is reshaping how scientists understand water deep inside ice giant planets such as Uranus and Neptune. Using high-power laser compression and ultrafast X-ray measurements, Kraus and his collaborators have shown that under extreme pressure and temperature, water enters a superionic state with a far more complex atomic structure than previously assumed
Andriambariarijaona, L., Stevenson, M. G., Bethkenhagen, M., Lecherbourg, L., Lefèvre, F., Vinci, T., Appel, K., Baehtz, C., Benuzzi-Mounaix, A., Bergermann, A., Bespalov, D., Brambrink, E., Cowan, T. E., Cunningham, E., Descamps, A., Cafiso, S. D. D., Dyer, G., Fletcher, L. B., French, M., … Ravasio, A. (2025). Observation of a mixed close-packed structure in superionic water. Nature Communications, 17(1), 374. https://doi.org/10.1038/s41467-025-67063-2
Under ordinary conditions, water exists as a liquid or solid ice with well-defined molecular arrangements. Inside large planets, however, pressures rise to millions of atmospheres and temperatures reach several thousand degrees Celsius. In this environment, water no longer behaves like a conventional fluid or crystal. Instead, it becomes superionic, a phase in which oxygen atoms form a rigid lattice while hydrogen ions move freely through it. This combination gives the material solid-like structure alongside liquid-like electrical conductivity.
Superionic water has attracted attention because of its potential role in generating planetary magnetic fields. Uranus and Neptune possess magnetic fields that are strongly tilted and offset from their rotation axes, unlike Earth’s. Many planetary models suggest that thick layers of electrically conducting water inside these planets could help explain this unusual behavior. Determining the precise structure of superionic water is therefore important for understanding how magnetic fields are produced and sustained in ice giants.
Earlier laboratory experiments succeeded in creating superionic water, but its atomic arrangement remained uncertain. Simulations and indirect measurements suggested that oxygen atoms might adopt a simple cubic structure, either body-centered cubic or face-centered cubic. These models assumed a relatively uniform and orderly lattice, largely because the experimental tools available at the time could not resolve finer structural details.
The new study challenges this simplified picture. By recreating planetary interior conditions using powerful lasers, the researchers were able to probe the atomic structure of superionic water with much higher precision. Experiments were carried out at the Matter in Extreme Conditions instrument at the Linac Coherent Light Source in the United States and at the HED-HIBEF instrument at the European XFEL in Germany. In both cases, water samples were rapidly compressed and heated, and their structure was captured using ultrafast X-ray diffraction techniques.
The measurements revealed that superionic water does not settle into a single, uniform crystal structure. Instead, it contains a mixture of face-centered cubic regions and hexagonal close-packed layers. These different arrangements coexist and overlap, producing a lattice that is locally ordered but globally irregular. The result is a hybrid structure with frequent defects and stacking variations, rather than a clean, repeating pattern.
This mixed arrangement helps reconcile earlier theoretical predictions with experimental observations. Advanced computer simulations had hinted that multiple oxygen frameworks might be energetically similar under superionic conditions, but direct experimental confirmation was lacking. The new data show that superionic water behaves more like conventional ice, which is known to exist in many different crystal forms depending on pressure and temperature, than previously thought.
From an engineering and physics perspective, the discovery provides important constraints for models of planetary interiors. Electrical conductivity, heat transport, and mechanical behavior all depend on atomic structure. A mixed and partially disordered lattice could influence how currents flow and how magnetic fields evolve over time inside ice giants. These factors are critical for interpreting data from planetary missions and for predicting the internal structure of similar planets beyond our solar system.
The work also demonstrates the growing role of large-scale experimental facilities in studying extreme states of matter. Conditions found deep inside planets cannot be reproduced using conventional laboratory equipment. By combining high-power lasers, X-ray free-electron lasers, and precise diagnostics, researchers are now able to investigate materials under pressures and temperatures once accessible only through theory.
Beyond planetary science, superionic materials are of interest for their unusual transport properties, including fast ion conduction. While the conditions required for superionic water are far from practical for terrestrial applications, understanding how ions move through partially ordered lattices may inform research into solid electrolytes and other advanced materials.
The study involved more than sixty researchers from Europe and the United States and was supported by joint funding from German and French research agencies. As experimental capabilities continue to improve, scientists expect to explore a wider range of pressures, temperatures, and compositions, further clarifying how simple compounds like water behave in some of the most extreme environments known.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
