New clues about superionic ice, possibly present in giant planets

Neptune and Uranus are two giant planets that have only been explored once, thanks to the NASA Voyager 2 expedition, almost 40 years ago. At the time, the probe registered strikingly unusual magnetic fields from the planets: unlike the other planets in the solar system, both Neptune and Uranus exhibit non-axisymmetric and non-dipolar magnetic fields. The origin of these peculiar fields could be explained by the presence of superionic ice in the interior structure of these ice giants.

Superionic ice is an exotic form of water ice that only exists under extreme pressures and temperatures.  In the superionic state, hydrogen ions from H2O molecules move almost freely within a solid oxygen cage. The oxygen atoms can adopt different arrangements, with the dominant form studied here exhibiting a face-centered cubic (FCC) lattice. In this structure, oxygen ions are arranged in a repeating cubic pattern with extra atoms at the cube's faces. The hydrogen ions diffuse within this structure, making this form of ice both solid (oxygen lattice) and liquid-like (mobile hydrogen), which is key to its unusual properties, such as its electrical conductivity.

Theoretical studies predicted the existence of superionic ice in the 80s, but it is only in recent years that laser induced shock experiments at the National Ignition Facility (NIF) have obtained data indicating the superionic nature of ice and confirmed a FCC structure. Researchers found that superionic ice transitions to an FCC oxygen lattice at pressures exceeding 100 GPa (gigapascals), corresponding to one millon times the atmospheric pressure, and at several thousand degrees, similar to conditions deep within Neptune and Uranus.

Discrepancies about temperature

However, different scientific groups disagree on the temperature at which the FCC superionic ice phase forms. Two publications, both using X-ray diffraction at synchrotron sources coupled with diamond anvils cells for generating extreme static pressures, came out 3 years ago with different conclusions. The transition lines showing the conditions under which FCC superionic ice becomes stable are strikingly different. At the highest pressure investigated, near 160 GPa, the first study places it around 2000 K, while the other paper states that it is at a higher temperature of around 3500K.  Resolving this discrepancy is crucial for understanding the deep interiors of Neptune and Uranus.

Now one of the two teams, led by the Commissariat à l’Energie Atomique (CEA) in collaboration with the Centre National de la Recherche Scientifique (CNRS) and Sorbonne University, which suggested the lower temperature transition line, has drawn the controversy to a close. “It is a challenge to replicate extreme planetary conditions in the lab, so we reproduced the experiment this time with a new geometry for the sample and exploiting the flux of the new EBS”, explains Alexis Forestier, researcher at CEA and corresponding author of the publication.

The team created a set-up of a sample confinement micro-oven inside the diamond anvil cell, which enabled temperature and pressure to be more homogeneous. The ice sample was a few micrometre in size, enclosed in a boron-doped diamond capsule and heated by lasers coming from two sides. “Before, you would get a large temperature gradient within the sample chamber, but we’ve now overcome this issue”, adds Gunnar Weck, researcher who has lead CEA experiments on superionic ice at ESRF since 2015.

Signature of the superionic transition

The results confirmed previous results, showing that the FCC phase is stable around 2000K beyond 150 GPa. This lower temperature transition line is in agreement with ab initio calculations and previous shock compressions experiments. “Sample preparation and optimisation plus the new developments of beamline ID27 in terms of metrology and the high flux enabled us to master the temperature measurements and acquire outstanding data”, says Gunnar Weck.

Phase diagram of H2O. The onset of superionic state in fcc ice in its metastable domain, deduced from the thermal expansion bump, is shown as green squares. The calculated onset of superionicity is shown as a green dashed line. Red dots indicates the structural bcc-fcc transition. The large red line shows our guess for the bcc-fcc phase boundary, which brings together the concordant experimental observations. Previous experimental data from static and dynamic studies are included, as well as calculated phase boundaries. Credits: Forestier, A., et al, PRL, 2025.

These improvements allowed to reveal an anomalously large thermal expansion within FCC ice when the protons start to rapidly diffuse across the crystal lattice of oxygen atoms. This new result constitutes a clear structural signature of the superionic transition in FCC ice, shedding light on the specifics of this transition and challenging further theoretical investigations.

The next step for the team is to pursue the exploration of superionic ice over a larger range of pressures, under conditions where theoreticians have already predicted new arrangements for the oxygen lattice.

Reference:

Forestier, A., et al, Physical Review Letters, 10.1103/PhysRevLett.134.076102

Text by Montserrat Capellas Espuny

Top image: View of Uranus from the Hubble mission. Credits: NASA.