Exotic ice could reveal mysteries of Neptune and Uranus’ magnetic anomalies

Ordinary ice – the one produced in domestic refrigerators and known scientifically by the name of Ih ice – is not the only crystalline phase of water. There are over 20 different possible stages. One of them, called “superionic ice” or “eighteenth ice”, is of special interest. Among other reasons, because it makes up a large part of the stuff of the planets Neptune and Uranus, also known as “ice giants”.

In the superionic crystalline phase, water loses its molecular identity (H2O); negative oxygen ions (O2-) are arranged in an extensive crystal lattice; and the protons, which constitute the positive hydrogen ions (H+), form a fluid that circulates through the lattice.

“The situation is analogous to that of a conductive metal, such as copper, with the big difference that, in the metal, the positive ions form the crystal lattice, while the electrons, carriers of the negative electrical charge, are relatively loose, circulating through the network”, says researcher Maurice de Koning, professor at the Gleb Wataghin Institute of Physics at the State University of Campinas (IFGW-Unicamp).

De Koning coordinated the study that resulted in an article featured on the cover of the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

As the researcher explains, superionic ice forms under extremely high temperatures, at the level of 5,000 kelvins (4,700 °C), and extremely high pressures, on the order of 340 gigapascals. This value is more than 3.3 million times greater than Earth’s standard atmospheric pressure. Therefore, it is impossible to have stable superionic ice in the terrestrial environment.

On Neptune and Uranus, however, the pressure resulting from the enormous gravitational fields of these giant planets makes it possible for large amounts of ice XVIII to exist in the inner layers closest to their respective cores. Seismographic measurements confirm that this actually happens.

“The electricity conducted by protons through crystalline networks of oxygens is closely linked to the question of why the axes of these planets’ magnetic fields do not coincide with their axes of rotation. They are, in fact, quite out of place,” says De Koning.

Measurements made by the Voyager 2 space probe, which approached these distant planets on its journey to the outer reaches of the Solar System and beyond, show that the axes of Neptune and Uranus’ magnetic fields form angles of 47 degrees and 59 degrees with their respective axes. of rotation.

Experiments and simulations

On Earth, an experiment reported on Nature magazine in 2019 it managed to produce a tiny amount of ice XVIII that remained for a nanosecond, that is, a billionth of a second, before breaking apart. This was accomplished by using laser-created shock waves launched onto a sample of water.

As the authors of the experiment described, six high-power laser beams were fired in a precise time sequence to compress, by means of shock waves, a thin layer of water encapsulated between two diamond surfaces. The shock waves reverberated between the two hard diamonds, providing a homogeneous compression of the water that resulted, for a short time, in the superionic crystalline phase.

“In the study carried out now, we did not do a real physical experiment, but we used computer simulation to investigate the mechanical properties of ice XVIII and find out how its deformations influence the behaviors observed on the planets Neptune and Uranus,” reports De Koning.

The researcher says that the study made use of the Density Functional Theory (DFT), a method derived from quantum mechanics and used in solid physics to solve complex crystalline structures. “We first investigated the mechanical behavior of a defect-free phase, which does not exist in the real world. Then we add defects to know what kinds of macroscopic deformations result from that,” he explains.

When talking about defects in crystals, the expression usually refers to point defects, characterized by the vacancy of ions or the intrusion of ions from other materials into the crystal lattice. However, that is not what this is about. The defect mentioned by De Koning is not punctual, but linear. It’s called “discordance” and it occurs when one phase of the crystal slips over another phase. The result is similar to what occurs when a rug is pushed across the floor lengthwise, producing a transverse ripple in the rug.

“In crystal physics, discordance was postulated in 1934. But it was only observed experimentally for the first time in 1956. It is a defect that explains a large number of phenomena. We usually say that it is for metallurgy what DNA is for genetics”, underlines the researcher.

In the case of superionic ice, the sum of dislocations produces a macroscopic deformation well known to mineralogists, metallurgists and engineers: shear. “In our study, we calculated, among other things, how much force it takes to break the crystal through shear,” says De Koning.

For this, the researcher and his colleagues needed to consider a very large cell of the material, with about 80,000 molecules. The calculations required extremely heavy and sophisticated computational techniques, employing neural network and machine learning, and composing various configurations based on DFT calculations.

“This was a very interesting aspect of our study, the integration of knowledge from several areas: metallurgy, planetology, quantum mechanics and high-performance computing”, concludes De Koning.

The work received support from the Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp) through a Post-Doctoral Scholarship granted to the first author, Filipe Matusalém de Souza, under the supervision of De Koning; a Thematic Project coordinated by Unicamp researcher Alex Antonelli; and the Center for Engineering and Computational Sciences (CCES), funded under the Research, Innovation and Dissemination Centers (CEPIDs) program.