Superionic ice contributes to the understanding of magnetic anomalies in Neptune and Uranus

Exotic water ice contributes to understanding of magnetic anomalies in Neptune and Uranus

Image from the XVIII Ice Simulation. Oxygen ions (red) occupy a regular crystal lattice, while protons (white) spread out like a liquid. Credit: Maurice de Kooning and Felipe Matuslim

Ordinary everyday ice, such as that produced by a refrigerator, is known as hexagonal ice (ice Ih), and it is not the only crystalline phase of water. More than 20 different stages are possible. One of them, called the “super-ice” or “ice eighteenth”, is of particular interest, among other reasons, because it is believed to make up much of Neptune and Uranus, the planets often referred to as “ice giants”.

In the superionic crystalline phase, water loses its molecular identity (H2O: negative oxygen ions (O2-) crystallize in a wide lattice, and the protons in the form of positive hydrogen ions (H+) form a liquid that floats freely within the oxygen lattice.

“The situation can be compared to a metallic conductor such as copper, with a significant difference in that positive ions Brazil.

De Koning led the study that resulted in an article published in Proceedings of the National Academy of Sciences (PNAS) and appeared on the cover of its November 8, 2022 issue.

He explained that super-ionizing ice forms at extremely high temperatures in the range of 5,000 K (4,700 degrees Celsius) and a pressure of about 340 gigapascals, or more than 3.3 million times the standard atmospheric pressure of Earth. Therefore, it is impossible for stable ionic supra-ice to exist on our planet.

However, it can exist on Neptune and Uranus. In fact, scientists are confident that large amounts of Ice XVIII lurk deep in their mantles, thanks to the pressure created by the huge gravitational fields of these giants, as confirmed by seismometric readings.

“The electricity that the protons conduct through the oxygen lattice is closely related to the question of why the magnetic field axis does not match the spin axis in these planets. It is, in fact, significantly skewed,” de Kooning said.

Measurements taken by the Voyager 2 space probe, which flew by these distant planets on its journey to the edge of the solar system and beyond, show that the axes of the magnetic fields of Neptune and Uranus form angles of 47 degrees and 59 degrees with the rotation of their respective axes.

Experiments and simulations

Down to earth experience reported in nature in 2019 He successfully produces a tiny amount of ice XVIII for 1 nanosecond (billionths of a second), after which the matter disintegrates. The researchers used laser-driven shock waves to compress and heat liquid water.

According to the paper in natureSix high-energy lasers were fired in a sequence designed to momentarily compress a thin laminated hydrophilic film between two diamond surfaces. The shock waves resonated between the two hard diamonds to achieve a homogeneous pressure of the water layer resulting in the ionic supercrystalline phase for a very short time.

“In this latest study, we didn’t do a real physics experiment but used it Computer simulation To investigate the mechanical properties of Ice XVIII and see how its deformations affect the phenomena seen to occur on Neptune and Uranus.”

A key aspect of the study was the publication of density functional theory (DFT), a method derived from Quantum mechanics They are used in solid state physics to solve complex crystal structures. “First of all, we investigated the mechanical behavior of a defect-free phase, which does not exist in the real world. Then we added defects to see what kinds of macroscopic distortions resulted,” he explained.

Crystalline defects are usually point defects characterized by ion vacancies or leakage of ions from other materials into crystal lattice. . Not so in this case De Koning was referring to linear defects known as “dislocations”, which result from angular differences between adjacent layers causing it to curl somewhat like a wrinkled rug.

“In crystal physics, dislocation was postulated in 1934, but was first observed experimentally in 1956. It is a kind of defect that explains many phenomena. We say that dislocation is mineralogy like the DNA of heredity,” said de Kooning.

In the case of ionic super-ices, the sum of the dislocations results in shear, a microscopic deformation familiar to metallurgists, metallurgists, and engineers. “In our study, we calculated, among other things, how much it is necessary to force the crystal to break up due to shear,” de Koonig said.

To this end, the researchers had to consider a relatively large cell of matter containing about 80,000 molecules. The calculations involved very heavy and complex computational techniques, including neural networks, machine learning, and the formation of various DFT-based configurations.

“This was the most interesting aspect of the study, integrating knowledge in mineralogy, planetary science, quantum mechanics and high-performance computing,” he said.

more information:
Filipe Matusalem et al, Plastic deformation of superionic water ice, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2203397119

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