Despite superficial appearances, the internal structure of ice giants like Uranus and Neptune is remarkably chaotic. Pressures millions of times greater than those at sea level, coupled with temperatures reaching thousands of degrees, give rise to highly peculiar materials.
A recent paper by researchers from the Carnegie Institution, published in Nature Communications, details an entirely new state of matter that could exist under these extreme conditions—a “quasi-one-dimensional superionic” phase.
Scientists have long understood that these ice planets are not composed of ordinary “ice” as we visualize it on Earth. Instead, they are hot, dense mixtures of water, ammonia, and methane.
However, replicating the conditions under which this slurry forms in a laboratory setting is nearly impossible. It would require terapascal pressures and temperatures high enough to melt most containment vessels.
Typically, to circumvent this issue, researchers turn to simulation—specifically, a modeling technique dubbed “Synthetic Uranus,” which mimics the environment of the seventh planet from the Sun, including pressure and temperature.
From prior chemical investigations, it was already known that common molecules like methane do not retain their familiar forms. Methane breaks down at pressures around 95 gigapascals, yielding hydrogen-rich materials alongside carbon allotropes such as diamond.
But even this simulation method has its limitations, ceasing to be accurate at even higher pressures.
To overcome this, the article employs a first-principles approach, allowing the quantum mechanics of the system to model the entire environment—as far as quantum mechanics permits modeling.
According to this simulation method, at pressures exceeding 1100 GPa, carbon and hydrogen form a stable compound, but with a highly unusual structure.
At this pressure, carbon atoms assemble into a rigid, fixed lattice shaped like a chiral helix—essentially, a microscopic, winding spiral staircase.
But the most intriguing behavior emerges upon heating. Normally, heating this crystalline structure causes it to transition into a liquid, allowing the atoms to move freely.
Yet, in certain other materials, such as water, an increase in temperature causes one set of atoms (oxygen, in water’s case) to remain in a crystalline solid state while the other set (hydrogen) begins to move freely. This state is known as the “superionic” state.
In the temperature range between 1000 and 3000 Kelvin, the new CH compound transitions into a superionic state, but with a twist. Instead of oxygen forming the lattice structure, as occurs in water, this crystalline framework is composed of carbon atoms.
The hydrogen atoms, confined by the carbon lattice, exhibit superionic diffusion along the spiral “staircase” (the z-axis) combined with rotational motion in the transverse (xy) plane.
Hydrogen atoms can easily travel up or down the ladder, but in other directions, they appear more likely to spin in place than to translate.
This unidirectional movement coupled with two-dimensional rotation led researchers to classify it as a hybrid type of “diffusive dimensionality”—the world’s first quasi-one-dimensional superionic state.
All of this is sound in theory, but what does it mean practically? The most notable consequence is that the material’s properties become anisotropic—meaning they change depending on the direction of measurement.
For instance, the material appears to conduct heat and electricity very well along the “staircase” axis, but less effectively along the other two axes. Furthermore, despite the presence of mobile hydrogen atoms (which carry a positive charge), electrical conductivity seems to still be determined by electron movement.
On a macro level, this helps substantiate theories about why the magnetic fields of Neptune and Uranus are so peculiar. Conventional models explain their tilted magnetic fields by assuming that hot superionic ices conduct heat and electricity equally in all directions.
However, with the emergence of this new quasi-one-dimensional superionic phase, that assumption is challenged and might actually align better with observational data gathered from the planets themselves.
Admittedly, the basic carbon-hydrogen material represents a massive simplification of the intricate chemical and thermal dynamics occurring within the cores of these worlds.
But the mere fact that we can model and understand how some of these materials might function in the real world shows that planetary science still has much to teach us about the workings of the Universe.
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