A time-integrated photograph of the X-ray diffraction experiment at the University of Rochester’s Laboratory for Laser Energetics. | Quanta Magazine
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Kodaikanal: It’s ice, but not quite. It isn’t cold but extremely hot, can exist only at extraordinarily high pressures, and is black.

Researchers in the US have created ‘superionic water ice’, a form of crystalline water that may help scientists better understand the universe, especially the solar system’s most distant planetary denizens, Uranus and Neptune, which are believed to be primarily formed of this.

The findings, derived by researchers from Lawrence Livermore National Laboratory, California, and Laboratory for Laser Energetics and Department of Mechanical Engineering, University of Rochester, New York, were published in the journal Nature last week.

The various forms of water ice

Water exists in many different crystalline forms, simply referred to as ice. The most common form of ice has a hexagonal arrangement of water molecules, and is designated ‘Ih’. Around 18 architectures of ice are already known to scientists, numbered from I to XVII, with ice I having two forms — ‘Ih’ and ‘Ic’.

Superionic water ice can now be designated ‘Ice XVIII’. It is neither solid, nor liquid, but kind of both. Its oxygen atoms are solidified into a cube, with the hydrogen atoms flowing freely like a liquid within a rigid oxygen cage.

“Because its water molecules break apart, it’s not quite a new phase of water,” physicist Livia Bove of France’s Pierre and Marie Curie University, who wasn’t involved in the research, was quoted as saying by Quanta magazine, an online science publication.

“It’s really a new state of matter,” she added.

It’s been three decades since computer simulations first indicated its existence.

Scientists were able to observe experimental evidence of its existence last year and also measured some of its macroscopic properties, like temperature and internal energy, but a new experiment was required to determine its atomic structure.

Design of the experiment

A thin layer of liquid water between two diamond anvils was compressed to pressures up to 1-4 million times that of Earth’s atmosphere.

Six giant lasers were employed to produce high pressure, by generating a series of shockwaves with progressively increasing intensity.

Simultaneously, the temperature within the sample was raised to about half that at the sun’s surface.

These extreme conditions forced the water sample to freeze into a heated ‘superionic’ ice phase, which the researchers wanted to photograph. But these pressure and temperature conditions couldn’t be maintained for more than a fraction of a second.

“Given the extreme conditions at which this elusive state of matter is predicted to be stable, compressing water to such pressures and temperatures and simultaneously taking snapshots of the atomic structure was an extremely difficult task, which required an innovative experimental design,” said physicist Federica Coppari at Lawrence Livermore National Laboratory, co-lead author of the paper, in a press statement about the study.

To work around this, researchers blasted a tiny piece of iron foil with 16 additional laser pulses. This created a hot plasma, which generated a precisely-timed flash of X-rays illuminating the superionic water ice in its stable form.

These flashes, upon striking the crystals, got diffracted and revealed the positions of oxygen atoms, thereby confirming that the compressed water was indeed frozen and stable.

A never-before-seen structure of superionic water ice — cubic crystals with oxygen atoms at each corner and in the centre of each face — was seen via X-rays.

“Computer simulations have proposed a number of different possible crystalline structures for superionic ice. Our study provides a critical test to numerical methods,” Coppari said.

“If you really want to prove that something is crystalline, then you need X-ray diffraction,” Christoph Salzmann of University College London, who discovered ices XIII, XIV and XV but wasn’t involved in this study, was quoted as saying in the Quanta article.

“All of this would not have been possible, say, five years ago. It will have a huge impact, for sure,” he said.

“Finding direct evidence for the existence of crystalline lattice of oxygen brings the last missing piece to the puzzle regarding the existence of superionic water ice,” Lawrence Livermore National Laboratory physicist and co-lead author Marius Millot was quoted as saying in the press statement.

“This gives additional strength to the evidence for the existence of superionic ice we collected last year,” Millot added.

Digging deeper into the universe

Although superionic water ice may not sound useful to the inhabitants of Earth, scientists think it might be the most common form of water present in the universe.

Up to 65 per cent of the constitution of icy planets in the solar system, like Uranus and Neptune, and similar exoplanets, is crystalline water. The conditions on these planets are presumed to be exactly right for superionic water ice to exist naturally.

Observations carried out by the NASA probe Voyager 2 showed that the magnetic fields emanating from Uranus and Neptune didn’t seem similar to Earth’s, which is produced by our rotating, molten iron outer core.

They looked lumpier and complex, with more than two poles, indicating a weaker process of generation. Their alignment was also not synchronised to their planets’ rotation.

Magnetic fields of astrophysical objects are primarily produced by a “dynamo”, like the liquid molten outer core of Earth. The weak and lumpier magnetic fields of Uranus and Neptune indicated their origin from a weak dynamo, which couldn’t be at their core.

Existing research implies that these icy planets may have mantles composed of superionic water ice. Their peculiar magnetic fields may have their origin from the flow of superionic water ice.

“Because water ice at Uranus’ and Neptune’s interior conditions has a crystalline lattice, we argue that superionic ice should not flow like a liquid such as the fluid iron outer core of the Earth,” Millot said.

“Rather, it’s probably better to picture that superionic ice would flow similarly to the Earth’s mantle (the layer next to a planet’s core), which is made of solid rock, yet flows and supports large-scale convective motions on very long geological timescales,” he added.

“This can dramatically affect our understanding of the internal structure and the evolution of the icy giant planets, as well as all their numerous extrasolar cousins.”

Mywish Anand is a winter intern at Kodaikanal Solar Observatory. He has a Master’s degree in Computational Physics from the Central University of Punjab, Bathinda.

Also read: The universe is expanding 9% faster than expected, but it’s unclear why


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