Scientists observe exotic ice that could exist on alien planets for the first time

Researchers have experimentally observed a rare phase of water known as plastic Ice VII for the first time, a form that was previously only predicted by theoretical models. This discovery sheds new light on how water behaves under extreme conditions and could help explain the composition of distant planets and moons.

Ice like no other

The study has been published in Nature. Unlike regular ice, plastic Ice VII requires extreme conditions to form. Scientists subjected water to pressures as high as 6 gigapascals (nearly 60,000 times atmospheric pressure) and temperatures up to 327°C (620°F) at the Institut Laue-Langevin (ILL) in France. The high-pressure, high-temperature environment forced water molecules into a distinct interwoven cubic structure, with hydrogen atoms behaving unusually.

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In this state, hydrogen atoms were observed moving erratically, leading to a phenomenon that had long puzzled researchers. The challenge in detecting plastic Ice VII lay in tracking these hydrogen movements, which required highly sensitive techniques rather than simply mapping the ice’s structure.

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Breakthrough using advanced techniques

To capture the elusive phase, the research team used quasi-elastic neutron scattering (QENS), a method that tracks the movement of tiny particles like hydrogen atoms. This confirmed a 17-year-old prediction that hydrogens within Ice VII swivel at a microscopic level when subjected to high temperatures and pressure.

However, an unexpected discovery emerged—rather than rotating freely, the molecules inside plastic Ice VII moved in staggered steps. Scientists believe this is due to the breaking and reforming of hydrogen bonds within the structure.

"The QENS measurements suggested a different molecular rotation mechanism for plastic Ice VII than the free rotor behaviour initially expected," explains Maria Rescigno, a physicist from the Sapienza University of Rome.

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Implications for distant worlds

This exotic ice could have once existed on Neptune, Jupiter’s moon Europa, and other icy worlds in our Solar System. Understanding how plastic Ice VII forms and behaves could provide insights into the geophysical processes of these celestial bodies.

Another intriguing question remains—how exactly does the transition to plastic Ice VII occur? Scientists are investigating whether the process is gradual or sudden. If the transition is continuous, it could mean plastic Ice VII serves as a precursor to an even rarer state of water called the superionic phase, where hydrogen moves freely through an oxygen crystal lattice.

"The continuous transition scenario is very intriguing," says physicist Livia Bove. "It hints that the plastic phase could be the precursor of the elusive superionic phase—another hybrid exotic phase of water predicted at even higher temperatures and pressures, where hydrogen can diffuse freely through the oxygen crystalline structure."