For years, textbooks have taught that water exists in three fundamental states—solid (ice), liquid (water), and gas (vapor or steam). However, beyond Earth’s familiar environment, in the extreme conditions of alien worlds, water can take on an entirely different form. Scientists have now provided the first direct evidence of a theoretical phase known as plastic ice VII, a high-pressure form of ice that blends properties of both liquid and solid states.
Breakthrough discovery under extreme conditions
An international team of researchers at the Institut Laue-Langevin (ILL) in France successfully created plastic ice VII by subjecting water to immense pressures of up to 6 gigapascals—roughly 60,000 times Earth’s atmospheric pressure—and heating it to 327°C (620°F). Their findings, published in Nature, provide long-awaited experimental confirmation of a 17-year-old theoretical prediction about how water molecules behave under such extreme conditions.
To observe this elusive phase, the scientists used quasi-elastic neutron scattering (QENS), a technique that tracks the movement of tiny particles such as hydrogen atoms. Their results confirmed that the hydrogen atoms within ice VII exhibit a microscopic swiveling motion when exposed to high temperatures and pressures.
A new form of ice with unique properties
Plastic ice VII is unlike any previously known form of ice. It possesses an interwoven molecular structure where the hydrogen atoms are somewhat disordered, giving it characteristics of both solid and liquid states.
“The ability of QENS to probe both the translational and rotational dynamics is a unique advantage for the exploration of such exotic phase transitions compared to other spectroscopic techniques,” said Maria Rescigno, a physicist involved in the study.
Interestingly, the research also suggests that plastic ice VII does not behave as expected. “The QENS measurements suggested a different molecular rotation mechanism for plastic ice VII than the free rotor behavior initially expected,” Rescigno added.
Implications for icy planets and moons
One of the biggest questions remaining is what happens when plastic ice VII “melts.” Some theories propose that while the hydrogen atoms move around, the larger molecular structure remains unchanged. Understanding this process could provide deeper insight into the geodynamics of icy planets and large moons.
Researchers believe this exotic form of ice may exist deep within icy planets such as Neptune and Uranus, as well as on Jupiter’s moon Europa. These findings could help explain the differentiation processes of large icy celestial bodies, offering new perspectives on planetary evolution and potentially even extraterrestrial water systems.
The discovery of plastic ice VII represents a major step forward in understanding water’s behavior under extreme conditions—a crucial piece in the puzzle of planetary science.
