Researchers have made a groundbreaking discovery in the field of planetary science by directly observing a hybrid phase of water known as plastic ice. This elusive form emerges at high temperatures and extreme pressures, exhibiting characteristics of both solid ice and liquid water. The team, led by physicist Livia Bove from Sapienza University of Rome, has documented these findings, which could significantly advance our understanding of icy moons and ocean worlds across the cosmos.
Plastic ice forms when temperatures soar above 177° C, and pressures exceed approximately 30,000 bars. Under these conditions, water molecules rearrange into a cubic crystal lattice, rotating at a speed akin to those found in liquid water. This dynamic behavior was first predicted 15 years ago through computer simulations that indicated ice VII, when subjected to such extreme environments, would exhibit free rotation of its molecules.
"Something intermediate between a liquid and a crystal, you can imagine that it is softer when you squeeze it," explained Livia Bove, shedding light on the unique nature of plastic ice.
The discovery holds immense implications for planetary science, particularly in understanding the evolution of icy moons into ocean worlds. Plastic ice may lurk at the bottom of massive oceans on exoplanets, some of which stretch thousands of kilometers deep. Such environments could potentially harbor life, making this research pivotal in the quest to discover habitable worlds beyond our solar system.
In plastic ice, as molecules rotate, they break hydrogen bonds with neighboring molecules only to swiftly form new ones. This constant reorientation results in a structure that bridges the gap between conventional ice and liquid water. In contrast, most ice on Earth's surface consists of water molecules organized into a hexagonal lattice.
Ice VII, a polymorph of ice with a dense cubic structure, forms at pressures above 20,000 bars. It has been discovered trapped within diamonds originating from Earth's mantle and is believed to exist inside other celestial bodies. The newly observed plastic ice VII may have played a role during the early formation stages of icy moons such as Europa and Titan within our own solar system.
The Institut Laue-Langevin in Grenoble, France, provided instrumental support for this study with its advanced tool capable of measuring molecular motions under extreme pressures. This facility enabled Bove and her colleagues to uncover the nuances of this hybrid phase of water.
Plastic ice joins a family of at least 20 known ice phases that manifest under various temperature and pressure conditions. These discoveries continue to expand our understanding of water's behavior under diverse environmental circumstances.
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