Wednesday, October 17, 2018

Exotic "Ice VII" May Form on Ocean Worlds

Ice VII (or "ice-seven") is an exotic form of ice that grows so rapidly it could, under the right conditions, freeze an ocean-world's worth of water in just a few hours. A team of researchers from Lawrence Livermore National Laboratory (LLNL) has recently uncovered the unusual process by which that freezing takes place. Their results were published in the American Physical Society’s journal Physical Review Letters.

Foreground: An artistic representation of a shock wave experiment on water that leads to freezing under the explosive compression. Background: An artistic representation of a hypothetical ocean world containing a deep liquid ocean with thousands of miles of ice VII beneath it, followed by a rocky or metallic core at the center of the planet.
Image Credit: Lawrence Livermore National Laboratory.
Science fiction fans will note that this sounds eerily reminiscent of Kurt Vonnegut’s 1963 novel Cat’s Cradle. Ice-nine, the subject of the fictional story, is a form of water that is a solid at room temperature. When a crystal of ice-nine touches regular water, the water freezes. The trouble is, of course, that entire oceans and all life forms freeze on contact with this dangerous substance.

Thankfully, ice VII poses little threat to life on Earth.

Ice VII is just one of the many different forms water can take when it solidifies: Ice I, the hexagonal structure that gives snowflakes their shape, is the kind that forms at ordinary pressures. But under high pressure, the molecules can arrange into different shapes—and at more than 10,000 times the pressure on the surface of the Earth, Ice VII is formed. Scientists can only create it in the lab by using shock waves or energetic laser pulses to compress a thin sample of water between two plates.

This phase diagram shows the relationship between temperature, pressure, and the state of water.
Image Credit: Wikimedia Commons
You're not going to find the more exotic kinds of ice here on Earth—but it’s possible that ice VII could pose a threat to life on other planets.

Among the many planets discovered outside of our solar system, scientists have started to narrow down the places most likely to contain life based on factors like temperature, atmosphere, and the likelihood of water. An especially intriguing category under consideration is the ocean worlds—planets that astronomers think have vast liquid oceans even deeper than our own.

While the existence of liquid water hints at the possibility of life, it also raises important questions about water under extreme conditions. “Water on these ocean worlds, under bombardment from other planetary bodies such as meteors or comets, undergoes intense changes for which life might not survive,” explains the project leader, LLNL’s Jonathan Belof. You might think that an ocean planet would be safer in the face of an asteroid strike—after all, the dust cloud kicked up by such an event is one of the main dangers it poses. But on a water world, there's a different threat: “The shock waves launched by the explosions from these planetary impact events can compress water to a pressure over 10,000 times that found on the Earth’s surface and cause the water to freeze into an exotic form known as ice VII,” says Belof.

This means that ocean worlds probably have a layer of ice VII below their liquid water, a layer that could be several hundreds of kilometers deep. “Our aim is to understand as much as possible about the high-pressure phase of water, ice VII, so that we can figure out if these planets really can support life and what the limits of habitability might be,” says Belof.

The first step toward achieving this goal was uncovering something that has long puzzled researchers: how ice VII forms. The details of compression experiments vary, as do the peak pressure at which the water is squeezed. The results vary too, in terms of how long it takes the ice to form and where the process seems to originate.

Theoretical models of freezing haven’t done a very good job of explaining the range of experimental results, so the LLNL researchers started here, combining theory, simulation, and an analysis of past experiments.

They began with the classic mathematical model of nucleation, the first step of freezing. During nucleation, some of the water molecules self-arrange into the structure of a solid—essentially forming ice crystals that, under the right conditions, grow and merge until the whole sample is frozen. From this model, the researchers looked at how nucleation occurs under different conditions and analyzed the results.

An image from a computer simulation of water molecules ordering to form ice VII.
Image Credit: Lawrence Livermore National Laboratory..

Their work indicates that at pressures high enough to initiate the formation of ice VII but lower than a certain threshold value, nucleation occurs at the water-plate interface and spreads inward toward the center of the sample over time. But at pressures greater than that threshold value, the nucleation occurs all throughout the sample—a rare occurrence that causes the ice to grow rapidly.

Although this analysis generally aligned with experimental results, the researchers wanted to see if their model would predict specific values that matched experimental data. So, they took their research a step further and built a comprehensive computer simulation. After a few months off to revise a key equation describing their system, they found that their nucleation-based simulation results matched the experimental data well, given two key conditions.

First, the growing ice crystal and the surrounding liquid water must be at different temperatures. Normally when ice forms, an ice crystal and the surrounding water is at the same temperature. However, in this case the rapid freezing rate and high pressure keep the ice and liquid water at different temperatures.

The second key condition is that the simulation needs to include a time lag related to ice cluster formation. When ice clusters form in the real world, you don’t know it until they're large enough to observe. To match experimental data then, the simulation needed to account for the time it takes an ice cluster to go from microscopic to macroscopic in size.

Together, these results explain the ice VII formation process and ties together the results of different experiments. “Our work shows that ice VII forms in a very unusual way – by popping into existence in tiny clusters of about 100 molecules and then growing extremely fast, at over 1000 miles per hour!,” says Belof. “Who knows, such growth rates might actually be occurring in the center of these ocean worlds,” he says.

This work also highlights the question of how systems behave when they are rocked by extreme conditions or, in the words of the first author of this research, Philip Myint, when they are “far from equilibrium.”

“When a system is driven strongly away from equilibrium, how will it respond? I think that most folks would be surprised to know that we have almost no understanding of that very fundamental problem,” says Myint. “If we can truly figure that out, it could open up entirely new discoveries and technologies. Our hope is that nucleation far from equilibrium, such as is in this case of shock compressed water, might be a small example to start us on that path.”

Kendra Redmond

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