Wed. Nov 30th, 2022
Artistic rendering of a shock wave experiment on water, used to form exotic ice VII in the laboratory, against the backdrop of a hypothetical ocean world.
enlarge / Artistic rendering of a shock wave experiment on water, used to form exotic ice VII in the laboratory, against the backdrop of a hypothetical ocean world.

Lawrence Livermore National Laboratory

Kurt Vonnegut’s 1963 novel cat crib introduced the world with the so-called “Ice Nine”, a fictional form of water that freezes at room temperature. If it even touches a drop of plain water, that too will freeze, and so on, spreading so quickly that it freezes everything that comes in contact with it.

Luckily for Earth, Ice-Nine doesn’t exist. But there is an exotic form of ice called “ice VII” that physicists can create in the lab. It is harmless in terrestrial conditions. But on an ocean world like Jupiter’s moon, Europa, under the right conditions, it could behave just like Ice-Nine, freezing an entire world within hours — with some important implications for the possibility of finding life on distant exoplanets. . Now we know more about how that special freezing process takes place, according to a recent article in Physical Assessment Letters.

It is the shape formed by the water molecules that determine which phase of ice you get. That ice in your glass of whiskey is technically ice Ih– “h” for hexagon, because that is the shape in which all oxygen atoms line up during freezing. But in theory there should be at least 17 different crystalline phases of water – which one you get depends on the pressure and temperature of a particular environment.

Under pressure

ice cream Ih will turn into ice-II if you apply enough pressure, with oxygen atoms arranged in a diamond shape. Keep increasing the pressure and you’ll get Ice-III, Ice-IV, Ice-V, Ice-VI and so on. Even at insanely high temperatures of hundreds of degrees Celsius, some form of ice is theoretically possible, provided the pressure is high enough.

Ice-VII was found lurking in diamonds mined from Earth's mantle earlier this year.
enlarge / Ice-VII was found lurking in diamonds mined from Earth’s mantle earlier this year.

Ice-VII’s oxygen atoms are arranged in a cubic shape, something that only occurs at pressures more than 10,000 times higher than those on the Earth’s surface. It was created in the lab by zapping thin water samples between plates with high-intensity shock waves or laser pulses. In 2017, physicists at Stanford University captured the formation of ice VII (in just six nanoseconds) on film for the first time.

And earlier this year Science reported the surprising discovery of naturally occurring ice VII lurking in diamonds mined deep in the Earth’s mantle. The temperatures there are high, as is the pressure; that’s how diamonds form in the first place. Those are not ideal conditions for ice VII, but it doesn’t arise there either. If water bubbles get trapped in diamonds as they form (in cavities called inclusions), that water is exposed to cooler temperatures as it gradually moves to the surface, still being subjected to high pressure.

That’s how to get bags of ice-VII into diamonds. “Water in diamonds is not unknown, but finding this very high-pressure form of water ice intact was really a coincidence,” study co-author George Rossman, a mineralogist at Caltech, told the Los Angeles Times.

Computer simulation showing water molecules arranging to form ice VII.
enlarge / Computer simulation showing water molecules arranging to form ice VII.

Lawrence Livermore National Laboratory

In this latest paper, physicists at Lawrence Livermore National Laboratory (LLNL) have come up with a new mathematical model based on the idea of ​​nucleation. Nucleation is what happens at the beginning of the freezing process, when a few water molecules begin to arrange themselves into a solid molecular structure. That forms small ice crystals. Make sure the conditions are just right, and those crystals will grow larger and combine with their neighbors, spreading out until all the water in any given sample is frozen.

“Ice-VII forms by emerging in small clusters of about 100 molecules and then growing extremely fast, at more than 1,000 miles per hour.”

It turns out that even if the pressure seems high enough to form ice-VII, if it’s not above a very specific threshold, the nucleation will spread inward to the center of the sample. Cross that threshold though and you’ll get a quick phase transition as the nucleation spreads throughout the sample, so the ice forms and spreads very quickly – an “Ice-Nine” scenario. The nascent ice crystal and the surrounding water must be at different temperatures for this rapid freezing rate to occur.

“Our work shows that ice-VII forms in a very unusual way — by coming into existence in small clusters of about 100 molecules and then growing extremely fast, at more than 1,000 miles per hour,” Co- said. author Jonathan Promise to Physics Buzz. These could simply be the conditions that exist on so-called “ocean worlds”: bodies that, like Earth, have an abundance of water. “Water on the ocean worlds, under bombardment from other planetary bodies such as meteors or comets, is undergoing intense changes for which life may not survive,” he says.

The shock waves from those explosions would be enough to compress all the water to just the right high pressure to freeze it into ice VII at a sufficient depth (several hundreds of kilometers). And if that ice VII spreads quickly to the surface, it could spell the demise of any life on the said exoplanet. “Our goal is to learn as much as possible about [ice-VII] so we can find out if these planets can actually support life and what the limits of habitability might be,” says Belof.

DOI: Physical Assessment Letters2018. 10.1103/PhysRevLett.121.155701 (About DOIs).

By akfire1

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