Research published earlier this month in Physical Review Letters suggests that an exotic phase of water, known as “ice VII” could grow at rates exceeding 1,000 miles per hour under the atmospheric conditions found on alien ocean worlds.
Water exists in three main phases (solid, liquid, and gas) and the phase it occupies is a function of atmospheric pressure and temperature. The solid form of water—ice—has several phases of its own, however most of them exist at extremely low temperatures (but some of them can exist at temperatures upwards of 1300 degrees fahrenheit so long as the pressure is high enough.
Ice phases are differentiated according to the shape of the atoms in the ice crystals. For example, the ice you use to cool down a drink is known as Ih. The “h” stands for hexagon, which is the shape formed by the oxygen atoms in this phase of ice. Water can be directly converted into five different ice phases (Ih, ice III, ice V, ice VI, and ice VII) out of the total 17 theoretical ice phases. Each increase in the ice number corresponds to an increase in pressure needed to form that phase.
Ice VII made headlines earlier this year when it was discovered trapped in diamonds that were formed over 400 miles beneath the Earth’s surface. This fortuitous discovery suggested that liquid water exists deep within Earth’s core, but it was also the first time that ice VII was seen outside of a lab.
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Now, theorists at Lawrence Livermore National Laboratories have demonstrated how water transitions to ice VII, a process known as “nucleation,” which helps explain how this exotic phase of ice forms on alien ocean planets.
To study water’s phase transition to ice VII, researchers at various laboratories typically use shock waves to briefly compress liquid water at pressures over 100,000 times greater than Earth’s atmospheric pressure at sea level. The issue, however, is that these different experiments contradict one another in explaining how water transitions into ice VII. For example, one experiment suggested that ice VII begins to form on the surface of its container and then works its way in until the whole sample is frozen, whereas most other experiments suggest that the ice forms homogeneously throughout the entire sample and at astoundingly fast rates. In one case, a sample of water transitioned to ice VII in just 10 nanoseconds.
“The conditions created by shock compression are unusual in that they produce an enormous driving force for the system to nucleate,” Philip Myint, a physicist at LLNL and the lead author of the paper said in a statement. “The liquid is driven away from equilibrium so quickly that it takes additional time for clusters to appear, a process known as transient nucleation.”
This explains, for the first time, how homogeneous nucleation occurred “almost instantaneously” in a number of past ice VII experiments. The reason that some experiments showed ice VII nucleation starting at the boundary between the container and water, while others showed it happening throughout the sample has to do with the pressure levels inflicted on the water, as well as the temperature of the sample. Up to a certain level of pressure, the ice VII will start forming at the edges of the sample and work its way in. Beyond that critical threshold, however, the ice will form homogeneously throughout the sample. Yet this will only occur if the liquid water is a different temperature than the ice crystals that are forming. In their simulations, the LLNL physicists also discovered that ice VII initially forms in 100-molecule clusters before rapidly spreading through the sample.
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The work done on ice VII formation at LLNL will help astrobiologists who are looking for life on exoplanets that are covered in water. Although water is a prerequisite for life as we know it, in some extreme cases astrophysical phenomena might result in a scenario where much of a planet’s oceans turn into ice VII and preclude the emergence of life.
“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,” Jonathan Belof, a physicist at LLNL told Physics Central . “The shock wave 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 ice VII.”