Kurt Vonnegut's 1963 novel Cat's Cradle introduced the world to so-called "Ice Nine," a fictional form of water that freezes at room temperature. If it so much as touches a drop of regular water, that will freeze, too, and so on, spreading so rapidly that it freezes everything that comes into contact with it.
Fortunately for Earth, Ice-Nine doesn't exist. But there is an exotic form of ice dubbed "ice VII" that physicists can create in the laboratory. It's harmless in terrestrial conditions. But on an ocean world like Jupiter's moon, Europa, it could behave just like Ice-Nine under the right conditions, freezing an entire world within hours—with some key implications for the possibility of finding life on distant exoplanets. Now we know more about just how that special freezing process occurs, according to a recent paper in Physical Review Letters.
ABSTRACT
The fundamental study of phase transition kinetics has motivated experimental methods toward achieving the largest degree of undercooling possible, more recently culminating in the technique of rapid, quasi-isentropic compression. This approach has been demonstrated to freeze water into the high-pressure ice VII phase on nanosecond timescales, with some experiments undergoing heterogeneous nucleation while others, in apparent contradiction, suggest a homogeneous nucleation mode. In this study, we show through a combination of theory, simulation, and analysis of experiments that these seemingly contradictory results are in agreement when viewed from the perspective of classical nucleation theory. We find that, perhaps surprisingly, classical nucleation theory is capable of accurately predicting the solidification kinetics of ice VII formation under an extremely high driving force (|Δμ/kBT|≈1) but only if amended by two important considerations: (i) transient nucleation and (ii) separate liquid and solid temperatures. This is the first demonstration of a model that is able to reproduce the experimentally observed rapid freezing kinetics.
Ice VII is a high-pressure form of ice that exists at pressures above 2.5 GPa (gigapascals) and temperatures below 130 K (Kelvin).
Ice VII is found in the interiors of large icy planets, such as Uranus and Neptune, and its nucleation process is crucial in understanding the formation and evolution of these planets. Additionally, since Ice VII can exist at high pressures and low temperatures, it has potential applications in materials science and high-pressure experiments.
Ice VII can be formed by compressing regular ice (Ice Ih) at high pressures and low temperatures. It can also be formed by decompressing Ice X, another high-pressure form of ice, at low temperatures.
The exact mechanism of Ice VII nucleation is still not fully understood. It is believed to involve a complex process of nucleation, growth, and transformation of different forms of ice, and is influenced by factors such as temperature, pressure, and impurities in the water.
Scientists use various experimental techniques, such as high-pressure cells and X-ray diffraction, to study the formation and properties of Ice VII. They also use computer simulations and theoretical models to understand the molecular dynamics involved in the nucleation process.