Abstract:
Ice usually means low temperature, but scientists have now confirmed that in extremely high-pressure environments, water can form a special kind of "ice" that still exists at temperatures exceeding 2000°C. A French research team recently successfully created a new type of ice in the laboratory that was previously theoretically predicted to exist - hexagonal close-packed superionic ice. This material is neither completely solid like ordinary ice nor can flow freely like liquid, and its unique structure may help explain the unusually complex magnetic fields inside ice giant planets such as Uranus and Neptune.
Water is one of the most common substances on earth and is indispensable for life. However, from a physical and chemical perspective, water actually has many very abnormal properties. For example, the volume of ordinary liquid water will expand when it freezes. The density of ice is lower than that of liquid water; water also has an unusually high surface tension and boiling point. If calculated purely based on the molecular weight of water molecules, water should even be more likely to exist in the form of a gas at room temperature.
When the environment shifts from the surface of the Earth to the interior of the planet, while simultaneously imposing extreme temperatures and pressures, water behaves in even more peculiar ways. Researchers simulated the extreme conditions inside Uranus and Neptune in the laboratory this time, and directly captured the crystal structure of hexagonal close-packed superionic ice for the first time.
The so-called "superionic ice" is a special state of matter between traditional solids and liquids. When water is subjected to enormous pressures of millions of atmospheres, the material is able to maintain a solid-like lattice structure even at very high temperatures. In this state, complete water molecules in the traditional sense no longer exist. The oxygen atoms are fixed in the crystal lattice position, while the hydrogen nuclei can move quickly between the crystal lattice composed of oxygen atoms like particles in the liquid.
Therefore, from a macroscopic perspective, this substance has some characteristics of both solid and liquid: oxygen atoms form a stable solid lattice, while hydrogen atoms are highly mobile. This unique structure also makes superionic ice highly conductive.
To create this extreme substance, researchers placed a very small sample of water between two diamond tips and then gradually applied pressure of up to 2.3 million atmospheres. For comparison, the pressure at the Earth's core is about 3.6 million atmospheres. The researchers then used high-power lasers to heat the compressed water to a temperature of 2,630 Kelvin, or approximately 2,357°C.
Under such extreme conditions, researchers used high-energy synchrotron radiation X-rays to conduct detailed scans of the samples and observe the structural changes of water in high-temperature and high-pressure environments from the atomic scale. The experiment finally captured superionic ice crystals in which oxygen atoms are arranged in a hexagonal close-packed structure, that is, an hcp structure.
The experiment also revealed the process by which this structure is formed. As the pressure and temperature continue to increase, the originally formed face-centered cubic, or fcc, structure will not remain stable forever. Slip will gradually occur between different layers inside the crystal, and eventually the original structure will be completely transformed into a more stable hexagonal close-packed structure, thus forming hcp superionic ice that has been the focus of theoretical predictions before.
The significance of this discovery is not just the creation of "ice of more than 2,000 degrees Celsius." Scientists have long believed that there may be huge superionic ice layers inside the ice giant planets such as Uranus and Neptune, and the special conductivity of this material may be closely related to the extremely unusual magnetic fields of the two planets.
The magnetic fields of Uranus and Neptune are significantly different from those of Earth. The Earth's magnetic field roughly forms a relatively regular structure around the planet's rotation axis, but the magnetic fields of Uranus and Neptune are significantly deviated from the planet's center and rotation axis, and their structures are complex and asymmetrical. Scientists have previously suggested that the area generating this abnormal magnetic field may not be located in the core of the planet, but in a conductive layer composed of high-voltage material inside it.
Superionic ice happens to have a very important property - it can conduct electricity. Because hydrogen nuclei can move freely between the fixed lattice of oxygen atoms, this material is able to develop electrical and hydrodynamic properties that are completely different from ordinary ice. If such a layer of material does exist inside Uranus and Neptune, it may be one of the important factors driving the strange magnetic fields of the two planets.
Scientists have previously obtained evidence of the existence of superionic ice through theoretical calculations and other high-pressure experiments, but this experiment further confirmed that this material can form a hexagonal close-packed structure under extreme conditions. The researchers believe that this result means that some models of the internal structure, conduction mechanisms, and material flow of ice giant planets may need to be revised.
More importantly, the temperature and pressure conditions inside Uranus and Neptune cannot occur naturally on the Earth's surface, so high-voltage laser experiments in the laboratory have become an important means of studying these distant worlds. By recreating planetary interiors on Earth, scientists can directly study states of matter that are inaccessible to spacecraft.
The research results have been published in Physical Review Letters. For planetary scientists, the confirmation of hexagonal close-packed superionic ice not only further reveals the complex forms that water can take on under extreme conditions, but also provides new physical basis for understanding the two most distant giant planets in the solar system.
In other words, the so-called "ice exceeding 2000°C" does not mean that ice cubes will not melt at high temperatures in the ordinary sense, but a completely new state of matter formed under huge pressure. In such an environment, water has completely broken away from the solid, liquid and gaseous behaviors we are familiar with every day, and has taken on a strange structure in which oxygen atoms are fixed and hydrogen atoms move freely. Such extreme states of matter are likely hidden deep in Uranus and Neptune billions of kilometers away.
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