Imagine a form of water so peculiar that it could potentially power the magnetic fields of massive planets. When subjected to extreme temperatures of several thousand degrees Celsius and immense pressures reaching millions of atmospheres, water transforms into a fascinating state known as superionic water. In this extraordinary condition, oxygen atoms establish a rigid solid framework, while hydrogen ions glide freely within this structure, leading to behaviors that starkly contrast with those of typical ice or liquid water.
This remarkable phase of water possesses excellent electrical conductivity, positioning it as a compelling candidate to explain the unusual magnetic fields detected around ice giant planets like Uranus and Neptune. These distant worlds are thought to harbor enormous reservoirs of water deep within their cores, suggesting that superionic water could be the prevalent form of water throughout much of our solar system.
The Enigmatic Structure of Superionic Water
The complexities surrounding the internal structure of superionic water have long puzzled scientists. While researchers have successfully synthesized superionic water in laboratory settings, its precise arrangement remained largely elusive. Previous studies suggested that oxygen atoms might organize themselves into one of two straightforward cubic configurations. These include a body-centered cubic arrangement, where an additional atom occupies the center of the cube, and a face-centered cubic arrangement, where atoms fill the centers of each face.
However, recent research unveils a much more intricate reality. Instead of adopting a singular, orderly pattern, the oxygen atoms come together to form a mixed structure that features regions of face-centered cubic arrangements alongside hexagonal close-packed layers. In these hexagonal sections, atoms tightly stack in repeating hexagonal patterns. When these hexagonal formations merge with cubic sections, the outcome is extensive structural disorder. Rather than exhibiting a neatly repeating lattice, the atoms create a hybrid and irregular sequence, detectable only through highly precise measurement techniques enabled by cutting-edge X-ray lasers.
Simulating Planetary Extremes in the Laboratory
To unveil these details, researchers executed two distinct experiments. One took place at the Matter in Extreme Conditions (MEC) instrument at LCLS in the United States, while the other was conducted at the HED-HIBEF instrument at the European XFEL. These powerful facilities allowed scientists to compress water to pressures exceeding 1.5 million atmospheres and heat it to several thousand degrees Celsius, all while capturing fleeting snapshots of its atomic structure within trillionths of a second.
The outcomes of these experiments align closely with advanced computational simulations, revealing that superionic water can adopt various structural forms, much like ordinary ice, which exists in multiple crystal phases depending on temperature and pressure. This study reinforces the notion that water, despite its apparent simplicity, continues to display unexpected and astonishing behaviors under extreme conditions. Furthermore, these insights refine our models of the internal structure and long-term evolution of ice giant planets, which are believed to be prevalent across the universe.
*Superionic water is a rare state of water that arises under extreme pressures and temperatures far beyond what we experience on Earth's surface. In this state, water behaves like a solid, yet the hydrogen ions are capable of moving freely within a rigid lattice of oxygen atoms. This exceptional combination grants superionic water the ability to conduct electricity. Scientists suspect that this unique form of water exists in the depths of large planets, where such extreme conditions naturally occur.
This research was made possible through a collaborative effort between the German Research Foundation (DFG) and the French research funding agency ANR, with over 60 scientists from Europe and the United States contributing to the experiments and resulting analyses.
