What is Ice XXI and why it matters
Ice XXI is a newly discovered phase of water that challenges our conventional understanding of ice. Unlike the familiar ice I that forms in freezers and ice cubes, Ice XXI arises under extreme conditions where pressure and rapid dynamic changes influence water’s solid state. This unexpected phase is notable for its tetragonal crystal structure and a repeating unit made up of 152 water molecules, setting it apart from all previously known forms of ice.
The experiment: using the world’s largest X-ray laser
Scientists carried out the discovery at the European XFEL facility in Germany, employing a diamond anvil cell to apply staggering pressures—up to 2 gigapascals, roughly 20,000 times Earth’s surface pressure. What makes this study extraordinary is the speed and control with which the pressure was applied and released. The team compressed the water in just 10 milliseconds and then decompressed it over about one second, repeating the cycle more than 1,000 times to map the crystallization pathways of H2O under such conditions.
During this rapid cycle, the facility’s powerful X-ray pulses captured about a million images per second. This allowed researchers to observe the formation and transformation of water’s crystal structure in real time, revealing the emergence of Ice XXI amid a series of transitional phases.
Why Ice XXI is a crucial piece in the ice puzzle
Water has a surprisingly rich phase diagram. While ice I is the common, everyday form, scientists have long theorized that many more crystalline structures exist under different pressures and temperatures. Ice XXI adds a valuable data point to this chart, illustrating that crystallization can follow multiple pathways under rapid pressure changes. Its large 152-molecule repeating unit and tetragonal symmetry indicate a highly organized, yet complex, arrangement of hydrogen-bonded water molecules.
The discovery underscores how dynamic pressure conditions—especially those explored with high-powered X-ray imaging—can reveal hidden states of matter that static experiments might miss. Ice XXI is thus not just a curiosity; it is a window into water’s behavior in environments far beyond Earth’s surface.
Implications for planetary science and beyond
Ice states are not mere scientific footnotes. The outer moons of planets and dwarf planets—where pressures can spike and fluctuate due to internal processes, tidal forces, or cryovolcanism—may host exotic ice phases that scientists have yet to observe directly. Ice XXI’s existence implies there could be a host of other, still-unknown ice forms waiting in the cold depths of icy worlds. Understanding these phases can inform models of planetary interiors, magnetic fields, and potential habitats for chemical reactions in subsurface oceans.
Moreover, the study demonstrates the power of combining diamond anvil cells with state-of-the-art X-ray lasers. As researchers push the boundaries of time-resolved crystallography, we can expect more surprises about how ordinary materials behave under extraordinary conditions.
Experts’ insights and future directions
Geun Woo Lee, a physicist at the Korea Research Institute of Standards and Science, highlighted the significance of observing multiple crystallization pathways in water. He notes that Ice XXI represents a fleeting but informative state that connects commonly known ice VI and other high-pressure phases, helping to map a more complete phase diagram of H2O under dynamic stress.
Looking ahead, researchers will likely explore how stable Ice XXI is under varied temperatures, how quickly it can transition to other phases, and whether similar room-temperature ice forms could exist on icy bodies in the solar system. The ongoing use of high-intensity X-ray pulses promises deeper insights into water’s complex behavior under pressure, potentially unlocking further surprises in the world’s most familiar molecule.
