What is Ice XXI and why it matters
Scientists have identified a remarkable new phase of water ice, dubbed Ice XXI, which forms at room temperature under extreme pressure. This discovery adds to the long list of ice polymorphs that arise when H2O is subjected to unusual conditions. While everyday ice is known as ice I, nature may host dozens more ice phases in the hidden corners of planets and icy moons.
Ice XXI stands out for its unique tetragonal crystal structure and a substantial unit cell comprised of 152 water molecules. Such a configuration puts Ice XXI in a distinct class from the well-known ice I and the high-pressure relatives that have captivated physicists for decades.
How Ice XXI was observed
The research team used a diamond anvil cell to press water to pressures reaching 2 gigapascals — roughly 20,000 times Earth’s sea-level pressure — in a blisteringly short timespan of 10 milliseconds. The pressure was then released in a controlled manner over about one second, and the process was repeated multiple times. Throughout these rapid compression cycles, the European XFEL facility in Germany supplied ultra-fast X-ray pulses that captured a torrent of data — about a million images per second — revealing how the ice crystalline structure evolved in real time.
According to physicist Geun Woo Lee of the Korea Research Institute of Standards and Science, these dynamic experiments revealed several crystallization pathways for water under extreme conditions. Ice XXI appears as an intermediary stage in the transformation of liquid water toward more exotic, higher-pressure phases such as ice VI. This finding underscores the fluid and complex nature of water’s solid states, even under conditions that far exceed typical laboratory environments.
Implications for planetary science
The discovery has broad implications beyond the lab. Ice phases are not just laboratory curiosities; they are central to understanding the interiors of icy moons and planets where pressure and temperature extremes prevail. Ice XXI could exist in the subsurface layers of worlds like Europa or Ganymede, potentially affecting the way heat is transported, the mechanical properties of the ice shell, or the dynamics of potential sub-surface oceans. By mapping the possible ice phases that water can adopt under high pressure, researchers can build more accurate models of planetary formation, geophysics, and even potential habitability in icy environments.
What lies ahead
Ice XXI is one piece of a larger puzzle: scientists expect that more unknown ice phases await discovery as researchers continue to push the boundaries of pressure, temperature, and chemical composition. The current results demonstrate that water’s solid-state landscape is richer and more intricate than previously thought, with multiple crystallization pathways that can lead to distinct, stable structures under dynamic conditions.
Concluding thoughts
While home freezers won’t conjure Ice XXI anytime soon, the experiment highlights a surprising truth: water’s solid form remains a frontier of science, especially under extreme conditions. The observation of Ice XXI not only expands the catalog of ice polymorphs but also prompts new questions about where such phases might exist in the cosmos and how they influence the behavior of icy worlds we may someday explore.
