Scientists have obtained the clearest observations to date of "superionic ice," a state of water where oxygen atoms form a solid crystal lattice while hydrogen atoms flow freely like a liquid. This discovery addresses a long-standing mystery regarding the internal composition of ice giant planets, Uranus and Neptune.
Experimental Findings
Using powerful X-ray lasers, researchers compressed water samples to pressures up to 180 gigapascals (1.8 million times Earth's atmospheric pressure) and heated them to thousands of degrees. Under these conditions, the team observed that oxygen atoms did not arrange into a simple, orderly pattern. Instead, they formed a complex structure that mixes two different crystal arrangements, which constantly interconvert.
Implications for Uranus and Neptune
Uranus and Neptune are believed to contain vast oceans of this superionic water in their deep interiors. Despite their designation as "ice giants," the extreme pressures and temperatures within these planets transform water into this superionic state. This unique state may account for puzzling features on these distant worlds, particularly their tilted, off-center magnetic fields that do not align with their rotational axes.
Resolving Past Contradictions
Previous experiments attempting to study water under these extreme conditions had produced conflicting results, with different crystal structures being reported at similar pressures and temperatures. The new measurements, published in Nature Communications, utilized ultrafast X-ray pulses lasting just 50 femtoseconds. This technique allowed researchers to capture diffraction patterns with high resolution, effectively "freezing" the water's structure before it could change, thereby resolving years of inconsistencies.
Water's Disordered Nature Under Pressure
At moderate pressures (below 120 gigapascals), experiments detected two different crystal structures coexisting. This suggests that the energy cost for forming either structure is nearly identical, leading water to crystallize into whichever pattern is locally favorable. As pressure increased beyond 150 gigapascals, one arrangement became dominant, but a significant portion (25-32%) of layers stacked in an alternative pattern, creating disorder within the crystal. Machine learning simulations, trained on quantum calculations, independently confirmed these disordered patterns.
Experimental Method
The experiments involved sandwiching thin water layers, approximately half the width of a human hair, between diamond windows. Laser pulses were used to generate shock waves that incrementally compressed the water. Each experiment lasted nanoseconds and destroyed the sample, necessitating repeated trials for verification.
Broader Significance
The nature of this stacking disorder, whether temporary or permanent, remains under investigation. If stable, such defects could influence how heat and electricity move through the material, which is critical for understanding planetary dynamo action and the generation of magnetic fields. The flow of charged hydrogen ions through the oxygen lattice is believed to contribute to these magnetic fields, and any structural features affecting this flow could alter their strength and shape.
This research reconciles prior contradictory measurements and validates theoretical calculations, strengthening models of planetary interiors. Water, despite its apparent simplicity, continues to reveal complex behaviors and structures under extreme conditions, with its high-pressure phase diagram remaining largely unexplored.