New Superionic Ice Phase Sheds Light on Ice Giant Magnetic Fields

Researchers have identified a new crystalline phase of superionic ice, a strange state of matter that exists under the crushing pressures of planetary interiors. This discovery provides a critical piece of the puzzle in understanding the internal composition and magnetic behavior of the solar system's ice giants.

Led by Alexis Forestier of the CEA, the research team observed a hexagonal close-packed (hcp) phase of ice by recreating extreme conditions in a laboratory setting. The team reported the "unambiguous observation of a novel H2O ice phase adopting an hcp oxygen sublattice." This specific phase becomes dominant at temperatures exceeding 1,800 Kelvin and pressures above 200 gigapascals—roughly 2 million times the atmospheric pressure on Earth.

The Nature of Superionic Ice

Superionic ice differs fundamentally from the ice found on Earth. In this state, oxygen atoms are locked into a rigid solid lattice, while hydrogen nuclei flow freely through the grid like a liquid. This unique duality allows the material to remain solid at temperatures hotter than the surface of the sun while remaining electrically conductive.

This latest discovery adds to a growing catalog of extreme water phases. Previous research has already identified other superionic structures, including the cubic Ice XVIII, discovered around 2019, and the body-centered cubic (bcc) Ice XIX. Together, these findings are refining the phase diagram of water under extreme pressure, showing that water can take on multiple distinct crystalline forms depending on the environment.

Implications for Ice Giants

Understanding these phases is essential for modeling the interiors of Uranus and Neptune. For decades, astronomers have been puzzled by the "off-kilter" multipolar magnetic fields of these planets, which differ significantly from the more symmetrical dipole field of Earth.

Because moving charged particles generate magnetic fields—a process known as dynamo theory—the electrical conductivity of superionic ice is a primary driver of planetary magnetism. The discovery of different crystalline phases, such as the shift from cubic to hexagonal structures, is critical because each phase possesses distinct electrical and mechanical properties. The specific arrangement of ice deep within a planet directly influences the strength and structure of its resulting magnetic field.

Future Research

While the hcp phase has been confirmed in the lab, scientists continue to work toward a complete map of how these materials are layered within actual planets. By integrating these laboratory findings into planetary models, researchers can better predict the internal dynamics of both our own solar system's ice giants and similar exoplanets across the galaxy. Future studies will likely focus on the transition points between these various superionic phases to determine exactly where one ends and the next begins in a planetary core.