X-ray diffraction finds superionic ice stacking its oxygen atoms hexagonally above 200 gigapascals

The pattern that comes out of a laser-heated diamond anvil cell reports where the oxygen atoms sit. Hydrogen is the part of superionic ice that moves: the nuclei flow through the solid oxygen grid like a liquid, and that flow is what makes the ice conduct electricity ^[6]. The diffraction fixes the geometry of the frame ^[4]. What the hydrogen does inside it comes from what was already understood about the superionic phase ^[6].

Nobody can retrieve a sample from inside Neptune, so the design is to reproduce the conditions on a bench ^[14]. The transition the team reports sits at 200 gigapascals and 1,800 kelvin ^[5], and the cell went to 230 gigapascals, which phys.org puts at more than 2 million times Earth's atmospheric pressure ^[3]. The hexagonal field is therefore documented across a band of roughly 30 gigapascals, about 15 percent above the boundary ^[13]. phys.org does not report how many heating runs produced that one corner of the phase diagram.

"We report the unambiguous observation of a novel H2O ice phase adopting an hcp oxygen sublattice," the authors wrote in the paper, which appeared in Physical Review Letters ^[9][8].

The link to the planets is older than the measurement. Superionic ice has been proposed as playing a role in generating the unusual magnetic fields of Uranus and Neptune ^[11]. For a structural result to feed that proposal, the two packings have to behave differently as conductors, and phys.org reports only that the hexagonal form may have different electrical and mechanical properties from the cubic one ^[12]. The paper's own statement of the planetary consequence, as phys.org quotes it, hedges the same way: "The presence of an fcc-hcp martensitic transition in the superionic regime of warm dense ice may have implications for planetary models of Uranus and Neptune" ^[10].

The paper's title goes further than the abstract's hedge, calling these pressures and temperatures conditions in ice giant planetary interiors ^[8].

Scientists knew this hot, conducting ice existed; the crystal shapes it forms were the open question ^[7]. A symmetry measured at a stated pressure and temperature can go into an equation of state. In my view the dynamo story stays where it was until someone measures conductivity in the hexagonal phase at 200 gigapascals and above.