The researchers then took the compressed ice, packed in carry-on luggage, to the University of Rochester where it was blasted by a pulse of laser light. That caused shock waves through the ice that lasted 10 to 20 billionths of a second, heating it to thousands of degrees and exerting a pressure more than a million times that of Earth’s atmosphere. Those conditions exist inside Uranus and Neptune and undoubtedly within numerous ice giants around other stars.

Earlier experiments by other groups had produced conductive water that could have been superionic, but those scientists could not determine if the current were carried by ions and not electrons. Here, Dr. Millot and his colleagues were able to capture the optical appearance of the ice. If electrons were moving around, it would have been reflective. (That is why metals are shiny.) Instead, the sample was opaque. That pointed to the movement of ions instead, indicating a superionic ice.

The superionic ice melted into a liquid at about 8,500 degrees Fahrenheit.

“It is a rather amazing experiment and the results are consistent” with theoretical and computational predictions, said Roberto Car, a chemistry professor at Princeton.

The superionic ice could help explain the lopsided, off-center magnetic fields of Uranus and Neptune, the solar system’s seventh and eighth planets that are known as ice giants and were visited briefly by NASA’s Voyager 2 spacecraft in the 1980s. Instead of Earth’s magnetic field generated at the core of the planet, the fields of those icy bodies may originate, in part, within shells of superionic ice inside their mantles.

Dr. Jeanloz said the agreement between experiment and prediction offered promise that scientists are beginning to understand the basic physics of how molecules in general behave under changing temperatures and pressures well enough for practical use.

“As one starts validating those kinds of predictions, it gives a hope that one could start thinking about engineering new materials,” Dr. Jeanloz said, “where you tell me what properties you want, and someone can use a computer now to figure out what kind of material, what kind of elements you have to put together, and how they’d have to be packed together to come up with those properties.”