NASA's Voyager 2 spacecraft—launched in 1977 and still in operation—is one of the agency’s two farthest-reaching and longest-running missions, the other being Voyager 1. The journey by Voyager 2 to the outer solar system and beyond has been marked by many scientific firsts, including the first and only study of the ice giants Neptune and Uranus by a space probe. Perhaps the most intriguing of Voyager 2’s findings about these two distant bodies is their tilted magnetic fields. Unlike Jupiter, Saturn, and the terrestrial planets, the magnetic axes of the seventh and eighth planets are offset from the axes around which they rotate—by roughly a third of Uranus’s radius and even more for Neptune. In addition, the fields themselves are asymmetrical.
The magnetic fields of most planets are generated through large-scale motion of electrically conductive fluids. On Earth, this dynamo effect occurs in the liquid iron outer core. Researchers have been split on the composition of the solar system’s two most distant planets—and thus the origin of their tilted and lopsided magnetic fields. Many scientists envision these planets with fully fluid-convecting interiors, while others have proposed a more complex structure, consisting of a thin, electrically conductive liquid shell surrounding a thicker mantle of ice. If, as the latter theory suggests, the fields originate not deep within the planets but in the liquid just beneath their gassy envelopes, this unusual arrangement could explain the planets’ strange magnetic nature.
The primary ingredient in the proposed fluid and solid layers may be water, thought to make up more than 60 percent of Uranus’s and Neptune’s masses. Subjected to intense pressure, the ice in these planets’ mantles would bear little resemblance to ice on Earth, appearing black rather than transparent and remaining solid even at temperatures approaching those of the Sun’s surface. This exotic phase of water has been dubbed superionic ice. The role of superionic ice in the formation of the ice giants’ odd magnetic fields was first posited nearly two decades ago and has been supported by a series of increasingly complex molecular dynamics simulations.
However, creating and characterizing superionic ice in a laboratory setting—an essential step towards validating the simulations—has until recently proven a virtually insurmountable hurdle. Now, through a novel combination of experimental techniques, a team of researchers from Lawrence Livermore, the University of California at Berkeley, and the University of Rochester have successfully produced superionic ice in the laboratory for a vanishingly brief moment. (Work at Livermore was supported by the Laboratory Directed Research and Development Program.) The properties of ice created in the laboratory agree well with simulations, lending credence to the superionic ice mantle theory.
It’s Just a Phase
Water can exist in three phases—solid (ice), liquid (water), or gas (steam). On Earth, the one oxygen and two hydrogen atoms of a liquid water molecule form a loose V shape. As ice, the atoms rearrange and combine with other water molecules to form a more widely spaced hexagonal crystal structure, as seen in snowflakes—which is why water expands as it changes from liquid to solid, the opposite of most substances. This familiar form of ice is the only solid phase of water found naturally on Earth’s surface but is just one of 18 known crystalline ice phases. The other phases, distinguished by their arrangements of hydrogen and oxygen atoms, form under different pressure and temperature conditions, some of which are difficult to replicate in a laboratory setting.