Neptune and Uranus have hearts of ice, but not like any ice you鈥檝e seen before. Now, a team of researchers has created for the first time the bizarre stuff that might occupy the frigid cores of these ice giants, potentially explaining their weird magnetic fields which can flip on and off daily.
It鈥檚 called superionic ice. It only occurs at temperatures matching those on the Sun, and pressures exceeding a million Earth atmospheres鈥� precisely the environment predicted at the centre of ice giants. When ice becomes superionic, the oxygen ions of the water molecules behave like a solid, staying in place to form a kind of lattice, while the hydrogen ions flow through it like a fluid.
This structure means superionic water ice can resist vastly higher temperatures than normal ice before melting. A team led by at Lawrence Livermore National Laboratory in California, made this ice in their lab, proving it actually exists 30 years after it was theorized.
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Magnetic ice
To do so, they started with ice VII, an exotic form of crystalline ice that only forms under intense pressure and is dense enough to sink in water.
They trained a powerful laser on one of two diamonds holding a cube of ice VII, generating a shock wave that travels through the diamond and compresses the ice. They found that the melting temperature was 4726掳C when under pressures equivalent to 2 million times the pressure of Earth鈥檚 atmosphere.
鈥淭his is almost the temperature of the surface of the Sun!鈥� says Millot. It is also hotter than the probable temperature deep inside Neptune and Uranus so that superionic ice could exist in those planets.鈥�
If Neptune and Uranus are full of this bizarre ice, it could also explain their unusual magnetic fields. The fluid-like hydrogen ions in superionic water ice can carry electrical charge, which makes the ice they inhabit good at conducting electricity and generating magnetic fields in the process.
鈥淧eople have predicted that the magnetic field we observed around those planets could be generated by a dyanamo inside 鈥� a thin, electrically conducting shell surrounding an insulating shell,鈥� Millot says. The new results support this model, he says, with the inner core formed of superionic ice, while the field forms in an outer, swirling layer of ionic fluid water.
Not so pure
Interiors of Neptune and Uranus are expected to be made of superionic ice with added nitrogen and carbon, a combination Millot says he鈥檇 like to test next to see if the superionic behavior continues.
鈥淭his is exciting and groundbreaking work,鈥� says , who studies the magnetic fields of the ice giants at the University of California, Santa Cruz. 鈥淕iven that the fluid, 鈥榠cy鈥� interiors of Uranus and Neptune are not pure water, but a mixture of water, methane and ammonia, I鈥檇 be very interested in future work to see if this superionic phase is maintained for such a fluid 鈥榠cy mix鈥� containing these other substances.鈥�
, who helps plan NASA鈥檚 missions to the ice giants at the Goddard Space Flight Center in Maryland, says this is a welcome study because 鈥渨e need to understand the properties of ices under more exotic conditions to better understand these planets.鈥�
Nature Physics
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