We might finally understand what’s going on inside Uranus and Neptune, and the answer is pretty surprising: They may each contain an ocean of water.
“We didn’t really know anything before,” about their interiors, said Adam Masters, a space and planetary scientist at Imperial College London. “So this hypothesis is very compelling.”
The idea about the two ice giant planets — so-called because of the freezing conditions in which they formed — was put forward by Burkhard Militzer, a planetary scientist from the University of California, Berkeley, and was published Monday in the Proceedings of the National Academy of Sciences. It could explain the strange magnetic fields of both worlds, which are unlike any other in the solar system.
Earth’s magnetic field is generated in its core, producing a clear north and south pole known as a dipole that roughly aligns with the axis of the planet. “It’s like there’s a big, giant bar magnet inside the planet,” said Heidi Hammel, an astronomer and planetary scientist at the Association of Universities for Research in Astronomy. “That’s true for Jupiter, Saturn, Earth and some of the moons of Jupiter as well.”
When NASA’s Voyager 2 spacecraft flew past Uranus in 1986, however, it discovered something unusual. “The magnetic field was hugely tilted and offset from the center of the planet,” Dr. Hammel said. At first, scientists thought the disordered field could be explained by a suspected giant impact early in Uranus’s life, which had knocked the planet on its side. But then the spacecraft flew past Neptune three years later and “its magnetic field was also significantly tilted,” Dr. Hammel said.
Dr. Militzer’s proposal aims to settle this debate. It is based on simulating the motion of 500 atoms to model the interiors of the two ice giants, and it suggests there is a layer of water about 5,000 miles thick inside the two planets sitting beneath their outer atmospheres.
“We think it’s an ocean,” Dr. Militzer said. “There’s hydrogen mixed in with it, and it has a high conductivity that’s important for the magnetic field.”
However, this ocean would have a pressure 60,000 times greater than that at Earth’s surface, so it would behave more like a supercritical fluid — a combination of gas and liquid — than like water on Earth.
Crucially this water would be separated, like oil and water, from a carbon-rich layer below that divides the ocean from each planet’s core. In the case of Uranus, the core is the size of Mercury, and Neptune’s is slightly larger, more Mars-size.
Previously, scientists had thought the interiors of the two planets would be more mixed. “The new thing is that the water separates from carbon,” Dr. Militzer said. Usually, he added, “when you pile hydrogen on top, the water layer dissolves.” But the ice giants might have formed with less hydrogen than Jupiter and Saturn because of their larger distance from the sun.
This layer of water, not the planetary cores, would then be responsible for producing the disordered magnetic fields of the two planets.
Dr. Militzer’s idea could explain a key difference between Jupiter and Saturn, which are mostly composed of hydrogen, and Uranus and Neptune. “The first step to understand our solar system is the difference between the gas giants and the terrestrial planets,” said Fran Bagenal, an astrophysics and planetary science professor at the University of Colorado, Boulder, and a member of the Voyager science team, “then the hydrogen planets and the ice giants.”
The finding could also inform a proposed return to Uranus next decade by NASA. That orbiting spacecraft could carry instruments to measure the interior structure and magnetic field of Uranus that would be able to investigate the presence of this water layer.
“It’s just another reason to go back,” Dr. Bagenal said.
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