What lies beneath the surface of Uranus and Neptune?

Diamond rain and super-ionic water are among the more dramatic theories proposed to explain the mysterious interiors of Uranus and Neptune, the ice giants of our solar system. 

However, a planetary scientist from the University of California, Berkeley, has introduced a compelling alternative: the interiors of these planets are layered, with materials separating like oil and water. 

This separation could explain the planets’ unusual magnetic fields and challenges earlier theories about their interiors.

A layered model for the ice giants

In a paper published in the Proceedings of the National Academy of Sciences, Burkhard Militzer, a professor of earth and planetary science at UC Berkeley, suggests that Uranus and Neptune have interiors composed of two distinct layers. 

Beneath the planets’ thick hydrogen-and-helium atmospheres, a deep ocean of water-rich material lies above a highly compressed, hydrocarbon-rich fluid composed of carbon, nitrogen, and hydrogen.

Militzer’s computer simulations show that under the extreme pressures and temperatures inside these planets, water (H₂O), methane (CH₄), and ammonia (NH₃) naturally separate. This separation occurs because hydrogen is squeezed out of methane and ammonia, leading to the formation of immiscible layers.

“We now have, I would say, a good theory why Uranus and Neptune have really different fields, and it’s very different from Earth, Jupiter, and Saturn,” Militzer explained. “It’s like oil and water, except the hydrogen-rich layer goes on top, and the heavier material stays below.”

Magnetic fields and convection

The theory provides a potential explanation for the irregular magnetic fields of Uranus and Neptune, which were discovered by NASA’s Voyager 2 mission in the 1980s. 

Unlike Earth’s strong, dipolar magnetic field – which results from convection in the planet’s liquid outer core – Uranus and Neptune have disorganized magnetic fields.

Convection, a process where hot material rises and cooler material sinks, generates magnetic fields in planetary interiors. For Uranus and Neptune, the lack of a large-scale, convecting layer in their interiors suggests that the materials within are layered and do not mix.

In Militzer’s model, the upper water-rich layer likely convects, creating the observed disorganized magnetic field, while the deeper, hydrocarbon-rich layer remains stable and stratified, preventing global convection.

Extreme conditions of Uranus and Neptune 

Militzer’s breakthrough came after a decade of research. Ten years ago, he used computer models with 100 atoms to simulate the behavior of elements in the planets’ interiors but could not replicate the formation of layers.

Last year, leveraging machine learning and more powerful computing tools, he increased the number of atoms in his model to 540. 

This allowed him to simulate the behavior of materials under the extreme pressures and temperatures inside Uranus and Neptune – up to 3.4 million times Earth’s atmospheric pressure and temperatures around 4,750 Kelvin (8,000°F).

“One day, I looked at the model, and the water had separated from the carbon and nitrogen,” Militzer said. “What I couldn’t do 10 years ago was now happening.”

The model revealed that as pressure increases with depth, hydrogen is squeezed out, forming a stable layer of carbon-nitrogen-hydrogen material, almost like a plastic polymer. This hydrocarbon-rich layer lies beneath the convecting, water-rich layer.

Gravity fields measured by Voyager 2 

When Militzer modeled the gravitational effects of a layered Uranus and Neptune, the results matched the gravity fields measured by Voyager 2 nearly 40 years ago. 

His model predicts that below Uranus’ 3,000-mile-thick atmosphere lies a water-rich layer about 5,000 miles thick. 

Beneath this layer is a hydrocarbon-rich layer of similar thickness, with a rocky core about the size of Mercury. Neptune, though more massive than Uranus, has a thinner atmosphere but similar water-rich and hydrocarbon-rich layers. Its rocky core is slightly larger, approximately the size of Mars.

Implications for exoplanets

If Militzer’s theory is correct, it has implications beyond our solar system. 

Planets similar in size to Uranus and Neptune, often referred to as sub-Neptune exoplanets, are among the most common types of planets discovered around other stars. These exoplanets may also have layered interiors with distinct chemical compositions and magnetic fields.

“If other star systems have similar compositions to ours, ice giants around those stars could well have similar internal structures,” Militzer noted.

The interiors of Uranus and Neptune 

Militzer hopes to collaborate with experimental physicists to recreate the extreme conditions of Uranus and Neptune’s interiors in laboratory settings. 

By testing the behavior of fluids with elemental proportions found in the protosolar system, researchers could verify whether immiscible layers naturally form.

Future space missions may also provide definitive proof. A proposed NASA mission to Uranus could include a Doppler imager to measure planetary vibrations. 

According to Militzer, a layered planet would vibrate at different frequencies than one with a fully convecting interior. His next project involves calculating these vibrations using his computational model.

The peculiarities of Uranus and Neptune 

Militzer’s findings challenge popular theories about Uranus and Neptune, such as the idea of diamond rain in the planets’ interiors or the exotic properties of super-ionic water.

“If you ask my colleagues, ‘What do you think explains the fields of Uranus and Neptune?’ they may say, ‘Well, maybe it’s this diamond rain, but maybe it’s this water property which we call super-ionic,’” Militzer said. 

“From my perspective, this is not plausible. But if we have this separation into two separate layers, that should explain it.”

Militzer’s layered model offers a comprehensive explanation for the peculiarities of Uranus and Neptune, from their magnetic fields to their gravitational signatures. 

By identifying the separation of water-rich and hydrocarbon-rich layers as the key factor, the study advances our understanding of the ice giants and opens new possibilities for exploring planetary interiors.

Future laboratory experiments and space missions could confirm these findings, not only shedding light on the mysteries of Uranus and Neptune but also providing insights into the structure of similar planets across the galaxy.

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