Uranus and Neptune are unique but superficially bland ice giants in our solar system. Planetary scientists have devised two proposals for what lies beneath their thick, bluish, hydrogen-and-helium atmospheres.
A planetary scientist at the University of California, Berkeley, proposes a new theory suggesting that these planet’s ineteriors are layered, with two distinct layers that don’t mix. This model offers a possible explanation for the planets’ unusual magnetic fields and challenges previous theories about their interiors.
The study suggests that beneath the cloud layers of these planets lies a deep ocean of water, followed by a highly compressed fluid of carbon, nitrogen, and hydrogen. Computer simulations indicate that under extreme temperatures and pressures, water, methane, and ammonia naturally separate into two layers, with hydrogen being squeezed out of the methane and ammonia in the deep interior.
The immiscible layers proposed for Uranus and Neptune could explain why these ice giants lack a magnetic field similar to Earth’s. This was one of the surprising findings from NASA’s Voyager 2 mission in the late 1980s, which revealed that the magnetic fields of Uranus and Neptune are unusual and do not resemble Earth’s well-defined dipolar field.
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Burkhard Militzer, a planetary scientist at the University of California, Berkeley, said, “We now have, I would say, a good theory about why Uranus and Neptune have really different fields, and they’re very different from Earth, Jupiter, and Saturn. We didn’t know this before. It’s like oil and water, except the oil goes below because hydrogen is lost.”
“If other star systems have similar compositions, ice giants around those stars could have similar internal structures. Planets about the size of Uranus and Neptune — so-called sub-Neptune planets — are among the most common exoplanets discovered to date.”
Burkhard Militzer, UC Berkeley, and NASA
As a planet cools, colder, denser material sinks while hotter, less dense fluid rises, a process known as convection. If the planet’s interior can conduct electricity, this convection can create a dipole magnetic field, like a bar magnet.
Earth’s magnetic field is generated by the convection of electrically conductive liquid iron in the outer core, forming a dipole that extends from the North to the South Pole. This is why compasses are drawn to the poles.
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Voyager 2 found that Uranus and Neptune lack a dipole magnetic field, showing instead disordered magnetic fields. This suggests that there is no large-scale convection in their deep interiors.
Over 20 years ago, two research teams proposed that the planets have immiscible layers, preventing convection and a global dipole field. However, they needed help determining what these non-mixing layers were composed of.
Burkhard Militzer, UC Berkeley
Ten years ago, Militzer attempted to solve the problem by running computer simulations with about 100 atoms, using the known proportions of carbon, oxygen, nitrogen, and hydrogen in the early solar system. At the extreme pressures and temperatures predicted for the planets‘ interiors—3.4 million times Earth’s atmospheric pressure and 4,750 Kelvin—he couldn’t find a way for immiscible layers to form.
However, last year, using machine learning, he simulated the behavior of 540 atoms and unexpectedly discovered that layers naturally formed as the atoms were heated and compressed.
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Militzer said, “One day, I looked at the model, and the water had separated from the carbon and nitrogen. What I couldn’t do 10 years ago was now happening. I thought, ‘Wow! Now I know why the layers form: One is water-rich, and the other is carbon-rich, and in Uranus and Neptune, it’s the carbon-rich system that is below. The heavy part stays at the bottom, and the lighter part stays at the top, and it cannot do any convecting.”
“I couldn’t discover this without having a large system of atoms and the large system I couldn’t simulate 10 years ago.”
As pressure and depth increase, more hydrogen is squeezed out, creating a stable, stratified carbon-nitrogen-hydrogen layer, similar to a plastic polymer. The upper water-rich layer likely undergoes convection, producing the disordered magnetic fields observed in Uranus and Neptune, while the deeper, hydrocarbon-rich layer does not.
When Militzer modeled the gravity of a layered Uranus and Neptune, the results closely matched the gravity fields measured by Voyager 2 nearly 40 years ago.
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Militzer predicts that beneath Uranus’ 3,000-mile-thick atmosphere, there is a water-rich layer about 5,000 miles thick, followed by a hydrocarbon-rich layer of similar thickness, with a rocky core about the size of Mercury.
Despite being more massive, Neptune is smaller in diameter and has a thinner atmosphere, but also has similar water-rich and hydrocarbon-rich layers. Its rocky core is slightly larger, about the size of Mars.
Using high-temperature and high-pressure experiments, Militzer plans to collaborate with colleagues to test whether layers form in fluids with the elemental proportions found in the early solar system. A proposed NASA mission to Uranus, equipped with a Doppler imager to measure planetary vibrations, could help confirm his model. A layered planet would vibrate at different frequencies than a convecting one, and Militzer intends to calculate how these vibrations would differ using his computational model.
Journal Reference:
- Burkhard Militzer et al., Phase separation of planetary ices explains nondipolar magnetic fields of Uranus and Neptune, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2403981121