- Uranus and Neptune are the ice giant planets in our solar system. They have deep atmospheres of hydrogen, helium and methane. But what are they like on the inside?
- Scientists thought Uranus and Neptune contain an unusual form of water that is part liquid and part solid, as well as methane and ammonia. Intense pressures might also turn carbon atoms into diamond material that rains down in the planets’ atmospheres.
- Instead, Uranus and Neptune might have deep oceans of water below their atmospheres, a new study suggests. Another layer rich in carbon would be below that, and the two layers can never mix together, like oil and water.
Are Uranus and Neptune water worlds?
Scientists have long thought that the ice giants – Uranus and Neptune – have a hot, dense fluid of icy water, methane and ammonia around their cores. But on November 25, 2024, scientists at the University of California, Berkeley, suggested a different scenario. Instead, they said the interiors of the two giant planets may be more layered, with deep global oceans of water beneath their atmospheres. This could also explain why both planets have unusually disorganized magnetic fields.
The researchers published their peer-reviewed findings in Proceedings of the National Academy of Sciences (PNAS) on November 25, 2024.
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The ice giants Uranus and Neptune
Uranus and Neptune are the two ice giants in our solar system. Unlike the gas giants Jupiter and Saturn, their interiors are thought to contain more icy water, methane and ammonia. The water may be in the form of superionic water, which is actually part solid and part liquid. This region is below the planets’ deep, thick atmospheres. Below that is a solid core.
Scientists have also theorized that their interiors contain superionic water ice, which is actually hot and part solid and part liquid. Or diamond rain, where pressures are so great that carbon atoms get squeezed into diamond and rain down in the atmosphere. Wild!
A layered interior with oceans?
But now, researcher Burkhard Militzer at UC Berkeley has proposed an alternative possibility. The interiors of both planets may be more distinctly layered. In this scenario, a global ocean of water is below each planet’s atmosphere. And below that would be a highly compressed fluid of carbon, nitrogen and hydrogen.
But just like oil and water, the two layers don’t mix and remain separate.
Previous studies have suggested the interiors of both planets contain icy water, methane and ammonia. The new study suggests that instead of remaining as one layer, this region divided into the two distinct layers. Essentially, hydrogen would be squeezed out of the methane and ammonia.
The study estimates that the water/ocean layer on both Uranus and Neptune is about 5,000 miles (8,000 km) thick. The carbon layer would be a similar thickness.
Lack of magnetic fields
Scientists have also long known that both Uranus and Neptune lack magnetic fields similar to Earth’s. The new study could explain why. Militzer said:
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. We didn’t know this before. It’s like oil and water, except the oil goes below because hydrogen is lost.
Voyager 2 found that both planets lack a dipole magnetic field, and instead only have disorganized magnetic fields. A dipole magnetic field is like what you see with a bar magnet. Earth’s dipole magnetic field is created by convection – the movement of heat through a fluid like air or water – in its interior, in the liquid outer iron core.
But since neither Uranus or Neptune have a dipole magnetic field, that means there’s no convection in their interiors. If there are two layers that don’t mix, that would prevent convection from occurring.
Simulating the planets’ interiors
Previously, Militzer used computer simulations of 100 atoms to try to figure out why the layers wouldn’t mix. The atoms had the same proportions of carbon, oxygen, nitrogen and hydrogen as the known composition of elements in the early solar system. The interiors of the two planets had an estimated 3.4 million times Earth’s atmospheric pressure and were 4,750 Kelvin (8,000°F) in temperature. But even in those simulations, he couldn’t determine what the layers would be composed of.
Then, last year, Militzer ran new simulations, but this time with 540 atoms. He found that in similar conditions in the interiors of Uranus and Neptune, the layers formed naturally. Militzer explained:
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 in the bottom, and the lighter part stays on 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.
Water on top, carbon on bottom
So basically, it was like the oil and water analogy. Except in this case, the water layer stayed on top instead of the bottom. Convection could occur in the upper water-hydrogen ocean layer, but not in the lower carbon-rich layer. Therefore, the water layer could produce the disorganized magnetic field, but not a full dipole magnetic field like Earth.
In addition, the gravity fields of Uranus and Neptune in the layered model matches what Voyager 2 actually measured decades ago.
The new model could explain the interior behavior of both Uranus and Neptune without the need for diamond rain or superionic water. As Militzer said:
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 superionic.’ From my perspective, this is not plausible. But if we have this separation into two separate layers, that should explain it.
Similarities to mini-Neptunes
The findings are reminiscent of some mini-Neptune type exoplanets, which are also thought to have deep water layers beneath thick hydrogen atmospheres. Others – called hycean worlds – might have solid surfaces, with deep global oceans and thick hydrogen atmospheres.
Bottom line: Uranus and Neptune might have deep water oceans beneath their thick atmospheres, with a carbon layer below them. Like oil and water, the two layers never mix.
Source: Phase separation of planetary ices explains nondipolar magnetic fields of Uranus and Neptune
Via University of Berkeley
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