Diamond Rain Theory of Uranus and Neptune Challenged

We might have been very wrong about exactly what lurks below the tranquil blue and green surfaces of Uranus and Neptune.

Previous theories have suggested a range of hidden properties of the two planets, including that these distant ice giants may hide downpours of diamonds. Now, a paper published Monday in the journal Proceedings of the National Academy of Sciences suggests that these planets may be made up of distinct layers, separated as strongly as oil and water. This bizarre structure could explain why Uranus and Neptune have such unusual magnetic fields.

The outermost layers of Uranus and Neptune consist mainly of clouds of hydrogen, helium and methane, which absorbs red light, giving these planets their blue hues. Their cores are thought to be composed of rocky and metallic material, likely a mix of silicates and iron. However, exactly what the rest of the atmosphere of these planets is made of, and how they are structured, has remained somewhat of a mystery to scientists.

In the paper, Burkhard Militzer, a planetary scientist at the University of California, Berkeley, suggests that below the outermost layer of clouds these planets may have a highly pressurized liquid mix of water (H2O), methane (CH4) and ammonia (NH3). This mixture may have separated into two layers, each about 5,000 miles thick, the uppermost being water-rich and the lower being hydrocarbon-rich.

This would occur as a result of the temperatures and pressures deep within the planets’ atmospheres, which would cause the hydrogen to be forced out of the methane and ammonia layer below.

This theory differs from a previous suggestion that extreme pressure and temperature conditions deep in the interiors of Uranus and Neptune could lead to the formation of diamonds that “rain” through their atmospheres.

This structure, which was modeled using computer simulations, would also explain the weirdness of the magnetic fields of Uranus and Neptune, both of which are nothing like our own. These magnetic fields are tilted significantly, compared with their rotation axes, and are disorganized, with no dipole like the Earth.

“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 said in a statement. “We didn’t know this before. It’s like oil and water, except the oil goes below because hydrogen is lost.”

In other planets, dipole magnetic fields are generated via the convection of electrically conductive fluids—in Earth’s case, molten iron and nickel in its core. However, measurements of Uranus and Neptune by Voyager 2 in 1986 found that neither had a dipole field, only disorganized magnetic fields.

Militzer suggests there is no global-scale convection but each of the immiscible layers within the planets could have its own convection current that could generate the disorganized magnetic fields observed.

Militzer has been pondering this conundrum for many years. But it was only recently, with the aid of machine learning, that his computer models finally found a way for these layers to form at the pressures and temperatures predicted for the planets’ interiors and also result in the observed magnetic field.

“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,” Militzer said. “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.'”

He continued: “I couldn’t discover this without having a large system of atoms, and the large system I couldn’t simulate 10 years ago.”

According to the paper, the upper water layer likely has a convection current that generates the sporadic magnetic field, but the carbon-nitrogen-hydrogen layer does not.

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

To confirm these models, we would need to send another spacecraft to study the ice giants, and it would take around 10 years to travel to Uranus alone.

Earlier this year, scientists said in a Nature Astronomy paper that the magnetic field properties of Uranus measured by Voyager 2 may not be a standard representation of the planet’s environment. This was because the planet had been hit by a solar storm only days before, resulting in the magnetic field being compressed to the degree that usually occurs only around 4 percent of the time.

“If the spacecraft had arrived only a few days earlier, the upstream solar wind dynamic pressure would have been [about] 20 times lower, resulting in a dramatically different magnetospheric configuration,” the researchers wrote in the Nature Astronomy paper.

“We suggest that discoveries made by the Voyager 2 flyby should not be assigned any typicality regarding Uranus’s magnetosphere.”

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References

Militzer, B. (2024). Disordered magnetic fields of Uranus and Neptune. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2403981121

Jasinski, J. M., Cochrane, C. J., Jia, X., Dunn, W. R., Roussos, E., Nordheim, T. A., Regoli, L. H., Achilleos, N., Krupp, N., & Murphy, N. (2024). The anomalous state of Uranus’s magnetosphere during the Voyager 2 flyby. Nature Astronomy. https://doi.org/10.1038/s41550-024-02389-3