When Voyager 2 passed by Uranus and Neptune four decades ago, astronomers were surprised to find no global dipole magnetic fields similar to what we see on Earth. The reason attributed to this is that these ice giants have distinct internal layers that remain unmixed, which hinders large-scale convection necessary for the formation of a dipole magnetic field. But what substances keep these layers separate? A scientist has simulated the interiors of these planets and discovered that under extreme pressure and temperature conditions, layers rich in water and hydrocarbons form naturally and do not mix.
What about diamond rain or super-ionic water?
These are just two of the intriguing ideas that planetary scientists have suggested regarding the mysteries hidden beneath the thick, bluish atmospheres composed of hydrogen and helium that envelop Uranus and Neptune, unique ice giants of our solar system.
A researcher from the University of California, Berkeley, has now presented a different theory: the interiors of both planets are layered, with the two layers not mixing like oil and water. This configuration could explain the peculiar magnetic fields of these planets, suggesting that previous models of their interiors might not be accurate.
In a study published this week in the journal Proceedings of the National Academy of Sciences, Burkhard Militzer contends that beneath the cloud layers lies a deep ocean of water, followed by a highly compressed mixture of carbon, nitrogen, and hydrogen. Simulations indicate that under the extreme temperatures and pressures within these planets, a blend of water (H2O), methane (CH3), and ammonia (NH3) would naturally segregate into two distinct layers, mainly because hydrogen would be forced out of the methane and ammonia present in the deep interior.
The existence of these immiscible layers can elucidate why neither Uranus nor Neptune possesses a magnetic field like that of Earth, a surprising revelation made by the Voyager 2 mission during the late 1980s.
“We now have a solid theory explaining why Uranus and Neptune exhibit significantly different magnetic fields, which stand in contrast to Earth, Jupiter, and Saturn,” Militzer, a professor of Earth and planetary science at UC Berkeley, stated. “This was previously unknown. It’s akin to oil and water, except the denser liquid goes below due to the loss of hydrogen.”
If other stars also have similar compositions to our solar system, Militzer suggests that ice giants orbiting those stars may share similar internal structures. Sub-Neptune planets, which are roughly the same size as Uranus and Neptune, are among the most frequently discovered exoplanets.
How Convection Creates Magnetic Fields
As a planet cools from the surface downwards, colder and denser materials sink, while bubbles of hotter fluids rise, similar to how boiling water behaves; this process is called convection. If the interior can conduct electricity, a thick layer of convecting material can generate a dipole magnetic field like that of a bar magnet. For instance, Earth’s dipole field, produced by its liquid outer iron core, creates a magnetic field that loops between the North and South Poles, causing compasses to point towards the poles.
However, Voyager 2 found that neither ice giant possess such a dipole field; instead, they have chaotic magnetic fields. This suggests that there is no convective motion present in a thick layer within the planets’ inner parts.
To clarify these observations, two separate research teams proposed over 20 years ago that the planets must have immiscible layers, inhibiting large-scale convection and the development of a global dipole magnetic field. However, convection within one layer could still yield a disorganized magnetic field. Yet, neither team could identify the materials that formed these non-mixing layers.
Militzer spent ten years trying to address this question using computer simulations involving around 100 atoms, reflecting the elemental makeup of the early solar system. Despite predicting high pressures (3.4 million times that of Earth’s atmosphere) and extreme temperatures (4,750 Kelvin or 8,000°F) for the planets’ interiors, he initially failed to uncover how these layers might develop.
Last year, however, utilizing machine learning, he managed to simulate the behavior of 540 atoms and was taken aback to find that layers naturally formed as the atoms were heated and compressed.
“One day, I examined the model, and noticed that the water had separated from the carbon and nitrogen. What had eluded me ten years ago was now occurring,” he recalled. “I thought, ‘This is remarkable! Now I understand why the layers form: One is water-rich, and the other is carbon-rich, with the carbon-rich layer positioned at the bottom. The heavier components settle down, and the lighter ones remain on top, preventing any convective movement.'”
“I couldn’t achieve this insight without simulating a larger system of atoms, which I was unable to do a decade ago,” he added.
The amount of hydrogen forced out rises with depth and pressure, leading to a stably stratified carbon-nitrogen-hydrogen layer, resembling a plastic polymer. While the upper, water-rich layer likely engages in convection to produce the observed disorganized magnetic field, the lower hydrocarbon-rich layer remains stable without convection.
When simulating the gravity of a layered Uranus and Neptune, Militzer found that his models closely matched the gravity fields recorded by Voyager 2 nearly 40 years ago.
“If you were to ask my colleagues what they think explains the magnetic fields of Uranus and Neptune, some might suggest diamond rain or the properties of superionic water,” he explained. “From my perspective, those ideas seem unlikely. Instead, the separation into distinct layers could be the key.”
Militzer predicts that beneath Uranus’ 3,000-mile-thick atmosphere, there lies a water-rich layer about 5,000 miles thick, followed by a similarly thick hydrocarbon-rich layer. Its rocky core is comparable in size to Mercury. Although Neptune is more massive than Uranus, it has a smaller diameter with a thinner atmosphere and similarly layered structures. Its rocky core is slightly larger than Uranus’s, roughly the size of Mars.
He aims to collaborate with colleagues to conduct laboratory experiments under extreme conditions to verify if layers can form in fluids with elemental proportions characteristic of the protosolar system. Furthermore, a proposed NASA mission to Uranus could provide further evidence, particularly if it includes a Doppler imager to study the planet’s vibrations. A planet with layered structures would vibrate differently than one with convective activity, Militzer noted. His next objective is to enhance his computational model to predict how these planetary vibrations might differ.
This research received support from the National Science Foundation (PHY-2020249) as part of the Center for Matter at Atomic Pressures.
