What happens when matter is squeezed so harshly that the rules of chemistry and physics seem to collapse? Deep below the biggest planet in our solar system, Jupiter, hydrogen the most primitive and widespread element in the universe goes through a remarkable transformation. Jupiter offers no firm surface on which to stand; instead, its immense size is a terrain of strange states of matter shaped by unimaginable pressures and temperatures.
At the top of Jupiter’s swirling atmosphere, hydrogen exists as a familiar gas. But as one descends, the pressure rises swiftly, soon surpassing what would crush a submarine or even a diamond. After a journey of about 1,000 miles down, the hydrogen is forced into a liquid state, forming what scientists call the largest “ocean” in the solar system though it is an ocean not of water, but of liquid hydrogen.
But the story grows even stranger. Drop another 20,000 miles, and the hydrogen is compressed so tightly that its electrons flee, turning the fluid into liquid metallic hydrogen a phase so alien that scientists only caught glimpses of its characteristics in the laboratory in the past few years. With diamond anvil cells and powerful lasers, researchers have recreated pressures of over two million atmospheres, observing what Carnegie’s Alexander Goncharov describes as an unexpected “dark hydrogen” phase a middle state that neither reflects nor transmits visible light, but can transmit heat in the infrared. “This observation would explain how heat can easily escape from gas giant planets like Saturn,” Goncharov explained in his team’s press release.
The phase transition from gaseous to liquid to metallic hydrogen in Jupiter is not abrupt. Instead, as shown by advanced machine learning simulations, the transition is smooth and continuous with no abrupt interfaces between the insulating and conducting layers. University of Cambridge’s Bingqing Cheng explains, “Within these bodies, the metal and insulating layers show a smooth density gradient between them, in place of a sharp transition.” Her lab’s neural network models, capable of simulating thousands of atoms at the speed of nanoseconds, overturned earlier theories that had predicted sudden phase transitions and instead provided a more nuanced view of Jupiter’s internal composition.
At the deep core of Jupiter, the pressure is an astonishing 100 million atmospheres and the temperature is a scorching 20,000 degrees Celsius hotter than the surface of the Sun. The composition of this core remains controversial. Emerging models, underpinned by NASA’s Juno mission measurements of gravity, suggest a “dilute” or “fuzzy” core an enriched zone in heavy elements, not a dense rocky core as once conceived. But researchers who modeled the formation and evolution of Jupiter have found that forming so fuzzy a core is “very challenging in the standard framework for Jupiter’s formation,” according to recent studies. In typical cases, heavy elements are segregated into the envelope, with only a very tiny, stable gradient persisting very deep in the planet.
The complexity of Jupiter’s interior has inspired the use of deep learning techniques to develop high-precision models. The NeuralCMS project, for example, uses neural networks trained on millions of planetary interiors to predict gravity moments and mass with high precision, achieving a major decrease in the computational time required to cross the vast parameter space of possible internal compositions (A&A, 2024).
In addition to its scientific mysteries, Jupiter is a keystone within the solar system. Its massive bulk plays a cosmic umbrella role by deflecting or capturing asteroids and comets that otherwise might be dangerous to Earth. If Jupiter’s screening had not been there, the frequency of catastrophic impacts on our planet might have been significantly higher a reality that underscores the importance of the planet in the evolution of life on Earth.
With ongoing scientific forays into the belly of Jupiter, both as space missions like Juno and Europa Clipper and in the guise of laboratory and computational advances, the planet still stands as an extreme conditions physics laboratory. Pursuit of understanding its unusual states of matter, its origin, and its solar system shielding function continues to inspire new scientific frontiers and technologies.
