Jupiter, the largest planet in our solar system, is a gas giant whose internal structure contrasts sharply with rocky planets like Earth. Its immense self-gravity compresses material into states rarely seen elsewhere. Jupiter is large enough to contain over 1,300 Earths and holds more than twice the mass of all other planets combined. This colossal size creates extreme internal pressures and temperatures that reshape the nature of matter, particularly its primary component, hydrogen. The journey into Jupiter’s interior is a descent into a high-pressure, high-temperature environment where familiar elements behave in exotic ways.
The Outer Layer: Molecular Hydrogen
The visible atmosphere, characterized by colorful bands and the Great Red Spot, is the beginning of Jupiter’s interior. This outermost layer, extending thousands of kilometers inward, is predominantly composed of molecular hydrogen and helium. As depth increases, pressure and temperature steadily rise, causing the hydrogen gas to compress.
Unlike terrestrial planets, there is no distinct boundary; the hydrogen gradually transitions into a supercritical fluid state. In this phase, the material is neither a true gas nor a true liquid, but possesses properties of both because the temperature and pressure are above the critical point.
The molecular hydrogen in this region remains electrically insulating, meaning it does not easily conduct electricity. This massive, turbulent envelope of hydrogen and helium fluid becomes hotter and denser with depth, setting the stage for the transformation of hydrogen beneath.
The Conductor: Liquid Metallic Hydrogen
The immense weight of the overlying molecular hydrogen subjects deeper layers to pressures reaching millions of times Earth’s atmospheric pressure. At approximately 20,000 kilometers below the cloud tops, this extreme compression forces a dramatic change: hydrogen atoms are squeezed so tightly that electrons are stripped from the nuclei, becoming unbound and free to move.
This results in the formation of liquid metallic hydrogen, a dense, electrically conductive fluid that behaves like a molten metal. The material remains liquid due to extremely high temperatures, estimated to be thousands of Kelvin. This layer constitutes the bulk of Jupiter’s volume and mass, acting as the planet’s vast, churning mantle.
The movement of this conductive liquid generates Jupiter’s powerful magnetic field, the strongest in the solar system. Deep convection currents within the metallic hydrogen create a planetary dynamo, converting the kinetic energy of the moving fluid into magnetic energy. This results in a magnetic field nearly fourteen times stronger than Earth’s at the equator, generated at a relatively shallow depth compared to Earth’s core.
Composition of the Inner Core
At the center of Jupiter, beneath the layer of liquid metallic hydrogen, lies the inner core. Data from the Juno mission suggests this region is not a compact, solid sphere as once theorized. Instead, the core is “dilute” or “fuzzy,” meaning its material is mixed and gradually transitions into the surrounding metallic hydrogen.
This central region is composed of a mixture of rock, ice, and heavy elements, such as silicates and water ice, compressed under extreme conditions (around 4,000 GPa and up to 20,000 Kelvin). The diffuse nature indicates that these heavy elements have been partially dissolved and mixed into the lower hydrogen envelope.
The total mass of this diffuse core is substantial, estimated to be between 7 and 25 times the mass of Earth. This massive, mixed structure challenges older models and implies a more complex formation history, suggesting the material has been stirred by convection currents over billions of years.
How We Know: Mapping Jupiter’s Interior
Directly observing Jupiter’s interior is impossible because the opaque atmosphere and immense pressure prevent any probe from descending far below the cloud tops. Scientists must deduce the internal structure by precisely measuring the planet’s external gravitational and magnetic fields. This methodology, known as “gravity science,” was a primary objective of NASA’s Juno spacecraft.
Juno employs highly sensitive hardware to perform these measurements. As the spacecraft orbits close to Jupiter, variations in the planet’s mass distribution cause minute changes in Juno’s velocity. The Deep Space Network on Earth tracks Juno’s radio signal, measuring tiny shifts in frequency (the Doppler effect) to determine the spacecraft’s velocity with extreme precision, down to fractions of a millimeter per second.
These velocity changes are used to create a detailed map of Jupiter’s gravitational field. Analyzing these subtle differences allows researchers to infer the density and distribution of mass within the layers below. The high-resolution gravity data gathered by Juno confirmed the metallic hydrogen layer and revealed the “fuzzy” nature of the inner core, departing significantly from previous theoretical models.