Jupiter's interior

Jupiter, the largest planet in the solar system, is often described as a “gas giant” planet. However, the interior structure of Jupiter is far more complex than a simple ball of gas. These interior characteristics are also responsible for many of the planet’s observed properties.

Overview

Jupiter is the largest planet in the solar system, with a mean diameter of about 139,000 kilometers. Jupiter’s mass is about 320 times more than Earth’s mass. This is greater than that of the rest of the planets combined. However, most of Jupiter’s mass is made up of the two lightest elements in the universe: hydrogen and helium. Jupiter is believed to be formed directly from the disk of material swirling together that formed the Sun. Thus, it is not a surprise to find that Jupiter’s composition is very similar to that of the Sun (a star), which is also mostly hydrogen and helium. However, there are significant differences between the structure of Jupiter and that of a star or even a failed star such as a brown dwarf. The interior structure of Jupiter has not been directly observed. It can only be inferred through mathematical modeling based on observations of the planet. Jupiter does not have a solid surface on which a spacecraft can land, and cloud layers in Jupiter’s upper atmosphere shield the interior of the planet from view.

Jupiter is observed to have an oblateness of 0.065 (its equatorial diameter is 6.5 percent greater than the pole-to-pole diameter). This observation constrains the size of any solid core that the planet may have. Mathematical models suggest that Jupiter may have a rocky core with a mass somewhere between eight and fifteen times the mass of Earth. Due to Jupiter’s large mass, though, that is only between 2.5 to 4.7 percent of Jupiter’s mass. Despite the core being several times the mass of Earth, the extreme pressure inside Jupiter, nearly seventy million atmospheres, compresses the core to a size on the order of that of Earth. The temperature at the core of Jupiter is believed to be perhaps 22,000 kelvins. It is not known if the core is solid or liquid.

Because Jupiter is in a region of the solar system in which a large number of icy bodies exist, a great number of these bodies must have impacted Jupiter over its history, starting at its formation. These ices, being heavier than the hydrogen and helium that make up the bulk of Jupiter, would have settled toward the deep interior of the planet. Planetary scientists believe that there may exist a layer of this material, perhaps 3,000 kilometers thick, on top of the core. The ices include frozen ammonia and methane rather than just water ice. The temperature and pressure deep inside Jupiter would ensure that this material is in a liquid state, though, rather than solid. Therefore, the term “liquid ices” is often used to describe this layer. Great pressure at this depth would make this material behave in ways quite different from the way the same material would behave on Earth.

Most of Jupiter is composed of hydrogen and helium. The outermost parts of Jupiter are gaseous; at great depths inside the planet, the pressure becomes great enough to compress these gases into a liquid state. That pressure is reached at a depth of about 1,000 kilometers below the cloud tops. However, there is no vast ocean of liquid hydrogen under Jovian skies the way that Earth’s water collects in oceans. On Jupiter, in fact, there is no clear boundary between the gaseous atmosphere and the liquid interior because the temperature and pressure inside Jupiter are well in excess of hydrogen’s critical point. Beyond the critical point of a substance, there ceases to be a definite phase transition between liquid and gas. Rather, the material takes on a state known as a supercritical fluid. At greater altitudes, the hydrogen in Jupiter is clearly gaseous. At much lower levels, it definitely has more liquid properties, but there is no obvious depth at which the hydrogen becomes liquid. Instead, with increasing depth, the hydrogen becomes more and more like liquid. Though Jupiter is called a “gas giant” planet, the majority of the planet’s composition is actually liquid.

At sufficient pressure and temperature, hydrogen takes on metallic properties. That means that it conducts heat and electricity like any other element on the left-hand column of the periodic table of elements. These conditions are met in Jupiter below a depth of about 7,000 kilometers below the planet’s cloud tops. Liquid metallic hydrogen exists from that depth all the way down to the liquid ices at the core. That means that the bulk of Jupiter’s mass is in a mantle composed of helium and liquid metallic hydrogen, possibly comprising about two-thirds of the planet.

Jupiter has the strongest magnetic field of any planet in the solar system. At its equator, Jupiter’s magnetic field is nearly fourteen times stronger than Earth’s magnetic field. Planetary magnetic fields are believed to be created by magnetohydrodynamics in a planet’s interior. A dynamo model of planetary magnetic fields shows that a suitable conductor moving in a magnetic field can regenerate that magnetic field, producing a long-lived magnetic field. However, this dynamo effect requires a highly conducting fluid in order to operate. It may be possible for part of Jupiter’s core to have a liquid iron region, but that would not be large enough to account for Jupiter’s magnetic field. Rather, Jupiter’s magnetic field originates primarily in its liquid metallic hydrogen mantle.

Unlike Earth, Jupiter radiates 1.6 times as much energy as it gets from the Sun. This surplus energy is produced by Kelvin-Helmholtz contraction. When planets form, a large amount of gravitation energy is released as the materials that form the planet come together. For fluid bodies such as Jupiter, as they radiate thermal energy into space, they contract somewhat. This contraction then compresses the material making up the planet, heating it further. Other gas giant planets besides Jupiter probably also had Kelvin-Helmholtz contraction after they formed, but they have long since stabilized at a point where such contraction no longer is a major source of thermal energy. Jupiter is at nearly the perfect mass to extend Kelvin-Helmholtz contraction to the longest time possible. If Jupiter had more mass, then it would have compressed faster until it reached a point of maximum compression. If Jupiter had less mass, like Saturn, then it would not have had sufficient gravity to keep contracting for as long as it has.

Knowledge Gained

Much has been learned about Jupiter through observations from Earth. The interior of the planet, of course, cannot be directly measured. Understanding the nature of matter has allowed astrophysicists to make theoretical models of Jupiter’s interior; however, it took measurements by spacecraft sent to Jupiter to actually begin to learn more about that planet’s interior structure.

Since Jupiter’s magnetic field is produced in the planet’s mantle, it rotates with the planet’s interior. Studies of the magnetic field show that Jupiter’s interior rotates once every nine hours, fifty-five minutes, and thirty seconds, somewhat more slowly than the rate of rotation of the cloud tops near the planet’s equator. Until spacecraft were able to approach Jupiter to measure its magnetic field, astronomers could only guess at its interior rotational period. Jupiter’s huge liquid metallic hydrogen layer produces a magnetic field that is so powerful that Jupiter has a magnetic field stronger than any other planet in the solar system, and Jupiter’s magnetosphere is the largest of any of the planets. The existence of Jupiter’s powerful magnetic field provided evidence of the metallic nature of hydrogen well before it was produced in the laboratory.

Most of the extrasolar planets discovered have been gas giant planets. As the largest gas giant planet in the solar system, studies of Jupiter help to reveal the nature of these planets. Gas giant planets can have masses greater or less than that of Jupiter. However, in size, Jupiter is about as large as a gas giant planet can be. If it had much more mass, gravity would compress it to a smaller volume. Less mass would, of course, make a smaller planet, but its lower gravity would compress the planet less. For example, Saturn has almost 30 percent of Jupiter’s mass but more than 80 percent of Jupiter’s diameter and more than 55 percent of Jupiter’s volume.

Studies of Jupiter’s composition have led astronomers to believe that Jupiter may have formed somewhat farther from the Sun than the distance at which it currently orbits. Interactions with other planets, notably Saturn, could have caused Jupiter to migrate inward. This planetary migration also explains observations of extrasolar gas giant planets that appear much closer to their stars than can be explained through the understanding of planetary formation.

In late 2008, a research team reported the results of computer simulations based on the properties of hydrogen-helium mixtures exposed to the extreme conditions of temperature and pressure deep inside Jupiter. Their computer simulation also incorporated a core accretion model. In a paper published in the Astrophysical Journal Letters (20 Nov. 2008), these researchers presented an argument that Jupiter’s core could be twice as big as previously thought. Their simulation predicted a rocky core perhaps amounting to 5 percent of Jupiter’s total mass, making the rocky core equivalent to fourteen to eighteen Earth masses. That core would have layers of metals, rocky material, methane ice, water ice, and ammonia ice. Like Earth’s core, the very center of Jupiter’s core would be composed of iron and nickel. This computer simulation could be applied to attempts to understand the cores of the other gas giant planets as well. However, being a computer model, additional observational data and further analysis would be needed before this intriguing claim could achieve complete acceptance from the planetary science community.

Context

Jupiter and Saturn, the two largest planets in the solar system, probably share a common origin and similar structure. Both formed largely from material that was coming together to form the Sun. Thus, studies of these two worlds allow astronomers to learn more about conditions in the early solar system. Understanding these planets helps astronomers to understand how other planets, including the “rocky” planets such as Earth, form. Jupiter also seems to be similar to exoplanets that have formed around other stars and, thus, a sort of laboratory for understanding those extrasolar planets.

However, Jupiter is still nearly 600 million kilometers away from Earth, even at its closest approach. Thus, detailed studies have required visits by spacecraft. In total, seven spacecraft have studied Jupiter. Launched in the early 1970s, Pioneer 10 and Pioneer 11, followed in the late 1970s by Voyager 1 and Voyager 2, eventually flew past Jupiter on their way to the outer solar system. Early in 2007, the New Horizons spacecraft flew past Jupiter on its way to Pluto and the Kuiper Belt. The Cassini spacecraft passed by Jupiter on December 30, 2000, on its way to Saturn. The Ulysses spacecraft flew by Jupiter in February 1992, using that planet’s gravity to send it into an orbit that permitted it to study the Sun’s polar regions. All of these spacecraft studied Jupiter as they went past. The Galileo spacecraft, however, was sent specifically to study Jupiter, orbiting that planet from 1995 to 2003. Galileo also sent an atmospheric probe into Jupiter, the only probe to enter the atmosphere of any of the gas giant planets. In 2011, NASA launched the Juno spacecraft, which reached Jupiter's orbit in July 2016. Juno orbited the planet at about 5,000 kilometers (3,000 miles) above Jupiter's cloud tops at its closest approach. In its initial mission, Juno orbited Jupiter thirty-five times during a fifty-three-day polar orbit, making observations that revolutionized the understanding of the planet. Juno’s initial mission was extended through 2025. The primary goal of this mission is to better understand Jupiter's formation, atmosphere, and interior structure.

Jupiter is the best studied of the gas giant planets, therefore, but it still holds many mysteries. Its interior must still be investigated through inferences from observations of Jupiter’s exterior and of the planet’s magnetic field. Debate continues over the exact nature of the planet’s interior structure as astronomers pursue additional studies to develop a detailed understanding of this planet.

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