Neptune's interior

Neptune is a gas giant planet, so its interior structure is completely different from that of the terrestrial planets and most similar to that of Uranus, another gas giant. The most likely structure consists of a relatively small rocky core, an icy layer, and a molecular hydrogen layer. The interior is then covered with an atmospheric layer of hydrogen, helium, and various other gases. Neptune’s interior lacks the metallic hydrogen layer found in Jupiter and Saturn.

Overview

It is not possible to conduct direct studies of planetary interiors, including Earth’s, so astronomers must resort to indirect clues to infer the interior structures and compositions of planets. These clues can include characteristics of the bulk density, the planetary magnetic field, seismic activity and waves for terrestrial planets, and heat flow from the interior.

The two major classes of planets in the solar system are the Jovian planets and the terrestrial planets. Jupiter is the prototype for the Jovian planets, which also include Saturn, Uranus, and Neptune. Earth is the prototype for terrestrial planets, which also include Mercury, Venus, and Mars. Earth’s moon is also very similar in structure to the terrestrial planets. In oversimplified terms, Jovian planets are big balls of gas, and terrestrial planets are small balls of rock and metal.

A planet’s bulk density is one clue to its interior composition and structure. Density is defined as the mass divided by the volume. Both an object’s mass and its volume depend on the object’s composition and how much of that material there is. For example, a boulder has a greater volume and mass than a pebble made of the same type of rock. However, the amount of material cancels out when dividing the mass by the volume. Therefore, the density of a boulder and the density of a pebble made of the same type of rock will be the same, even if the boulder is as large as a planet. Density is a property of the type of material, but not how much material there is, so the density of a planet can tell us something about its composition. Water has a density of one thousand kilograms per meter cubed (kg/m³). Planets with approximately this density are primarily icy materials or gas. Planets with a density of approximately two thousand to three thousand kg/m³ are typically made of rocky materials. Planets with a density of seven thousand to nine thousand kg/m³ would be primarily metallic in composition. Planets with densities between these ranges are mixtures of materials. Neptune has a density of 1,600 kg/m³. It must, therefore, be mostly ice or gas. This density, the highest of the Jovian planets, is slightly greater than the density of Uranus and significantly greater than the densities of Jupiter and Saturn. Therefore, the rocky cores of Neptune and Uranus constitute a larger portion of their total mass than do the cores of Jupiter or Saturn. Because Neptune’s density is slightly greater than that of Uranus, the two planets must have differences in their interior compositions despite their very close similarities.

Seismic activity can give us clues to the interiors of only the terrestrial planets (which are solid and, therefore, have seismic activity). Neptune is a gas giant planet with no potential for seismic activity, so this potential clue does not apply to Neptune.

Because the Jovian planets are gas, astronomers can use the ideal gas law for theoretical calculations of interior properties. This equation relates the temperature, density, and pressure of the gas.

Planetwide magnetic fields give us clues to the planets’ interiors. All magnetic fields are ultimately produced by some type of electric current, so for a planet to have a magnetic field, its interior must contain a liquid or gas layer that can conduct electricity. The planet’s rotation helps set up the electric currents in this layer. A composition of iron or other ferromagnetic materials can enhance the magnetic field produced by the interior electric currents but is not essential. Neptune does have a strong magnetic field that is comparable in strength to those of Saturn and Uranus. Like that of Uranus, Neptune’s magnetic field axis is tilted significantly from its rotational axis. The tilt is 46.9 degrees in the case of Neptune. The center of Neptune’s magnetic dipole is displaced significantly from the center of the planet. This displacement is even greater than for Uranus. No other planets have such displacements or tilts of their magnetic dipoles. If the icy layer is not completely solid but at least partially molten, then the water can conduct electric currents and produce a magnetic field. Impurities in the water increase its ability to conduct electricity. Hence, we infer that Neptune’s icy layer is not completely solid, and the molten regions are not symmetric about the center of the planet.

With the exception of Uranus, the Jovian planets emit more energy than they receive from the Sun. Neptune emits about three times as much energy as it receives from the Sun. This extra energy must come from somewhere, and the only place available is the planet’s interior. Therefore, the interiors of the Jovian planets, including Neptune, must be hot. When a gas giant planet forms, its own gravity compresses the gas. A gas heats up when it is compressed, so the interiors of gas giant planets should be hot. Neptune, therefore, has a hot interior, as expected from the behavior of gases and confirmed by the heat flowing outward from the interior to the surface. The existence of large storms and weather on Neptune provides additional evidence that heat energy, which is needed to power storms, flows upward from the interior.

Putting all these clues together, astronomers can infer the interior structure of Neptune. The innermost layer is a rocky core. Neptune’s rocky core is approximately the size of Earth and has a temperature of about seven thousand kelvins (6,727 degrees Celsius)—for comparison, the surface of the Sun is 5,778 kelvins. The mass of Neptune’s rocky core is most likely a little less than ten times the mass of Earth, but estimates range from four to fifteen times Earth’s mass. The rocky core is more massive than Earth, even though it is about the same size because the high temperatures and pressures in the core of the planet increase the density of the rock.

Above the rocky core, Neptune has an icy mantle consisting primarily of water, methane, and ammonia ice that is not completely frozen. The ice is slushy rather than completely frozen—hence, convection currents can flow in this mantle. The convection currents carry heat from the interior to the surface. They also help form Neptune’s magnetic field. As the slushy material circulates, it moves in the Sun’s extended magnetic field. This motion induces electric currents in the slush. The slushy water ice has ammonia dissolved in it, so it conducts electricity very well. These induced electric currents induce Neptune’s magnetic field. Electric currents induce magnetic fields, and moving or changing magnetic fields induce electric currents. If the regions where the magnetic field is generated are far from the center of the planet and distributed asymmetrically around the planet, then the magnetic field would be tilted and displaced, as the Voyager 2 data show.

The outermost layer of Neptune’s interior is molecular hydrogen. The temperature, pressure, and density of the molecular hydrogen layer increase with depth. The icy layer can form despite high temperatures because the freezing temperature of water increases as the pressure increases.

Neptune’s interior structure is similar to that of Uranus but dissimilar from the interiors of the other gas giants, Jupiter and Saturn, which have a metallic hydrogen layer in addition to these layers. Neptune and Uranus do not have metallic hydrogen layers because the pressure in the hydrogen layers is not high enough for hydrogen to become metallic hydrogen rather than molecular hydrogen.

Knowledge Gained

Most of what was initially known about Neptune and its interior came from the Voyager 2 mission. After flying by Jupiter and Saturn, Voyager 1 flew out of the plane of the solar system. Voyager 2 flew on to Uranus and Neptune. The Neptune flyby was in 1989. Examples of the knowledge of Neptune gained from the Voyager mission include measuring the planet’s magnetic field, taking more accurate measurements of Neptune’s mass and, therefore, its density, and discovering its surface storms.

The Great Dark Spot that the Voyager discovered on Neptune was a large storm, similar in nature to the Great Red Spot on Jupiter. Since the Voyager mission, the Hubble Space Telescope has been able to take pictures with enough resolution to allow astronomers to follow weather patterns on Neptune. Storm systems on Neptune change. Weather and large storm systems require energy to drive them. On Earth, the energy comes from the sun, but Neptune is too far from the Sun for solar energy to drive the storms. Therefore, the needed energy must come from the interior of the planet.

The interior differences among the four Jovian planets provide us with clues to their formation. Jupiter, Saturn, Uranus, and Neptune all have rocky cores of approximately the same size and mass. Jupiter and Saturn, however, have much more hydrogen and helium surrounding their rocky cores. This observation suggests that Uranus and Neptune formed much later than Jupiter and Saturn, at a time when the protoplanetary disk had dissipated more of its hydrogen and helium gas. At the greater distance from the Sun, the small planetesimals that had to merge to form the cores of Uranus and Neptune were farther apart and, therefore, took longer to merge.

Neptune, like the other planets, is differentiated into layers. The denser materials are closer to the center. Differentiation in solid terrestrial planets tells us that they were at one time liquid because denser materials would sink only in a fluid, not in a solid. Smaller satellites that were never liquid are not differentiated.

In 2015, New Horizons passed Neptune taking pictures of the planet’s back side. These images provided scientists with additional data with which to analyze Neptune. Launched in 2021, the James T. Webb Space Telescope has provided scientists with more images of Neptune, its satellites, and its rings, as well as provided scientists with visual data to continue their study of the planet which will, hopefully, lead to further answers regarding Neptune’s interior.

Context

Comparing the interiors of other planets casts light on Earth’s interior, just as knowledge of Earth’s interior provides insights into other planets. These comparisons of how planetary interiors develop provide clues to the origins of the planets and to the processes that shape planetary interiors and compositions.

Voyager 2 studied the outer planets Uranus and Neptune. New Horizons sent back images, and the James T. Webb Space Telescope has also provided images of Neptune for scientists to study. Through this technology, scientists are learning more about Neptune. As technology advances, our knowledge of the planet and its interior will increase dramatically. It is very likely to completely change our ideas about Neptune’s interior.

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