Saturn's interior

Saturn, the second largest planet in the solar system, is famous for its ring system, but the key to understanding the planet is to understand its interior structure. Though it is similar to Jupiter and likely shares a similar origin, Saturn is not just a smaller version of Jupiter. While Saturn is called a gas giant planet, its interior is more than just a big ball of gas.

Overview

Jupiter and Saturn are the two largest planets in the solar system. Both planets are believed to have formed directly out of the accretion disk of material surrounding the Sun during its formation. As a result, both planets would be expected to have similar compositions and structures. There are important differences between Jupiter and Saturn, however. Because Saturn formed directly from the material accreting to form the Sun, it is expected to have a composition similar to that of the Sun and Jupiter. Indeed, Saturn is composed primarily of hydrogen and helium, the main constituents of Jupiter and the Sun.

Saturn is the second largest planet in the solar system, having about 30 percent of Jupiter’s mass, yet that mass is spread over a volume that is about 60 percent that of Jupiter. This gives Saturn a much lower density than that of any other planet in the solar system. One reason for this low density is that hydrogen, the main component of Saturn and Jupiter, is very compressible. Saturn’s lower gravity, due to its lower mass, compresses its hydrogen less than does Jupiter. Thus, Saturn is much more distended than Jupiter.

Though Saturn’s primary composition is hydrogen and helium, it has also accumulated heavier elements through collisions with smaller bodies since it formed. Some of the material is iron and silicates (rocks), and these materials sink to the center of the planet to form a core. Such a core would be under extreme pressure and temperature, with the temperature of the core is probably near ten thousand kelvins. Since Saturn is in the outer solar system, many of the bodies colliding with it would also contain ices—not only water ice but also frozen carbon dioxide, frozen methane, and frozen ammonia. These ices, heavier than hydrogen, also sink deep into the planet. However, at the pressure and temperature at that depth, the ices are in a liquid state. The term “liquid ices” is often used to describe the state of these fluids.

Saturn has an oblateness of 0.98 (its equatorial diameter is 9.8 percent greater than the pole-to-pole diameter), making Saturn the most oblate of the solar system’s planets. Though this level of oblateness constrains the possible size of Saturn’s core, there is some disagreement among planetary scientists as to the core's actual size. Many models suggest that Saturn has a core that is about ten to fifteen times the mass of Earth. This is similar in mass to Jupiter’s core, but it is less compressed, having a diameter perhaps in excess of twenty thousand kilometers. Though Saturn’s core is about the size of Jupiter’s core, it is a much larger percentage of Saturn than Jupiter’s core is of Jupiter. A layer of liquid ices of perhaps up to ten thousand kilometers thick likely sits on top of the core.

The vast majority of the rest of Saturn is composed of hydrogen and helium. These are in a gaseous state in the outermost parts of the planet. Clouds of ammonia ice and other ices exist in the upper few hundred kilometers. Below about two thousand kilometers beneath the clouds, the pressure is so high that hydrogen is compressed into a liquid state. However, the temperature and pressure of Saturn’s interior are well beyond the critical point of hydrogen. The critical point in chemistry is the temperature and pressure at which there is no clear distinction between liquid and gas fluid states. Therefore, there is no clear boundary between the gaseous upper portion of Saturn and the liquid hydrogen inner portion. As depth increases, the hydrogen gradually becomes more and more liquid-like. The bulk of Saturn is in this liquid state, even though it is called a “gas” giant planet.

Hydrogen begins to exhibit typical metallic properties, such as electrical conductivity, when it is under sufficient pressure and temperature. On Saturn, these conditions are believed to be met near twenty-five thousand kilometers beneath the cloud layers. Thus, from the top of the liquid ices layer out to nearly 55 percent of Saturn’s radius, the hydrogen is in this liquid metallic state. Jupiter also has a liquid metallic hydrogen mantle, but on that planet, this layer is far larger than on Saturn. As with Jupiter, the magnetohydrodynamics of Saturn’s liquid metallic hydrogen is the source of the planet’s magnetic field. Planetary magnetic fields are believed to be produced through the motion of a highly conducting fluid through a magnetic field. The conductor then reinforces the existing field, creating a rather stable and permanent planetary field. This is the dynamo model of planetary magnetic fields. Because Saturn’s liquid metallic hydrogen layer is far smaller than Jupiter’s, Saturn’s magnetic field is much weaker than Jupiter’s, being only about 3 percent as strong. Even so, it is still nearly six hundred times stronger than Earth’s magnetic field.

Saturn, like Jupiter, radiates more energy than it gets from the Sun, almost 2.9 times what it gets from the Sun. Jupiter radiates energy through kelvin-Helmholtz contraction, a form of hydrostatic contraction that permits a fluid body to compress, generating thermal energy. Saturn has much less mass than Jupiter. Theory indicates that kelvin-Helmholtz contraction is unable to produce the level of thermal energy needed to account for observations of Saturn. However, Saturn’s temperature is lower than Jupiter’s. At the temperature and pressure of Saturn’s liquid hydrogen layers, helium, another major component of Saturn, precipitates out. Droplets of helium, heavier than hydrogen, sink toward the planet’s deeper interior. The sinking helium releases gravitational energy in the form of heat, warming the planet. This thermal energy is nearly double the energy that the planet gets from the Sun, resulting in Saturn’s observed emissions.

Knowledge Gained

Most observations of Saturn have, of necessity, been made from Earth. The interior of the planet, however, cannot be studied from afar. Spacecraft sent to Saturn have yielded more data, but there is still considerable debate about the nature of Saturn’s interior. Mathematical models of Saturn’s interior are made based on observed characteristics, but more research is needed. The exact nature of Saturn’s interior is, therefore, still the object of much speculation.

Because Saturn is shrouded in clouds, astronomers are unable to measure Saturn’s rotational rate directly using observations from Earth. The planet’s magnetic field should rotate with the planet. In 1981, Voyager 2 measured a rotational rate of a bit over 10 hours, 39 minutes. In 2007, however, scientists at the National Aeronautics and Space Administration’s (NASA’s) Jet Propulsion Laboratory released a new finding of 10 hours, 32 minutes, and 35 seconds for Saturn’s rotational rate using data from the Cassini orbiter. The discrepancy between these measurements may be explained by the fact that it is unusually difficult to determine the rotational rate for Saturn because the planet’s magnetic axis is nearly the same as its rotational axis, providing little change in the magnetic field for spacecraft to measure as the planet rotates.

Saturn and Jupiter both likely formed from the cloud of gas swirling together to form the Sun. Thus, these two gas giants would be expected to have very similar compositions to the sun. Indeed, like the Sun, Jupiter and Saturn are both composed mostly of hydrogen and helium. Saturn’s atmosphere, however, contains quite a bit less helium than either the Sun or Jupiter. This finding was a mystery until astronomers found that Saturn radiates far more energy than it gets from the Sun. The explanation that helium precipitating to lower levels of Saturn could heat the planet also explains why the upper portions of Saturn, which can be observed, are deficient in helium.

Hydrogen and helium are the primary constituents of Saturn. Both Jupiter and Saturn are somewhat enriched in elements heavier than hydrogen and helium when compared with the Sun. This is expected because both planets have, over the several billion years since the planets’ formation, accreted planetesimals, asteroids, and comets. Saturn is considerably more enriched in these heavier elements than Jupiter, however, and the reason is unclear. Saturn may have simply accreted a greater percentage of these bodies, or it may have collected less hydrogen and helium when it formed. Further research is needed to answer the question of why Saturn and Jupiter have this compositional difference.

Studies of Saturn and Jupiter have led some astronomers to theorize that these planets may have formed somewhat closer together than they currently are in the solar system. Jupiter migrated closer to the Sun, and Saturn migrated somewhat farther from the Sun to its current position.

Context

The two largest planets in the solar system, Jupiter and Saturn, probably formed in the same manner, at about the same time, in the same part of the disk of material swirling together to form the sun. Thus, they would be expected to be very similar. Indeed, they are similar, but there are important differences between them. Understanding those differences will cast light on the conditions under which gas giant planets form. In turn, understanding the formation of the gas giants will improve our understanding of the formation of other planets in the solar system, such as Earth, as well as the planetary systems of other stars.

The planet Saturn, because of its great distance from Earth—more than 1.2 billion kilometers at its closest—has mostly been studied via telescopes here on Earth. However, four spacecraft have investigated Saturn up close. The first was Pioneer 11, which passed closest to Saturn on September 1, 1979. Then, two Voyager spacecraft visited Saturn, with Voyager 1 passing closest to Saturn on November 12, 1980, and Voyager 2 flying past Saturn on August 25, 1981. No spacecraft visited Saturn until more than two decades later, when the Cassini orbiter entered orbit around Saturn on July 1, 2004. Cassini studied Saturn and its satellites until 2017, when it ended its mission by diving into Saturn’s icy atmosphere. Although Cassini carried the Huygens probe to study Saturn’s satellite Titan, no atmospheric probe was carried to study Saturn’s atmosphere or interior, so all studies of Saturn’s interior must be made by inference from observations of Saturn’s exterior and of the planet’s magnetic field. This means that scientists do not yet have a firm grasp of Saturn’s interior, and further research is needed to fully understand this planet’s interior structure. Still, Cassini did provide scientists with new data about Saturn’s interior during its final dive, and this data regarding the planet’s atmosphere and core is being studied. Scientists have also relied on the study of Saturn’s rings to shed light on the interior of Saturn.

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