III.

Composition and Structure of Jupiter

Astronomers were able to accurately determine Jupiter’s mass even before 1900. They calculated the gravitational force that Jupiter exerts on its satellites by measuring their movements around the planet over an extended period. Because the gravitational force exerted by a planet is proportional to its mass, they could deduce Jupiter’s mass. Spacecraft flying by Jupiter have made more detailed studies of Jupiter’s gravitational field possible, giving clues about the planet’s inner structure. These spacecraft have also relayed close-up images of the clouds and information about the composition of Jupiter’s outer layers. Putting all of this data together, astronomers have assembled a detailed picture of Jupiter’s composition and structure.

A.

Composition of Jupiter

The fact that Jupiter’s radius is 11.2 times larger than Earth’s means that its volume is more than 1,300 times the volume of Earth. The mass of Jupiter, however, is only 318 times the mass of Earth. Jupiter’s density (1.33 g/cm³) is therefore less than one-fourth of Earth’s density (5.52 g/cm³). Jupiter’s low density indicates that the planet is composed primarily of the lightest elements—hydrogen and helium.

Galileo, a National Aeronautics and Space Administration (NASA) spacecraft composed of an orbiter and a planetary probe, arrived at Jupiter in 1995. The probe, which entered the atmosphere near 6° north, measured high winds and a puzzling lack of water molecules deep in Jupiter’s atmosphere. It also found that the ratio of the amount of hydrogen present to the amount of helium present was similar to the ratio that has been determined for the outer envelope of the Sun. This similarity in the hydrogen-helium ratio supports the theory that Jupiter and the Sun formed from the same cloud of material (See also Planetary Science).

B.

Structure of Jupiter

When a spacecraft flies by a planet, the gravitational field of the planet causes the spacecraft to accelerate. This change in speed and direction can be detected as a slight shift in the frequency of the radio signals that the spacecraft is sending back to Earth (see Doppler Effect). Scientists have analyzed radio signals from several spacecraft that have passed Jupiter and have combined their results with studies of Jupiter's composition to create computer models of the planet. The computer models predict that Jupiter's outer layer, composed of a gaseous mixture of hydrogen, helium, and traces of hydrogen-rich compounds such as ammonia, methane, and water vapor, is about 1,000 km (about 600 mi) thick. Beneath this layer, the pressure is so great and the atmosphere is so hot and compressed that the hydrogen and helium atoms do not behave as a gas, but as what physicists call a supercritical fluid. Supercritical fluids form at high temperatures and pressures and have properties similar to those of both gases and liquids. The supercritical zone extends 20,000 to 30,000 km (12,000 to 19,000 mi) into Jupiter, which is about one-fourth to one-third of the radius of the planet.

Beneath the supercritical fluid zone, the pressure reaches 3 million Earth atmospheres. At this depth, the atoms collide so frequently and violently that the hydrogen atoms are ionized—that is, the negatively charged electrons are stripped away from the positively charged protons of the hydrogen nuclei. This ionization results in a sea of electrically charged particles that resembles a liquid metal and gives rise to Jupiter’s magnetic field. This liquid metallic hydrogen zone is 30,000 to 40,000 km (19,000 to 25,000 mi) thick—about half the radius of the planet—and extends to the molten rock core at Jupiter's center. The molten rock core occupies a sphere with a radius of about 10,000 km (about 6,000 mi)—about one-fourth of Jupiter's total radius—and has a mass perhaps 10 to 15 times the mass of Earth.

C.

Evolution of Jupiter

According to current theories, an enormous disk of dust and gas encircled the Sun as it formed about 4.6 billion years ago. The material in this disk eventually formed the planets, moons, and asteroids of the solar system. Mineral particles and metal-rich grains in this disk combined with icy comet-like fragments to form seeds for larger bodies. The largest fragments swept up the most dust and surrounding gases and became the planets. Planets such as Jupiter and Saturn that attained masses greater than 14 times the mass of Earth had sufficient gravity to attract and hold hydrogen and helium atoms, which constituted most of the disk material. These planets became gas giants. Planets with weaker gravity, such as Earth and Mars, could not hold hydrogen and helium and so remained smaller and mainly rocky. Eventually, nearly all of the matter of the disk was concentrated in a few bodies: the planets and their moons. Jupiter was the largest of these bodies.

Despite the planet’s large size, Jupiter is far too small to become a star. The pressure and temperature at Jupiter’s core are not high enough to cause sustained fusion of hydrogen—the process that makes a star shine. Even though Jupiter contains more than twice as much mass as all the other planetary bodies in the solar system combined, it would need to have about 80 times its current mass for sustained fusion to occur.

Many puzzles remain about how Jupiter and similar giant planets form. Scientists have been surprised by the orbits of some Jupiter-like planets discovered around other stars. In some cases, the giant planets orbit closer in around these stars than Mercury orbits our Sun. Other Jupiter-like planets have extremely eccentric orbits, unlike Jupiter's nearly circular orbit. These findings and other research suggest that Jupiter and the other giant planets in our solar system may not have formed in their present orbital positions. Jupiter may have formed closer to the Sun and moved outward, or Jupiter may have moved inward. If Jupiter had a shifting orbit, its movements may have greatly affected the formation and orbits of other planets in the solar system.

How quickly Jupiter formed is another puzzle. Planets are thought to form by accretion (clumping together) of smaller bodies while stars form by gravitational collapse at the center of a gas cloud. Accretion is thought to be a relatively slow process—too slow perhaps for Jupiter to have retained so much gas. A faster way for a giant planet to take shape might be the sudden collapse of a disk of dust and gas into a large body. NASA’s planned Juno probe to Jupiter will study the size of Jupiter's rocky core, which may provide a clue to how Jupiter became a planet.