Jupiter was known to the ancient astronomers and was aptly named after the ruler of the Roman gods. It is by far the largest of all the planets, having a mass 318 times that of the earth, and is vast enough to contain more than 1,300 globes the size of our planet. Of the total extrasolar mass in the solar system, more than 70 per cent is contained within this one planet. Despite these statistics, however, Jupiter’s density is only 1.33 times that of water, a reflection of the fact that the planet’s composition differs radically from that of the earth; it is a gaseous, hot terrestrial planet.
Through the telescope, Jupiter appears as a yellow, oblate disk, crossed with orange-red bands that often show signs of turbulence. Observation of the movement of atmospheric features, such as the Great Red Spot, reveals the rapid axial rotation of this massive planet: Jupiter has the fastest rotation period of any planet. This is the reason for the relatively large ellipticity shown by the disk—the equatorial radius is 44,685 miles (71,900 kilometers) and is 2,670 miles (4,300 kilometers) greater than the polar radius.
Bursts of radio emission from the planet were detected in 1955, and steady synchrotron emission at shorter wavelengths was picked up a few years later. Because synchrotron radio emission results from charged particles moving at high speeds within a magnetic field, it was proposed that Jupiter itself must be trapping electrons in its own magnetic field. Not until Pioneer 10, the American Jupiter probe, flew past Jupiter in December 1973 was this idea proved to be true. Jupiter has the most intense magnetic field of any planet— about five times the strength of the earth’s field. When it was passing through the planet’s radiation belts, Pioneer 10 was exposed to radiation levels 400 times the lethal dose for humans from the electrons trapped there.
The Jovian atmosphere
Before the Pioneer and Voyager expeditions to Jupiter, it was thought from spectroscopic observations that the planet’s atmosphere contains methane, ammonia, and hydrogen as well as other gases in much smaller quantities. It is now believed that Jupiter’s atmosphere consists of about 84 per cent hydrogen and 15 per cent helium. The latter has been calculated not on the basis of direct observation, but on determinations of molecular weights.
Such a chemical composition is intriguingly similar to that of the sun. In fact, it has been estimated that if Jupiter had been formed from material that was dense enough for the planet to have fifty times its present mass, fusion reactions may well have started up within it and it would have evolved into a star. The solar system would then have been a binary star system.
The clouds that lie within Jupiter’s atmosphere are stacked mainly at three different levels. Each is at a different temperature, and so has a different chemical composition, which determines its color. Uppermost, with a temperature of —234° F. ( —148° C), are the white clouds, probably composed of solid ammonia; the intermediate clouds at —40° F. ( — 40° C), are tinged brown by polymerized ammonium hydrosulfide; the lower visible levels are so deep that the atmosphere above scatters their light to make them appear blue—just as Earth’s daytime sky looks blue.
The Great Red Spot, the most prominent of Jupiter’s features, remains a mystery despite the observations made by the Pioneer and Voyager space probes. A number of theories have been put forward that suggest how this feature, measuring about 25,000 miles (40,200 kilometers) by 20,000 miles (32,000 kilometers), could have survived for at least the century over which it has been observed. (It is at least 300 years old if seventeenth-century observations refer to the same spot.) The periphery of the Spot rotates counterclockwise and, as it is in the southern hemisphere of the planet, this rotational direction indicates that the Spot is a high-pressure zone. The red color is probably due to phosphorus, which is produced when phosphine (brought up from below) is dissociated by sunlight.
The Great Red Spot and smaller white ovals lie between zones where the wind blows in opposite directions. These winds have speeds of up to about 300 miles (500 kilometers) per hour. The zones adjacent to them have speed differences of up to 400 miles (650 kilometers) per hour. The zones themselves correlate with the position of the colored bands, and remain relatively fixed in latitude. The spots are rotated by the shear between adjacent windflows, in a movement similar to rollers turning beneath a moving load.
The internal structure of Jupiter
Earth-based observations have been important in determining the internal structure of Jupiter, by giving values for the size, shape, and mass
of the planet. For example, the mass of Jupiter’s gas relative to that of its solid core can be estimated from observations of the planet’s satellites.
These satellites respond to the gravitational attraction of the whole planet. The flattening of the globe due to its rotation rate results in a torque that causes the whole orbit of a satellite to revolve, or precess, around the planet. From observations of the precession rate, astronomers can calculate the invisible gravitational flattening of the whole planet, whereas direct observation of Jupiter reveals the flattening of only the outer, gaseous layer. This layer is affected by the centrifugal force resulting from Jupiter’s rapid rotation to a greater extent than is the underlying solid core. The relative masses of the two regions can be calculated from the precessing orbits of the satellites and from the observed degree of flattening of the planet.
Earth-based observations have revealed another important clue to the internal structure of Jupiter, which is that the planet emits about twice the amount of heat it receives from the sun. Convection is assumed to be the process by which this heat is transported from the interior of Jupiter to the surface layers. Theoretical models demonstrate that most of the planet is composed of hydrogen, and that only the uppermost few thousandths of Jupiter are gaseous. Moving down the atmosphere toward the center of the planet, the hydrogen becomes liquid as the temperature rises 34,000° F. (19,000° C) ata depth of about 8,080 miles (13,000 kilometers). The temperature continues to rise toward the core until at a depth of about 37,000 miles (60,000 kilometers), at the outer layer of the silicate core, the temperature reaches 43,000° F. (24,000° C).
Jupiter’s magnetic field
The source of Jupiter’s intense magnetic field is probably an internal dynamo, driven by the planet’s fast rotation and its high-temperature regions that conduct heat. Jupiter’s magnetic field is in the opposite direction to the earth’s, so a terrestrial compass would point south rather than north.
The strong magnetic field, combined with the relative weakness of the solar wind near the planet—Jupiter is 5.2 astronomical units from the sun—endows the planet with a very large magnetosphere, with a magnetotail that extends away from the sun to at least the distance of Saturn.
The outer region of the magnetosphere, which is influenced by plasma pressure and rotation-induced centrifugal forces, is diskshaped. All of the Galilean satellites, especially Io, are deeply embedded in this field. In addition, Io seems to be eroded by interaction with the high-energy particles within the field and it is also the source of the plasma that inflates the magnetosphere.
Before the Voyager missions to Jupiter, 13 satellites were known to orbit about the planet, extending from the innermost one, Amalthea, at 112,590 miles (181,200 kilometers) from Jupiter, to Sinope, which, at a distance of 14,900,000 miles (24,000,000 kilometers), takes 758 days to complete a single orbit.
The ring system
The probes found three more satellites close in to the planet, but one of the most surprising discoveries, made by Voyager 1, was the detection of a faint ring around Jupiter, split into three bands. This discovery makes Jupiter the nearest planet to the earth to exhibit a ring system.
Composed mostly of micron-sized particles and extending from about 1.7 to about 1.8 of a Jupiter radius, the principal ring is relatively thick, 18 miles (29 kilometers), with a width of 4,000 miles (6,400 kilometers), and is encapsulated in a much thicker halo. Furthermore, there appears to be a tenuous sheet structure that extends down from the main ring to the upper layers of the planet’s atmosphere. A ring system such as Jupiter’s must have a constant supply of new material, because particles are continually lost by atmospheric and other drag