Contents

• Observation by Spacecraft.
• Composition, Structure, and Magnetic Field.
• Jupiter’s Satellites and Rings.

JUPITER,In astronomy, fifth planet from the sun, and the largest planet in the solar system. Named for the ruler of the gods in Roman mythology, Jupiter has 1316 times the volume of earth but is only 318 times more massive. Thus, the mean density of Jupiter is about one-fourth that of earth, indicating that the giant planet consists mainly of gas rather than the metals and rocks of which the earth and other inner planets are composed.

Orbiting the sun at an average distance of 778,412,020 km (483,682,810 mi), Jupiter makes a complete revolution in 11.857 earth years but takes only 9.925 hr to rotate once on its axis. This rapid rotation causes an equatorial bulge that is apparent in telescopic views of the planet. The rotation is not uniform. The banded appearance of Jupiter reflects the presence of strong atmospheric currents that lead to different rotation periods at different latitudes. These bands are made more apparent by the pastel colors of the clouds themselves, including the famous ocher-colored oval called the Great Red Spot. The colors come from traces of compounds formed by ultraviolet light, lightning discharges, and heat. Some of these compounds may be related to organic molecules that formed on the ancient earth as a prelude to the origin of life.

The first spacecraft to approach Jupiter was Pioneer 10,, launched in 1972 by the U.S. National Aeronautics and Space Administration (NASA); in 1973 its measurements revealed the planet's intense radiation and magnetic field. The following year Pioneer 11 provided more detailed images and the first close look at Jupiter's polar regions. Scientific knowledge of the Jupiter system increased enormously in 1979 with the successful visits by NASA’s Voyager 1 and 2.. Many previously unknown features of the planet were documented, including the thin ring around the planet and the extreme turbulence around the Great Red Spot; the predominance of H₂ in the makeup of the atmosphere was confirmed; and detailed photographs of Jupiter's four large moons, suggesting that three of them have an ice crust, were transmitted.

More knowledge about Jupiter and its system was provided by the mission of the Galileo spacecraft. Launched in 1989 by the U.S., it traveled through the inner solar system viewing the dark side of the moon, taking the first close-up of an asteroid, and viewing the comet Shoemaker-Levy 9. In July 1994, astronomers were treated to an unprecedented celestial show when a series of about 21 fragments of the comet smashed into Jupiter, an event predicted a year earlier. The impacts produced brilliant fireballs and left dark "bruises" on the planet's surface; one of the largest fragments produced a plume of gas about 1900 to 2600 km (1200 to 1600 mi) high and left a dark spot about the size of earth. Data from the collisions provided a greater understanding of the composition of Jupiter, particularly its upper atmosphere. Galileo's instrument-loaded probe plunged into the planet's atmosphere in December 1995, transmitting data that included the first direct measurements of Jupiter's atmosphere, temperature, density, and composition before the probe melted. Galileo then moved into orbit around Jupiter. During the next seven years it made multiple flybys of Jupiter's largest moons-Io, Europa, Ganymede, and Callisto-and transmitted high-resolution images and valuable data. Galileo also observed a satellite of the asteroid Ida; the International Astronomical Union named it Dactyl after Dactylos, the son of Ida and Jupiter. Following a November 2002 flyby of Amalthea, one of Jupiter's inner moons, NASA mission controllers set the aging spacecraft on a collision course with Jupiter, and Galileo's 14-year mission came to an end in September 2003. Additional data was provided by the Cassini spacecraft, which, while en route to Saturn, made its closest approach to Jupiter in December 2000; and by the Ulysses spacecraft, a joint NASA and European Space Agency mission, which, while studying the sun, followed a trajectory that took it past Jupiter in 1992 and 2004.

At least 87 percent of Jupiter's atmosphere is molecular hydrogen, H2, with helium, He, constituting most of the remainder. The interior must have essentially the same composition as the atmosphere in order to yield the low observed density. It appears that this huge world is made mostly from the two lightest and most abundant elements in the universe, a composition similar to that of the sun and other stars. Jupiter may therefore represent a direct condensation of a portion of the primordial solar nebula-the great cloud of interstellar gas and dust from which the entire solar system formed about 4.6 billion years ago.

Jupiter radiates about twice as much energy as it receives from the sun. The source of this energy is apparently a very slow gravitational contraction of the entire planet, rather than the nuclear fusion that powers the sun. Jupiter would have to be almost 100 times larger to have enough mass to ignite a nuclear furnace.

Jupiter's turbulent, cloud-filled atmosphere is cold, although the probe from the Galileo spacecraft in 1995 indicated a hotter, drier atmosphere than previously believed. With hydrogen so abundant, hydrogen-based molecules, such as methane, ammonia, and water, predominate. Periodic temperature fluctuations in Jupiter's upper atmosphere reveal a pattern of changing winds like that in the equatorial region of earth's stratosphere. Photographs of sequential changes in Jovian clouds suggest the birth and decay of giant cyclonic storm systems in the atmosphere; Galileo's probe gave evidence of winds up to 644 km per hour (400 mph).

Ammonia freezes in the low temperature of Jupiter's upper atmosphere (--125° C/--193° F), forming the white cirrus clouds-zones, ovals, and plumes-seen in many photographs of the planet transmitted by the Voyager spacecraft. At lower levels, ammonium hydrosulfide can condense. Colored by other compounds, clouds of this substance may contribute to the widespread tawny cloud layer on the planet. The temperature at the tops of these clouds is about --50° C (about --58° F), and the atmospheric pressure about twice the sea-level atmospheric pressure on earth. Through holes in this cloud layer, radiation escapes from a region where the temperature reaches 17° C (about 63° F). Still deeper, warmer layers have been detected by radio telescopes that are sensitive to cloud-penetrating radiation.

Although only the barest skin of the planet is directly visible, calculations show that the temperature and pressure continue to increase toward the interior, reaching values at which hydrogen first liquefies and then assumes a metallic, highly conducting state. A core of earthlike material may exist at the center.

TThe Jovian magnetic field is generated deep within these layers. At the surface of Jupiter, this field is 14 times stronger than earth's. Its polarity is the opposite of earth's, so a terrestrial compass taken to Jupiter would point south. This field is responsible for the huge belts of trapped charged particles that circle the planet out to a distance of 10 million km (about 6.2 million mi).

Sixteen principal satellites of Jupiter have so far been discovered; by early 2004, 47 smaller moons had also been sighted (23 of them in 2003), bringing the total number to 63. The four largest moons were discovered in 1610 by the Italian astronomer Galileo. They were subsequently named after mythological paramours of Jupiter (or Zeus in the Greek pantheon): Io, Europa, Ganymede, and Callisto. This tradition has been followed in naming some of the other moons.

Modern observations have shown that the mean densities of the largest moons follow the trend apparent in the solar system itself. Io and Europa, close to Jupiter, are dense and rocky like the inner planets. Ganymede and Callisto, at greater distances, are composed largely of water ice and have low densities. During the formation of both planets and satellites, proximity to the central body (the sun or Jupiter) evidently prevented the more volatile substances from condensing.

Callisto is almost as big as Mercury, and Ganymede is bigger than Mercury. If they orbited the sun, they would be considered planets; internal activity on Ganymede recorded by Galileo suggested that the moon has its own magnetic field. The presence of complex organic molecules-basic ingredients for life-was detected by Galileo on the surfaces of Callisto and Ganymede. The icy crusts of these two bodies are marked by numerous craters, the record of an early bombardment, probably by comet nuclei, similar to the asteroidal battering that scarred earth's moon. In contrast, the surface of Europa is extremely smooth. It is evidently covered by a layer of water ice that emerged from Europa's interior after the early bombardment. A layer of liquid water is thought to lie beneath the ice, and images returned by Galileo suggested that it may be only 1 to 2 km (less than 1.25 mi) beneath the surface of Europa's ice crust. An intricate network of shallow cracks covers the icy surface.

The most remarkable satellite is unquestionably Io. Its surface has a bizarre appearance: yellowish, brown, and white areas dotted by black features. Io is racked by volcanism that is driven by the dissipation of tidal energy in the satellite's interior. Ten volcanoes were erupting during the spacecraft Voyager’s flybys in 1979, and in 1997 Galileo sent back images of Ra Patera in eruption. Sulfur dioxide issues from the vents and condenses on the surface, forming a local, transient atmosphere. The white regions are solid SO2; the other markings are presumably caused by other sulfur compounds.

The remaining moons are very much smaller and less well studied than the four Galilean satellites. Many of the outer moons may represent captured bodies.

Close to the planet is a faint system of rings. The material in these rings must be continuously renewed, since it is visibly moving in toward the planet. It may be produced by the disintegration of small moonlets imbedded within it. The satellite Metis is just at the outer boundary and could be one source of ring material.

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