the chief ancient Roman and Italian god. Like Zeus, the Greek god with whom he is etymologically identical (root diu, “bright”), Jupiter was a sky god. One of his most ancient epithets is Lucetius (“Light-Bringer”); and later literature has preserved the same idea in such phrases as sub Iove, “under the open sky.” As Jupiter Elicius he was propitiated with a peculiar ritual to send rain in time of drought; as Jupiter Fulgur he had an altar in the Campus Martius, and all places struck by lightning were made his property and were guarded from the profane by a circular wall.

the most massive planet of the solar system and the fifth in distance from the Sun. It is one of the brightest objects in the night sky; only the Moon, Venus, and sometimes Mars are more brilliant. Jupiter is designated by the symbol ♃.

Planetary data for Jupiter Planetary data for JupiterWhen ancient astronomers named the planet Jupiter for the Roman ruler of the gods and heavens (also known as Jove), they had no idea of the planet's true dimensions, but the name is appropriate, for Jupiter is larger than all the other planets combined. It takes nearly 12 Earth years to orbit the Sun, and it rotates once about every 10 hours, more than twice as fast as Earth; its colourful cloud bands can be seen with even a small telescope. It has a narrow system of rings and more than 60 known moons, one larger than the planet Mercury and three larger than Earth's Moon. Some astronomers speculate that Jupiter's moon Europa may be hiding an ocean of warm water—and possibly even some kind of life—beneath an icy crust.

Knowledge about the Jovian system grew dramatically after the mid-1970s as a result of explorations by three spacecraft missions—Pioneers (Pioneer) 10 and 11 in 1973–74, Voyagers (Voyager) 1 and 2 in 1979, and the Galileo orbiter and probe, which arrived at Jupiter in December 1995. The Pioneer spacecraft served as scouts for the Voyagers, showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment. The greater number and increased sophistication of the Voyager instruments provided so much new information that it was still being analyzed when the Galileo mission began. The previous missions had all been flybys, but Galileo released a probe into Jupiter's atmosphere and then went into orbit about the planet for intensive investigations of the entire system over several years. Yet another view of the Jovian system was provided in 2000 by the flyby of the Cassini spacecraft on its way to Saturn. Observations of the impacts of the fragmented nucleus of Comet Shoemaker-Levy 9 with Jupiter's atmosphere in 1994 also yielded information about its composition and structure.Yet another look at the Jovian system was provided in late 2000 and early 2001 by the flyby of the Cassini spacecraft on its way to Saturn. Observations of the impacts of the fragmented nucleus of Comet Shoemaker-Levy 9 with Jupiter's atmosphere in 1994 also yielded information about its composition and structure.

Planetary data for JupiterJupiter has an equatorial diameter of about 143,000 km (88,900 miles) and orbits the Sun at a mean distance of 778 million km (483 million miles). The table (Planetary data for Jupiter) shows additional physical and orbital data for Jupiter. Of special interest are the planet's low mean density of 1.33 grams per cubic cm—in contrast with Earth's 5.52 grams per cubic cm—coupled with its large dimensions and mass and short rotation period. The low density and large mass indicate that Jupiter's composition and structure are quite unlike those of Earth and the other inner planets, a deduction that is supported by detailed investigations of the giant planet's atmosphere and interior.

Even a modest telescope can show much detail on Jupiter. The region of the planet's atmosphere that is visible from Earth contains several different types of clouds (cloud) that are separated both vertically and horizontally. Changes in these cloud systems can occur over periods of a few hours, but an underlying pattern of latitudinal currents has maintained its stability for decades. It has become traditional to describe the appearance of the planet in terms of a standard nomenclature for its alternating dark bands, called belts, and bright bands, called zones. The underlying currents, however, seem to have a greater persistence than this pattern.

Jupiter has no solid surface—hence, no topographic features—and the planet's large-scale circulation is dominated by latitudinal currents. The lack of a solid surface with physical boundaries and regions with different heat capacities makes the persistence of these currents and their associated cloud patterns all the more remarkable. The Great Red Spot, for example, moves in longitude with respect to all three of the planet's rotation systems, yet it does not move in latitude. The white ovals found at a latitude just south of the Great Red Spot exhibit similar behaviour; white ovals of this size are found nowhere else on the planet. The dark brown clouds, evidently holes in the tawny cloud layer, are found almost exclusively near 18° N latitude. The blue-gray or purple areas, from which the strongest thermal emission is detected, occur only in the equatorial region of the planet.

The true nature of Jupiter's unique Great Red Spot was still unknown at the start of the 21st century, despite extensive observations from the Voyager and Galileo spacecraft. On a planet whose cloud patterns have lifetimes often counted in days, the Great Red Spot has survived as long as detailed observations of Jupiter have been made—at least 300 years. There is some evidence that the spot may be slowly shrinking, but a longer series of observations is needed to confirm this suggestion. Its present dimensions are about 20,000 by 12,000 km (12,400 by 7,500 miles), making it large enough to accommodate both Earth and Mars. These huge dimensions are probably responsible for the feature's longevity and possibly for its distinct colour.

Atmospheric abundances for JupiterThe presence of methane and ammonia in Jupiter's atmosphere was deduced in this way in the 1930s, while hydrogen was detected for the first time in 1960. (Although 500 times more abundant than methane, molecular hydrogen has much weaker absorption lines because it interacts only very weakly with electromagnetic waves.) Subsequent studies led to a growing list of new constituents, including the discovery of the arsenic compound arsine in 1990. The table (Atmospheric abundances for Jupiter) includes a list of Jupiter's atmospheric constituents and their abundances as determined by Earth-based, spacecraft, and atmospheric probe observations as of 2002.

Atmospheric abundances for JupiterThe elemental abundances in Jupiter's atmosphere can be compared with the composition of the Sun (see the right two columns of the table (Atmospheric abundances for Jupiter)). If, like the Sun, the planet had formed by simple condensation from the primordial solar nebula that is thought to have given birth to the solar system, their elemental abundances should be the same. A surprising result from the Galileo probe was that all the globally mixed elements that it could measure in the Jovian atmosphere showed the same approximately threefold enrichment of their values in the Sun, relative to hydrogen. This has important implications for the formation of the planet (see below Origin of the Jovian system (Jupiter)). Spectroscopy from Earth reveals a large spread in the values of other elements (phosphorus, germanium, and arsenic) not measured by the probe. The abundances of the gases from which these elemental abundances are derived depend on dynamical phenomena in Jupiter's atmosphere—principally chemical reactions and vertical mixing. The significance of the helium and neon depletions is discussed in the section The interior (Jupiter), below.

In addition to measuring atmospheric composition, the Galileo probe carried instruments to measure both the temperature and pressure (atmospheric pressure) during its descent into the Jovian atmosphere. This profile is illustrated in the figure-->, which includes the locations of the different cloud layers if they had occurred where they were expected. Notably, temperatures higher than the freezing point of water (273 K, 32 °F, 0 °C) were measured at pressures just a few times greater than sea-level pressure on Earth (about one bar). This is mainly a consequence of Jupiter's internal energy source, although some warming would occur just through the trapping of infrared radiation by the atmosphere via a process comparable to Earth's greenhouse effect.

Atmospheric abundances for JupiterThe list of atmospheric abundances in the table (Atmospheric abundances for Jupiter) is certainly not complete. For example, astronomers expect monosilane (silane) to be present in the deep atmosphere, along with many other exotic species. Other nonequilibrium species should occur in the higher regions, accessible to future atmospheric probes, as a result of chemical reactions driven by lightning discharges or solar ultraviolet radiation, or at the poles (where, for example, benzene has been detected) by the precipitation of charged particles.

In 1994 Comet Shoemaker-Levy 9 (Shoemaker-Levy 9, Comet), which had been discovered the previous year, crashed into Jupiter's atmosphere after breaking up into more than 20 fragments. The successive explosions were observed by telescopes on Earth's surface, the Earth-orbiting Hubble Space Telescope, and the Galileo spacecraft en route to Jupiter. Only Galileo saw the explosions directly because they occurred on the back side of Jupiter as seen from Earth. Nevertheless, the fireballs produced by the largest fragments rose above the planet's limb, and the resulting black smudges in Jupiter's cloud layers were visible even in small telescopes as Jupiter's rotation brought them into view. Spectroscopic studies revealed that the impacts had produced or delivered many chemicals such as water, hydrogen cyanide, and carbon monoxide, substances that exist on Jupiter but in much smaller concentrations. The excess carbon monoxide and hydrogen cyanide remained detectable in the upper atmosphere several years after the event. In addition to its intrinsic interest, the collision of a comet with Jupiter stimulated detailed studies of the effects that cometary impacts would have on Earth.

The nonthermal component of the continuous decimetre radiation is interpreted as synchrotron emission (synchrotron radiation)—that is, radiation emitted by extremely high-speed electrons moving in the planet's magnetic field within a toroidal, or doughnut-shaped, region surrounding Jupiter—a phenomenon closely analogous to that of Earth's Van Allen belts (Van Allen radiation belt). The maximum emission occurs at a distance of two planetary radii from the centre of the planet and has been detected from Earth at 178–5,000 megahertz and by the Cassini orbiter at 13,800 megahertz, the operating frequency of the spacecraft's radar instrument. The intensity of the emission and its plane of polarization (the plane in which the oscillations of the radio emission lie preferentially) vary with the same period. Both effects are explained if the axis of the planet's magnetic field is inclined by about 10° to the rotational axis. The period of these variations is the rotation period designated as System III (see above Basic astronomical data (Jupiter)).

Just as charged particles trapped in the Van Allen belts produce auroras on Earth when they crash into the uppermost atmosphere near the magnetic poles, so do they also on Jupiter. Cameras on the Voyager and Galileo spacecraft succeeded in imaging ultraviolet auroral arcs on the nightside of Jupiter. The Hubble Space Telescope also captured images of far-ultraviolet auroras on the planet's dayside. In addition, Earth-based observations have recorded infrared emissions from H₃⁺ ions at both poles and imaged the associated polar auroras. Evidently protons (hydrogen ions, H⁺) from the magnetosphere spiral into the planet's ionosphere along magnetic field lines, forming the excited H₃⁺ ions as they crash into the atmosphere dominated by molecular hydrogen. The resulting emission produces the auroras. It is observable from Earth at wavelengths where methane in Jupiter's atmosphere has very strong absorption bands and thereby suppresses the background from reflected sunlight. The relation of the ultraviolet and infrared auroras, the detailed interaction of Io's flux tube with Jupiter's ionosphere, and the possibility that ions from Io's torus are impinging on the planet's atmosphere remain active topics of research.

Atmospheric abundances for JupiterThe source of internal heat has not been completely resolved. The currently favoured explanation invokes a combination of the gradual release of primordial heat left from the planet's formation and the liberation of thermal energy from the precipitation of droplets of helium in the planet's deep interior, as is also known to occur on Saturn. The lower helium abundance in Jupiter's atmosphere relative to the Sun (see table (Atmospheric abundances for Jupiter)) supports this latter deduction. The first process is simply the cooling phase of the original “collapse” that converted potential energy to thermal energy at the time when the planet accumulated its complement of solar nebula gas (see below Origin of the Jovian system (Jupiter)).

The first objects in the solar system discovered by means of a telescope—by Galileo in 1610—were the four brightest moons of Jupiter, now called the Galilean satellites (Jupiter). The fifth known Jovian moon, Amalthea, was also discovered by visual observation—by Edward Emerson Barnard (Barnard, Edward Emerson) in 1892. All the other known satellites were found in photographs or electronic images taken with Earth-based telescopes or by the cameras on the Voyager spacecraft. Jupiter's multicomponent ring was detected in Voyager images in 1979.

Moons of JupiterData for the known Jovian moons are summarized in the table (Moons of Jupiter). Roman numerals are assigned to the first 16 known moons in order of their discovery. The orbits of the inner eight moons have low eccentricities and low inclinations; i.e., the orbits are all nearly circular and in the plane of the planet's equator. Such moons are called “regular.” The orbits of the dozens of moons found beyond Callisto have much higher inclinations and eccentricities, making them “irregular.” The two innermost moons, Metis and Adrastea, are intimately associated with Jupiter's ring system, as sources of the fine particles and as gravitationally controlling “shepherds.” Amalthea and Thebe also contribute to the ring system by producing very tenuous gossamer rings slightly farther from the planet. There may well be additional, undiscovered small moons close to Jupiter. There almost certainly are more distant irregular moons than those so far detected.

Galileo proposed that the four Jovian moons he discovered in 1610 be named the Medicean stars, in honour of his patron, Cosimo II de' Medici (Cosimo II), but they soon came to be known as the Galilean satellites in honour of their discoverer. Galileo regarded their existence as a fundamental argument in favour of the Copernican model (Copernican system) of the solar system, in which the planets orbit the Sun. Their orbits around Jupiter were in flagrant violation of the Ptolemaic system, in which all celestial objects must move around Earth. In order of increasing distance from the planet, these satellites are called Io, Europa, Ganymede, and Callisto, for figures closely associated with Jupiter in Greek mythology. The names were assigned by the German astronomer Simon Marius (Marius, Simon), Galileo's contemporary and rival, who likely discovered the satellites independently. There proved to be a particular aptness in the choice of Io's name: Io—“the wanderer” (Greek iōn, “going”)—has an indirect influence on the ionosphere of Jupiter, as discussed above.

Although approximate diameters and spectroscopic characteristics of the Galilean moons had been determined from Earth-based observations, it was the Voyager missions that indelibly established these four bodies as worlds in their own right. The Galileo mission provided a wealth of additional data. Before Voyager it was known that Callisto and Ganymede are both as large as or larger than the planet Mercury; that they and Europa have surfaces covered with water ice; that Io's orbit is surrounded by a torus of atoms and ions that include sodium, potassium, and sulfur; and that the inner two Galilean moons have mean densities much greater than those of the outer two. This density gradient from Io to Callisto resembles that found in the solar system itself and seems to result from the same cause (see below Origin of the Jovian system (Jupiter)). The density values suggest that Io and Europa have a rocky composition similar to that of the Moon, whereas roughly 50 percent of Ganymede and Callisto must be made of a much less dense substance, water ice being the obvious candidate.

The icy surface of this satellite is so dominated by impact craters (meteorite crater) that there are no smooth plains like the dark maria (mare) observed on the Moon. In other words, there seem to be no areas on Callisto where upwelling of material from subsequent internal activity has obliterated any of the record of early bombardment. This record was formed by impacting debris (comet nuclei and asteroidal material) primarily during the first 500 million years after the formation of the solar system in much the same way that the craters on the Moon were produced. The unmodified appearance of the surface is consistent with the absence of a differentiated interior. Evidently no tidally induced global heating and consequent melting occurred on Callisto, unlike the other three Galilean moons. The Galileo spacecraft revealed that craters smaller than 10 km (6 miles) are hidden by drifts of fine, dark material resembling a mixture of clay minerals.

Unlike Callisto, Ganymede, an equally icy satellite, reveals distinct patches of dark and light terrain. This contrast is reminiscent of the Moon's surface, but the answer to which terrain came first—dark or light—is exactly reversed. In contrast to the Moon, the dark regions on Ganymede are the older areas, showing the heaviest concentration of craters. The light regions are younger, revealing a complex pattern of parallel and intersecting ridges and grooves in addition to unusually bright impact craters typically surrounded by systems of rays. This manifestation of active crustal movement and resurfacing is accompanied by clear evidence of internal differentiation. Unlike Callisto, Ganymede has an iron-rich core and a permanent magnetic field that is strong enough to create its own magnetosphere and auroras. The trace components identified in Ganymede's icy surface include a smaller amount of the same claylike dust found on Callisto and the same traces of solid carbon dioxide, hydrogen peroxide, and sulfur compounds, plus evidence for molecular oxygen and ozone trapped in the ice.

The surface of Europa is totally different from that of Ganymede or Callisto, despite the fact that the infrared spectrum of this object indicates that it, too, is covered with ice. There are few impact craters on Europa—the number per unit area is comparable to that on the continental regions of Earth, indicating that the surface is relatively recent. Some scientists think the surface is so young that significant resurfacing is still taking place on the satellite. This resurfacing evidently consists of the outflow of water from the interior to form an instant frozen ocean.

Seen through a telescope from Earth, Io appears reddish orange, while the other moons are neutral in tint. Io's infrared spectrum shows no evidence of the absorption characteristics of water ice. Scientists expected Io's surface to look different from those of Jupiter's other moons, but the Voyager images revealed a landscape even more unusual than anticipated.

Volcanic (volcanism) fissures, instead of impact craters, dot the surface of Io. Nine volcanoes (volcano) were observed in eruption when the two Voyager spacecraft flew by in 1979, while the closer encounters by Galileo indicated that as many as 300 volcanic vents may be active at a given time. The silicate lava emerging from the vents is extremely hot (about 1,900 K 【3,000 °F, 1,630 °C】), resembling primitive lavas on early Earth. This unprecedented level of activity makes Io the most tectonically active object in the solar system. The surface of the satellite is continually and completely replaced by this volcanism in just a few thousand years. Various forms (allotropes) of sulfur appear to be responsible for the black, orange, and red areas on the moon's surface, while solid sulfur dioxide (sulfur oxide) is probably the main constituent of the white areas. Sulfur dioxide was detected as a gas near one of the active volcanic plumes by Voyager's infrared spectrometer and was identified as a solid in ultraviolet and infrared spectra obtained from Earth-orbital and ground-based observations. These identifications provide sources for the sulfur and oxygen ions observed in the Jovian magnetosphere and prove that Io's volcanic activity is the source of its torus of particles.

The energy for this volcanic activity requires a special explanation, since radioactive heating is inadequate for a body as small as Io. The favoured explanation is based on the observation that orbital resonances with the other Galilean satellites perturb (perturbation) Io into a more eccentric orbit than it would assume if only Jupiter controlled its motion. The resulting tides developed by the gravitational contest over Io between the other satellites and Jupiter release enough energy to account for the observed volcanism. The interior contains a dense, iron-rich core, which probably produces a magnetic field. The interactions of Io with Jupiter's magnetosphere and ionosphere are so complex, however, that it has been difficult to distinguish the satellite's own field from the current-produced fields in its vicinity.

Moons of JupiterThe only other Jovian moon that was close enough to the trajectories of the Voyager spacecraft to allow surface features to be seen was Amalthea. So small that its gravitational field is not strong enough to deform it into a sphere, it has an irregular, oblong shape (see table (Moons of Jupiter)). Like Io, its surface exhibits a reddish colour that may result from a coating of sulfur compounds released by Io's volcanoes. In addition to providing new images of Amalthea, the Galileo orbiter was able to view the effect of impacts on Thebe and Metis. All three of these inner moons are tidally locked, keeping the same face oriented toward Jupiter. All three are some 30 percent brighter on their leading sides, presumably as a result of impacts by small meteoroids. Amalthea has a remarkably low density, implying a highly porous structure that probably resulted from internal shattering by impacts.

Moons of JupiterBefore the turn of the 21st century, eight outer moons were known, comprising two distinct orbital families (as can be seen in the table (Moons of Jupiter)). The more distant group—made up of Ananke, Carme, Pasiphae, and Sinope— has retrograde orbits around Jupiter. The closer group—Leda, Himalia, Lysithea, and Elara—has prograde orbits. (In the case of these moons, retrograde motion is in the direction opposite to Jupiter's spin and motion around the Sun, which are counterclockwise as viewed from above Jupiter's north pole, whereas prograde, or direct, motion is in the same direction.) In 1999 astronomers began a concerted effort to find new Jovian satellites using highly sensitive electronic detectors that allowed them to detect fainter—and hence smaller—objects. When in the next few years they discovered a host of additional outer moons, they recognized that the two-family division was an oversimplification. There must be well more than 100 small fragments orbiting Jupiter that can be classified into several different groups according to their orbits. Each group apparently originated from an individual body that was captured by Jupiter and then broke up. The captures could have occurred near the time of Jupiter's formation when the planet was itself surrounded by a nebula that could slow down objects that entered it. These small moons may be related to the so-called Trojan asteroids (Trojan planets), two groups of minor planets that share Jupiter's orbit. The Trojans occupy regions 60° ahead of and behind the position of the planet in its orbit. These regions are the L4 and L5 equilibrium points in Lagrange's solution to the three-body problem (see celestial mechanics: The three-body problem (celestial mechanics)).

As the Pioneer 10 spacecraft sped toward its closest approach to Jupiter in 1974, it detected a sudden decrease in the density of charged particles roughly 125,000 km (78,000 miles) from Jupiter, just inside the orbit of its innermost moon, Metis. This led to the suggestion that a moon or a ring of material might be orbiting the planet at this distance. The existence of a ring was verified in 1979 by the first Voyager spacecraft when it crossed the planet's equatorial plane, and the second spacecraft recorded additional pictures, including a series taken in the shadow of the planet looking back at the ring toward the direction of the Sun. The ring was many times brighter from this perspective. Evidentally most of the ring particles scatter light forward much better than in the reverse direction (toward Earth). It was therefore no surprise that Earth-based observations failed to discover the ring before Voyager. The forward scattering implies that most of the particles are very small, in the micrometre size range, rather like the motes of dust seen in a sunbeam on Earth or the fine particles on car windshield, which show the same optical effect.

The ring exhibits a complex structure that was elucidated by images obtained with the Galileo spacecraft in 1996–97. It consists of four principal components: an outer gossamer ring that fades into invisibility beyond the orbital radius of the satellite Thebe (222,000 km); an inner gossamer ring bounded by the orbit of Amalthea (181,000 km); the main ring, about 30 km thick, that extends inward from the orbits of Adrastea (129,000 km) and Metis (128,000 km) to an inner edge at 123,000 km; and a toroidal halo of particles with a thickness of 20,000 km that extends from the main ring inward to 92,000 km. (One kilometre is about 0.62 mile.)

Atmospheric abundances for JupiterGiven the planet's large proportion of hydrogen and its huge mass, it has been traditional to assume that Jupiter formed by condensation from the primordial solar nebula. This hypothesis implies that the elements should all be present on Jupiter in the same proportions that they occur in the Sun. However, the most recent evidence (see table (Atmospheric abundances for Jupiter)) indicates that the elemental proportions on Jupiter differ from the solar values.

Atmospheric abundances for JupiterCurrent models for Jupiter's origin suggest instead that a solid core of about 10 Earth masses formed first as a result of the accretion of icy planetesimals (planetesimal). This core would have developed an atmosphere of its own as the planetesimals released gases during accretion. As the mass of the core increased, it would have become capable of attracting gases from the surrounding solar nebula, thus accumulating the huge hydrogen-helium envelope that constitutes Jupiter's atmosphere and fluid mantle. The accumulating envelope would have mixed with the outgassed atmosphere from the core. Thus, the presently observed enrichment of the most abundant heavy elements in this envelope, compared with solar values (see table (Atmospheric abundances for Jupiter)), reflects the concentration of such elements in the core. The mass spectrometer on the Galileo probe (see above The atmosphere (Jupiter)) showed that these heavy elements are enriched by the same factor of about three. For this enrichment to include volatile substances like argon and molecular nitrogen requires that the icy planetesimals must have formed at temperatures of 30 K (−400 °F, −240 °C) or less. Just how this happened remains a puzzle, and its solution may ultimately help explain the presence of giant planets (planet) that have been detected very close to their stars in other planetary systems.

The Jovian system—the planet Jupiter and its moons, magnetosphere, and rings—is discussed in detail and compared with the other giant planets in (in order of increasing difficulty) J. Kelly Beatty, Carolyn Collins Petersen, and Andrew Chaikin, The New Solar System, 4th ed. (1999); David Morrison and Tobias Owen, The Planetary System, 3rd ed. (2003); and Imke de Pater and Jack J. Lissauer, Planetary Sciences (2001). A comprehensive, popular-level review of knowledge of the Jovian system, including early results from the Galileo space probe, is Reta Beebe, Jupiter: The Giant Planet, 2nd ed. (1997). Jupiter's moons are discussed in the context of all the moons of the outer solar system in David A. Rothery, Satellites of the Outer Planets: Worlds in Their Own Right, 2nd ed. (1999). Detailed descriptions of the Galileo mission and its findings are provided in David M. Harland, Jupiter Odyssey: The Story of NASA's Galileo Mission (2000); and Daniel Fischer, Mission Jupiter: The Spectacular Journey of the Galileo Spacecraft (2001). The original reference for visual observations of Jupiter with telescopes of moderate size is Bertrand M. Peek, The Planet Jupiter, rev. by Patrick Moore (1981). A more recent account that includes results from the Voyager space probes is John H. Rogers, The Giant Planet Jupiter (1995). Details regarding the Voyager missions may be found in David Morrison and Jane Samz, Voyage to Jupiter (1980).Tobias Chant Owen