Jupiter is the fifth planet from the Sun and the largest planet within the solar system. It is two and a half times as massive as all of the other planets combined, and is classified as a gas giant planet. Jupiter shares the gas giant classification with three other planets in the Solar System, namely Saturn, Uranus, and Neptune. Together, these four planets are sometimes referred to as the Jovian planets (Jovian being the adjectival form of Jupiter, derived from the Latin genitive of the noun).

When viewed from Earth, Jupiter can reach an apparent magnitude of -2.8, making it the third brightest object in the night sky. The planet was known to astronomers of ancient times. The Romans named it after Jupiter, the principal God of Roman mythology (the name being a reduction of 'Deus Pater', 'God father' ^[1]), during the era of Classical Antiquity.

The planet Jupiter is primarily composed of hydrogen, with a smaller portion of helium and a rocky core. Due to its rapid rotation the planet possesses a slight but noticeable bulge around the equator, giving it an oblate appearance. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that has been existent since at least the seventeenth century. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons that were first discovered by Galileo Galilei in 1610.

Jupiter has been explored on several occasions by robotic spacecraft; most notably during the early Pioneer and Voyager fly-by missions, and later by the Galileo orbiter. Future targets for exploration include the possible ice-covered liquid ocean on the Jovian moon Europa.

Overview

Jupiter is usually the fourth brightest object in the night sky (after the Sun, the Moon and Venus);^[2] however at times Mars appears brighter than Jupiter.

Jupiter is 2.5 times more massive than all the other planets combined; so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. A volume equal to 1,300 Earths only contains 318 times as much mass.^[2]

Along with the Sun, the gravitational influence of Jupiter has helped shape the Solar System. The orbits of most of the system's planets lie closer to Jupiter's orbital plane than the Sun's equatorial plane (Mercury is the only planet which is closer to the Sun's equator in orbital tilt), the majority of short-period comets belong to Jupiter's family (a result due to both Jupiter's mass and its relative speed), the Kirkwood gaps in the asteroid belt are mostly due to Jupiter, and the planet may have been responsible for the Late Heavy Bombardment of the inner solar system's history.^[3] Jupiter has been called the solar system's vacuum cleaner,^[4] due to its immense gravity well.

Extrasolar planets have been discovered with much greater masses. There is no clear-cut definition of what distinguishes a large planet such as Jupiter from a brown dwarf star, although the latter possesses rather specific spectral lines. Currently, if an object of solar metallicity is 13 Jupiter masses or above, large enough to burn deuterium, it is considered a brown dwarf; below that mass (and orbiting a star or stellar remnant), it is a planet.^[5] Jupiter is thought to have about as large a diameter as a planet of its composition can; adding extra mass would cause the planet to shrink due to increased gravitational compression. The process of further shrinkage with increasing mass would continue until stellar ignition is achieved.^[6] This has led some astronomers to term it a "failed star". Although Jupiter would need to be about seventy-five times as massive to become a star, the smallest red dwarf is only about 30% larger in radius than Jupiter.^[7][8]

In spite of this, Jupiter still radiates more heat than it receives from the Sun. This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 3 cm each year.^[9] When they were younger and hotter, Jupiter and the other gas giant planets were much larger than they are today. However, because of its lower mass and weaker gravitational pull, Saturn would expand more rapidly than Jupiter with increasing heat; therefore, Saturn must have formerly been even larger than Jupiter.

Jupiter's rotation is the solar system's fastest, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. The planet is also perpetually covered with a layer of clouds, composed of ammonia crystals and possibly ammonium hydrosulphide. Its best known feature is the Great Red Spot, a storm larger than Earth which was likely first observed by Giovanni Domenico Cassini and Robert Hooke four centuries ago. Indeed, mathematical models suggest that the storm is stable and may be a permanent feature of the planet.^[10] In 2000, three small spots merged to form a larger spot named Oval BA, which later acquired a red hue very similar to that of the Great Red Spot.^[11]

Historical observations

The planet Jupiter has been known since ancient times and is visible to the naked eye in the night sky. The Romans named the planet after the Roman god Jupiter (also called Jove). The astronomical symbol for the planet is a stylized representation of the god's lightning bolt. (♃ is found at Unicode position U+2643.)

The Chinese, Korean, Japanese, and Vietnamese refer to the planet as the wood star, 木星,^[12] based on the Chinese Five Elements. In Vedic Astrology, Hindu astrologers refer to Jupiter as Brihaspati, or "Guru" which means the "Big One". In Hindi, Thursday is referred to as Guruvaar (day of Jupiter). In the English language Thursday is rendered as Thor's day, with Thor being identified with the Roman god Jupiter.

In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa, Ganymede and Callisto (now known as the Galilean moons) using a telescope, the first observation of moons other than Earth's. This was also the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory placed him under the threat of the Inquisition.^[13]

In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch refractor at Lick Observatory in California. The discovery, a testament to his extraordinary eyesight, made him quickly famous. The moon was later named Amalthea.^[14]

Physical characteristics

Composition

Jupiter's upper atmosphere is composed of about 93% hydrogen and 7% helium by number of atoms, or approximately 75% hydrogen and 24% helium by mass, with the remaining 1% of the mass accounted for by other substances. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements, by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulphide, neon, oxygen, phosphine, and sulphur. The outermost layer of the atmosphere contains crystals of frozen ammonia.^[15][16] Through IR and UV measurements benzene (at a relative mixing ratio of 2x10^-9 to hydrogen) and other hydrocarbons have also been found.^[17]

This atmospheric composition for hydrogen and helium is very close to the composition of the solar nebula. Galileo probe results show that neon in Jupiter's upper atmosphere is unexpectedly only about a tenth as abundant as in the sun (20 parts per million by mass, vs. 200 parts per million for the Sun) [3]. This depletion may be due to the fact that the ice which carried heavier elements into Jupiter during formation was still too warm to hold all of its nebular neon (this requires temperatures below 17 Kelvins). Abundances of heavier inert gases in Jupiter's atmosphere are about 2 to 3 times solar abundance.

On the basic of spectroscopy, Saturn is thought to have a quite similar composition to Jupiter, but Uranus and Neptune have relatively much less hydrogen and helium. However, very good abundance numbers for heavier elements are lacking for the other outer planets, due to lack of data from atmospheric entry probes from these.

Shape and structure

Jupiter is an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles. This is caused by Jupiter's rapid rotation which causes the equator to bulge outward.^[18]

There is still some uncertainty regarding the interior structure of Jupiter. One model shows a homogeneous model with no solid surface; the density may simply increase gradually toward the core. Alternatively Jupiter may possess a dense, rocky core with a mass of up to twelve times the Earth's total mass.^[19] The core region is surrounded by dense metallic hydrogen, with further layers of liquid hydrogen and gaseous hydrogen. There may be no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.^[20][18]

Cloud layers

Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation, an effect first noticed by Giovanni Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere; three "systems" are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10º N to 10º S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's "official" rotation.^[21]

Jupiter is wrapped by bands of clouds of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon.

The outer atmosphere of Jupiter consists of two decks of clouds; a thick, lower deck and a thin, clearer region. The coloration of the bands is caused by the altitude at which clouds become visible. The darker coloration of the belts is likely caused by upwellings of colorful compounds that mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.^[2]

The only spacecraft to have descended into Jupiter's atmosphere and take scientific measurements is the Galileo probe (see Galileo mission). It sent an atmospheric probe into Jupiter upon arrival in 1995, then itself entered Jupiter's atmosphere and burned up in 2003.

The Great Red Spot

The Great Red Spot is a persistent anticyclonic storm on the planet Jupiter, 22° south of the equator, which has lasted at least 340 years. The storm is large enough to be visible through Earth-based telescopes. It was probably first observed by Giovanni Domenico Cassini, who described it around 1665.^[22]

This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million miles) from Jupiter. Cloud details as small as 160 km (100 miles) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.

The oval object rotates counterclockwise, with a period of about 6 days.^[23] The Great Red Spot's dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth's size.^[24] The tops of this storm is about 8 km above the surrounding cloudtops.^[25]

Storms such as this are not uncommon within the turbulent atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last hours or centuries.

Before the Voyager missions, astronomers were uncertain of the nature of Jupiter's Great Red Spot. Many believed it to be either a solid or a liquid feature on the planet's surface as this appears consistent with the observable turbulence patterns.^[26] However, even before Voyager proved that the feature was a storm, there was strong evidence that the spot cannot be associated with any deeper feature on the planet's surface, as it has been proven that the Spot rotates differentially with respect to the rest of the atmosphere, sometimes faster and sometimes more slowly, so that during its recorded history it has traveled several times around the planet with regard to any possible fixed rotational marker below it.

Recent examination of Jupiter's surface by a NASA research team using the Hubble telescope has shown that several smaller storms located near to the Giant Red Spot have, in the last twelve months, amalgamated, increased in intensity and changed colour from white to red.^[27] This data on Oval BA, nicknamed Red Spot Jnr.^[28] was published in the astronomical journal Icarus by planetary scientist Amy Simon-Miller in October 2006. Further investigations using spectroscopy have been delayed due to the movement of Jupiter behind the Sun and it is yet to be verified if the phenomena is caused by increased wind speed, comparable to the 400 mph speeds in the larger anomaly, drawing similar materials from deeper in the atmosphere and exposing them to sunlight.

Planetary rings

Jupiter has a faint planetary ring system composed of three main components: an inner torus of particles known as the halo, a relatively bright main ring, and an outer "gossamer" ring.^[29] The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter's orbit due to its strong gravitational pull. The orbit of the material decays towards Jupiter and new material is added by additional impacts.^[30] In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the gossamer ring.^[31]

Magnetosphere

This planet's broad magnetic field is 14 times as strong as the Earth's, ranging from 4.2 gauss at the equator to 10–14 gauss at the poles, making it the strongest in the solar system (with the exception of sunspots.)^[2] This field is believed to be generated by eddy currents within the metallic hydrogen core. The field traps a sheet of plasma particles from the solar wind, generating a highly energetic magnetosphere around the planet that extends outward to a radius of 150–200 times the radius of Jupiter. Electrons from this plasma sheet ionize the torus of sulfur dioxide generated by the tectonic activity on the moon Io. Hydrogen particles from Jupiter's atmosphere are also trapped in the magnetosphere. Electrons within the magnetosphere generate a strong radio signature that produces bursts in the range of 0.6–30 GHz.^[32]

Exploration of Jupiter

Since 1973 a number of automated spacecraft have visited Jupiter. Flights to other planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Reaching Jupiter from Earth requires a delta-v of 9.2 km/s,^[33] which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit.^[34] Fortunately, gravity assists through planetary flybys can be used to reduce the energy required to reach Jupiter, albeit at the cost of a significantly longer flight duration.^[33]

Fly-by missions

Spacecraft	Closest approach	Distance
Pioneer 10	December 3, 1973	130,000 km
Pioneer 11	December 4, 1974	34,000 km
Voyager 1	March 5, 1979	349,000 km
Ulysses	February 1992	409,000 km
	February 2004	240,000,000 km
Cassini	August 18, 1999	10,000,000 km

Beginning in 1973, several spacecraft have performed planetary fly-by maneuvers that brought them within observation range of Jupiter. Pioneer 10 obtained the first ever close up images of Jupiter and the Galilean moons, studied its atmosphere, detected its magnetic field, observed its radiation belts and found that Jupiter is mainly liquid.^[35] Six years later the Voyager missions vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close up images of the planet's atmosphere.^[15]

The next mission to encounter Jupiter, the Ulysses solar probe, performed a fly-by maneuver in order to attain a polar orbit around the Sun. During this pass the probe conducted studies on Jupiter's magnetosphere. However, since there are no cameras onboard the probe, no images were taken. A second fly-by six years later was at a much greater distance.^[36]

In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet. On December 19, 2000, the Cassini spacecraft, captured a very low resolution image of the moon Himalia, but it was too distant to show any surface details.^[37]

The New Horizons probe, en route to Pluto, will flyby Jupiter for a gravity assist. Closest approach will be February 28, 2007. While at Jupiter, New Horizon's instruments will refine the orbits of Jupiter's inner moons, particularly Amalthea. The probe's cameras will measure plasma output from volcanoes on Io and study all four Gallilean moons in detail.^[38] Imaging of the Jovian system began September 4, 2006.^[39]

Shoemaker-Levy comet

During the period July 16 to July 22, 1994, over twenty fragments from the comet Shoemaker-Levy 9 hit Jupiter's southern hemisphere, providing the first direct observation of a collision between two solar system objects. Although not a probe from Earth, the impact did provide data on the composition of Jupiter's atmosphere.^[40] It is thought that due to Jupiter's large mass and location near the inner solar system it receives the most frequent comet impacts of the solar system's planets.

Galileo mission

So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter on December 7, 1995. It orbited the planet for over seven years, conducting multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 as it approached Jupiter in 1994, giving a unique vantage point for the event. However, while the information gained about the Jovian system from Galileo was extensive, its originally-designed capacity was limited by the failed deployment of its high-gain radio transmitting antenna.^[41]

An atmospheric probe was released from the spacecraft in July, 1995, entering the planet's atmosphere on December 7. It parachuted through 150 km of the atmosphere, collecting data for 57.6 minutes, before being crushed by the pressure to which it was subjected by that time (about 22 times Earth normal, at a temperature of 153 ^oC). [4] It would have melted thereafter, and possibly vaporized. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, in order to avoid any possibility of it crashing into and possibly contaminating Europa.^[41]

Future probes

NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2010.^[42]

Because of the possibility of a liquid ocean on Jupiter's moon Europa, there has been great interest to study the icy moons in detail. A mission proposed by NASA was dedicated to study them. The JIMO (Jupiter Icy Moons Orbiter) was expected to be launched sometime after 2012. However, the mission was deemed too ambitious and its funding was cancelled.^[43]

Natural satellites

Jupiter has at least 63 moons. For a complete listing of these moons, please see Jupiter's natural satellites. For a timeline of their discovery dates, see Timeline of discovery of Solar System planets and their natural satellites.

The four large moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.

Galilean moons

The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbours at the same point in every orbit it makes.

The tidal force from Jupiter, on the other hand, works to circularize their orbits.^[44] This constant tug of war causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing.

Regular moons	Inner group	The inner group of four small moons all have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree.
	Galilean moons[46]	These four moons, discovered by Galileo Galilei and by Simon Marius in parallel, orbit between 400,000 and 2,000,000 km, and include some of the largest moons in the solar system.
Irregular moons	Themisto	This is in a group of its own, orbiting halfway between the Galilean moons and the next group.
	Himalia group	A tightly clustered group of moons with orbits around 11,000,000-12,000,000 km from Jupiter.
	Carpo	Another isolated case; at the inner edge of the Ananke group, it revolves in the direct sense.
	Ananke group	This group has rather indistinct borders, averaging 21,276,000 km from Jupiter with an average inclination of 149 degrees.
	Carme group	A fairly distinct group that averages 23,404,000 km from Jupiter with an average inclination of 165 degrees.
	Pasiphaë group	A dispersed and only vaguely distinct group that covers all the outermost moons.

References and Notes

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