Jupiter is the fifth planet from the sun and the largest in the solar system. Named after the ruler of the gods in Roman mythology, Jupiter has 1400 times the volume of Earth but only 318 times more mass. Thus, the mean density of Jupiter is about one-fourth that of Earth, indicating that the giant planet must consist of gas rather than the metals and rocks of which the Earth and other inner planets are composed.
Once every 11.9 Earth years, Jupiter makes a complete orbit around the sun at a mean distance 5.2 times greater than one astronomical unit even though it rotates once on its axis every 9.9 hours, causing a bulge at its equator visible with a telescope (Jupiter's rotation is not uniform). Colourful latitudinal bands, atmospheric clouds, and storms illustrate Jupiter's dynamic weather systems; the cloud patterns change within hours or days.
The Great Red Spot is a complex storm moving in a counter-clockwise direction. At its outer edge, material appears to rotate in four to six days; near the center, motions are small and nearly random in direction. An array of other smaller storms and eddies can be found throughout the banded clouds. Aurora emissions, similar to Earth's Northern Lights, have been observed at Jupiter's poles and appear to be related to material from Io that spirals along magnetic field lines eventually falling into Jupiter's atmosphere. Cloud-top lightning bolts, similar to superbolts in Earth's high atmosphere, have also been observed.
Composition, Structure, and Magnetic Field
Scientific knowledge of the Jupiter system increased enormously with visits to the system by spacecraft launched by NASA. In 1979, Voyagers 1 and 2 passed close to Jupiter on their way out of the solar system, taking photographs and measurements more detailed than those from Earth. In 1995, the Galileo spacecraft neared Jupiter and launched a probe toward the planet. Before the probe was incinerated in Jupiter's atmosphere, it sent new information about the planet's atmosphere back to Earth. Galileo continued to return data to Earth from its orbit until late 1997.
Earlier spectroscopic observations from Earth demonstrated that most of its atmosphere is molecular hydrogen (H2). Samples tested by the Galileo probe indicated that 80 percent of the Jovian atmosphere is H2, with helium (He) constituting almost all of the remaining 20 percent. Jupiter's interior must have essentially the same composition in order to yield the observed low density. Apparently, this huge world must be made mostly of the two lightest and most abundant elements in the universe, a mixture similar to that of the Sun's 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.
Scientists also collected new information about Jupiter when fragments of the dying Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions stirred up the planet's atmosphere, heating interior gases to incandescence – the point at which they produce light – and bringing them to the surface. Scientists captured detailed images of these gases with telescopes located on the earth and in space, and used spectroscopy to analyze the gases in order to verify and expand knowledge about the composition of the planet's atmosphere.
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 80 times larger to have enough mass to ignite a nuclear furnace. Therefore its turbulent, cloud-filled atmosphere is very cold. Due to the abundance of hydrogen, the 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 those 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. Galileo probe results indicate that strong winds (faster than 650 km/h, or 400 mph) blow through the atmosphere at all cloud depths, suggesting that the winds are caused by heat escaping from Jupiter's depths (unlike Earth's winds, which are caused by heating from the Sun or from condensed water vapor). Lightning on Jupiter is quite different to that on Earth; striking about ten times less often on Jupiter, Jovian lightning bolts have about ten times more energy than lightning bolts on Earth. With a lower percentage of water molecules than does the Sun, Jupiter appears much drier than scientists anticipated.
Two known cloud layers of ammonia and ammonium hydrosulfide, and at least one theorized cloud layer made of water vapor, exist in Jupiter's atmosphere. Ammonia freezes in the low temperature of Jupiter's upper atmosphere (-125°C or -193°F), forming the white cirrus clouds-zones, ovals, and plumes seen in many photographs transmitted by the Voyager spacecraft. At lower levels, ammonium hydrosulfide condenses. Coloured by other compounds, clouds of this substance may contribute to the widespread sand-colored cloud layer on the planet. The temperature at the top of these clouds is about -50°C (about -58°F) and the Jovian atmospheric pressure is about twice the sea-level atmospheric pressure on earth. Through holes in this cloud layer, radiation escapes from a region that may be a layer of water vapor clouds (where the temperature reaches 17°C or 63°F). Still deeper, warmer layers have been detected by radio telescopes that are sensitive to cloud-penetrating radiation. Scientists had hoped that the Galileo probe would pass through the first three layers of clouds (ammonia, ammonium hydrosulfide, and water vapor), but the probe hit the atmosphere in an unexpectedly clear area where only the ammonium hydrosulfide layer was present.
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 solid, earthlike material may exist at the center. The 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 that 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 million miles). One of these belts, between Jupiter's ring and the outermost atmosphere, was discovered by the Galileo probe and came as a surprise to scientists. It is about ten times as strong as the earth's Van Allen radiation belts and contains mysterious high-energy helium ions from an unknown source. 
Jupiter's Satellites and Rings
As of January 1, 2006, Jupiter has an official number of 63 moons. The four Jovians were first observed in 1610 by Galileo. Between the Jovians and Jupiter are the 'ring' moons, and beyond these are two sets of four moons discovered before 2000. Together, these 16 satellites for the longest time stood alone. Then, a team of University of Hawaii astronomers discovered a group of eleven moons in 2000, and eleven more in 2002 so that Jupiter, by January of 2003, had forty satellites. And during 2003, 23 new irregular retrograde satellites were discovered and are now being classified. But by far the most unique sets of moons are the rings and the Jovians.
The four rings – Metis, Adrastea, Thebe, and Amalthea – are constantly struck by meteoroids collisions that shower Jupiter's rings with more debris and dusts. The Jovians each have a unique characteristic: Io is the smallest Jovian and the most volcanic body in the solar system, Europa is constantly the subject of a gravitational tug of war between Jupiter and the Jovians that heats its interior and lends credit to the submerged ocean theory, Callisto has the most heavily cratered surface of any body in the solar system, and Ganymede is larger than Mercury and exhibits two distinct surface types.
Closer to the planet, the Voyager spacecraft discovered a faint system of rings with material that must be continuously renewed and is probably produced by the disintegration of small moonlets imbedded within the rings. The satellite Metis, just at the outer boundary, could be one source of ring material.
Potential prospects for life on Jupiter
Aside from Mars, the Jovian system is the most likely place in the solar system to sport life. Though the planet itself cannot support life and most of its moons cannot either, the four moons discovered by Galileo offer some hope. In particular, the icy moon Europa is the prime candidate for life. Many NASA scientists at JPL and other laboratories suspect with strong evidence that an ocean is hidden under the moon's solid and icy surface. NASA is currently working through its budget in an attempt to find a way to send a probe to Europa that would ultimately dig deep beneath its surface. And if life does not exist on Europa, the larger Jovian satellites Callisto and Ganymede offer an environment potentially suitable for future human colonization.
