Jupiter is the fifth planet from the Sun, and the largest planet in the Solar System. It is named for the ruler of the gods in Roman mythology.

Facts and Figures

Jupiter has 1,400 times the volume of Earth, but is only 318 times more massive. The diagram below shows just how large Jupiter is -- all eight of the other planets in the Solar System could fit within Jupiter, with space to spare.

View from Earth

Jupiter has been known since prehistoric times as a bright "wandering star." In 1610, Galileo pointed his telescope at the sky and discovered four of Jupiter's moons -- Io, Europa, Ganymede and Callisto, now commonly referred to as the Galilean Moons.

Through telescopes, Jupiter is a striking sight, displaying a series of yellow-brown bands, parallel to the equator, that are separate zones of the atmosphere and rotate at different speeds. The slower zones near the two poles take six minutes longer to complete one revolution than does the speedy equatorial zone.

When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (Venus is seldom visible in a dark sky). The four Galilean Moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small telescope.

Exploration

The first spacecraft to visit Jupiter was Pioneer 10, in 1973, followed by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years.

Composition, Structure, and Magnetic Field

Jupiter is about 90% hydrogen and 10% helium, with traces of methane, water, ammonia and "rock." It probably has a core of rocky material, but the exact composition of that core remains a mystery. Our knowledge of the interior of Jupiter is highly indirect -- Galileo's atmospheric probe only penetrated about 150 km below the uppermost layers of the atmosphere.

Above the core lies the main bulk of the planet, in the form of liquid metallic hydrogen. This substance consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices."

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. Water, carbon dioxide, methane and other simple molecules are also present in very small amounts.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, data from the Galileo atmospheric probe indicates that there is much less water than expected. The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun, but it now appears that the actual concentration is much less than the Sun's. The high temperature and density of the uppermost parts of the atmosphere were also surprising.

Jupiter, like the other gas planets, has high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance. The bands have been known for some time, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, but the details of those reactions remain elusive.

The Great Red Spot has been seen by Earthly observers for more than 300 years. The "spot" is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Infrared observations and the direction of its rotation suggest that the Great Red Spot is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. How such a structure can persist for so long remains a mystery.

Jupiter's magnetosphere extends more than 650 million kilometers (well past the orbit of Saturn). Its magnetic field is generated deep with its atmosphere and is responsible for the huge belts of trapped charged particles that circle the planet out to a distance of 10 million kilometers.

Jupiter's Rings

Jupiter has rings like Saturn's, but much fainter and smaller. They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that it was at least worth a quick look to see if any rings might be present. Unlike Saturn's, Jupiter's rings are dark. They are probably composed of very small grains of rocky material. Particles in Jupiter's rings probably don't stay there for long, due to atmospheric and magnetic drag. The Galileo probe found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons. The inner ring is broadened by interactions with Jupiter's magnetic field.

Jupiter's Satellites As of February 2004, Jupiter has 63 known satellites, the vast majority of which have yet to be named. Io and Europa, the two moons closest to the planet, are dense and rocky like the inner planets. Ganymede and Callisto are composed largely of water ice and have low densities. Callisto is almost as big as Mercury, and Ganymede is bigger than Mercury. In fact, if they orbited the Sun instead of Jupiter, they would be considered planets. The icy crusts of these two bodies are marked by numerous craters, similar to those found on the Earth's moon. In contrast, the surface of Europa is extremely smooth. Europa appears to be covered by a layer of water ice that emerged from its interior after its early bombardment by comet nuclei and other space debris, and a layer of liquid water may still lie beneath the ice. An intricate network of shallow cracks covers the icy surface. The most remarkable satellite is Io. Its surface is covered with yellowish, brown, and white areas dotted by black features. It is racked by volcanism that is driven by the dissipation of tidal energy in the satellite's interior. Ten volcanoes were erupting during the Voyager flybys in 1979, and evidence for later eruptions has since been detected. 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 Galilean Moons. Satellite Distance from Jupiter Radius Mass Discoverer Date Metis Adrastea Amalthea Thebe Io Europa Ganymede Callisto Leda Himalia Lysithea Elara Ananke Carme Pasiphae Sinope 128,000 km 129,000 km 181,000 km 222,000 km 422,000 km 671,000 km 1,070,000 km 1,883,000 km 11,094,000 km 11,480,000 km 11,720,000 km 11,737,000 km 21,200,000 km 22,600,000 km 23,500,000 km 23,700,000 km 20 km 10 km 98 km 50 km 1,815 km 1,569 km 2,631 km 2,400 km 8 km 93 km 18 km 38 km 15 km 20 km 25 km 18 km 9.56e16 kg 1.91e16 kg 7.17e18 kg 7.77e17 kg 8.94e22 kg 4.80e22 kg 1.48e23 kg 1.08e23 kg 5.68e15 kg 9.56e18 kg 7.77e16 kg 7.77e17 kg 3.82e16 kg 9.56e16 kg 1.91e17 kg 7.77e16 kg Synnott Jewitt Barnard Synnott Galileo Galileo Galileo Galileo Kowal Perrine Nicholson Perrine Nicholson Nicholson Melotte Nicholson 1979 1979 1892 1979 1610 1610 1610 1610 1974 1904 1938 1905 1951 1938 1908 1914

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Science.--Astronomy.--Solar System.

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