Jupiter and Earth compared.  Courtesy NASA/JPL

Jupiter has been known since ancient times and during the nineteenth and twentieth centuries was studied by astronomers all over the world by telescopes.  It was only towards the last quarter of the twentieth century that it was observed by very powerful telescopes including the Hubble Space Telescope and was visited by a number of spacecraft.

Jupiter was first visited by Pioneer 10  in 1973 and later by Pioneer 11,Voyager 1, Voyager 2 and Ulysses. The most significant mission was the Galileo Mission which lasted for eight years and studied the planet and its moons extensively.  This mission is dealt with in the section of this web-site entitled 'The Moons of Jupiter'.

At perihelion or in simple language, the nearest distance of Jupiter to the Sun is  740,742,598 kilometers (4.951 astronomical units).   At aphelion or in simple language the furthest distance from the Sun is 816,081,455 kilometers ( 5.455 astronomical units)

This true-colour simulated view of Jupiter taken by NASA's Cassini spacecraft on 7 December 2000. The resolution of the high resolution image is about 144 kilometres per pixel. Jupiter's moon Europa is casting the shadow on the planet. NASA/ JPL /University of Arizona.  From ESA micromedia library.

Jupiter is by far the most massive planet in the Solar System and is an example of a Gas Giant. The mass of Jupiter is 318 times that of Earth and it is more than twice the mass of all the other planets put together

It is by no means a perfect sphere and squashed at the poles.  It's equatorial diameter is  142,984 kilometres - its polar diameter is 133,709 kilometers.  Its 'surface area is 6.14 X 10 which is 120 times that of the Earth^.  Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)

Jupiter does not have a solid surface and  the gaseous material simply gets denser with depth (the radii and diameters quoted for Jupiter are for a levels corresponding to a pressure of 1 atmosphere) What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level).

Jupiter overall composition is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn is the other Gas Giant in the Solar System and has a similar composition.  Uranus and Neptune are sometimes referred to wrongly as Gas Giants - however they are better referred to Ice Giants.

Our knowledge of the interior of Jupiter and Saturn is highly indirect and is likely to remain so for some time. (The data from Galileo's atmospheric probe only went  down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet which consists of metallic hydrogen. This exotic form of the most common  element is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

 Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest, driest (low water vapour content) and least cloudy areas on Jupiter at that time.

The approximate accepted composition of the upper cloud levels are:-

 86%            Hydrogen  

 ~14%           Helium                                                                                                     
0.1%            Methane
0.1%            Water Vapour
0.02%          Ammonia
0.0002%      Ethane
0.0001%      Phosphine
<0.00010%  Hydrogen Sulphide

Data from the Galileo atmospheric probe also indicate 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 much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere

Jupiter and the other gas planets have 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 coloured bands that dominate the planet's appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 640kph/400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colours seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulphur and phosphorus whose compounds and elemental allotropic forms take on a wide variety of colours, but the details are unknown.

Picture Courtesyc NASA/JPL

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)

Photo Jupiter's Magnetosphere Credits: NASA/JPL/Johns Hopkins University Applied Physics Laboratory

The vast magnetosphere of charged particles whirling around Jupiter, normally invisible, can be imaged by a new type of instrument aboard the Cassini spacecraft and is seen here. Three features are sketched in for context: a black circle showing the size of Jupiter, lines of Jupiter's magnetic field, and a cross-section of the Io torus, a doughnut-shaped ring of charged particles that originate from volcanic eruptions on Jupiter's moon Io and circle Jupiter at about the orbit of Io. Jupiter's magnetosphere is the largest object in the solar system. If it glowed in wavelengths visible to the eye, it would appear two to three times the size of the Sun or Moon to viewers on Earth. Cassini's ion and neutral camera detects neutral atoms expelled from the magnetosphere, deriving information about their source. This image was taken shortly after Cassini's closest approach to Jupiter, about 10 million kilometres from the planet on 30 December 2000.

Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travellers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being.
 

Jupiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after travelling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based observatories and from the Galileo spacecrft

Unlike Saturn's, Jupiter's rings are dark.. They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice.

In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)

The Galileo Probe

No web-site on  Jupiter would be complete without a discussion of the Galileo Space Probe.