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Image: Reta Beebe, Amy Simon & NASA
Jupiter is the largest planet in the solar system. It has a diameter 11 times that of the Earth and a mass (more than 300 times that of the Earth) which is greater than twice the sum of all the other planets; yet its mass is less than one thousandth that of the Sun. It orbits the Sun at a distance of about 500 million miles (780 million km) in just under 12 years.

Internal structure

The structure of Jupiter is very unlike that of the Earth. The visible 'surface' of Jupiter is, in fact, the uppermost layers of clouds of methane and ammonia. The interior of Jupiter is made up of a solid core of material similar to that of the Earth with a diameter of about 24,000 km. Surrounding this, with a diameter of about 100,000 km, is a metallic mixture of hydrogen and helium. On the Earth we know these two as gases which at very low temperatures can be liquefied; in Jupiter's interior the pressure is so high that the hydrogen takes up a state in which it behaves like a metal. Outside this metallic hydrogen zone is a shell of liquid molecules, mainly hydrogen and helium, with the cloudy atmosphere, about 1000km deep, above. The Great Red Spot (GRS) has been shown to be an enormous anti-cyclonic system, which has lasted at least 300 years. The size of the GRS spans from 12,000km by 25,000km – this is large enough to hold two Earth's. Observations have been made in Infrared of the direction of rotation indicates that it's an area of high pressure.

The temperatures in Jupiter's atmosphere are very cold ranging from -130°C at the top of the clouds to 30°C about 70km below.


From the Earth Jupiter can be seen, even in a small telescope, to show a disc with polar flattening. This is due to the very rapid rotation; cloud features are seen to pass around the planet in about 10 hours. Across the disc several bands of dark and light clouds can be seen with a giant feature called 'The Great Red Spot' being visible during each rotation.

The pictures returned by the Voyager probes have shown the complexity of the structures within these bands. The Red Spot has been shown to be an enormous anti-cyclonic system which has lasted at least 100 years. The clouds are composed of methane and ammonia with the colours being due to different compounds of sulphur, nitrogen and possibly phosphorus.

Jupiter is one of the brightest objects in the night sky. A combination of its orbital period, 11.9 years, and that of the Earth means that we see Jupiter at opposition (its closest to the Earth) every 13 months. It is then very bright; the only star-like objects that can appear brighter being Venus, Mars and the very rare close novae or supernovae.


Io passing over Jupiter
Io passing over JupiterImage: J. Spencer & NASA
With a pair of binoculars, or a small telescope, it is possible to see that Jupiter is a small disk. It is also possible to see the four moons discovered by Galileo. These are the four largest of Jupiter's family of 16 satellites which all lie within the magnetsphere extending 650km into space. The diameters of the biggest, Io, Europa, Ganymede and Callisto range between 3000 and 5000 km whereas the smallest, Leda has a diameter of only 10 km and the fifth largest, Amalthea is less than 300 km across.

The names of all of Jupiter's satellites come from mythological lovers of Jupiter, except Amalthea who was his nurse.

Io, the closest of the four large moons to Jupiter, is the most fantastic. Because of tidal forces of Jupiter and the other moons the surface moves in and out by some 100 metres. This generates a lot of heat which causes a peculiar form of vulcanism in which volcanoes emit fountains of sulphur compounds from subsurface liquid sulphur magma. Several of these volcanoes were seen in eruption by the Voyager probes.

Europa, Ganymede and Callisto are all covered in water-ice but each shows different structures indicating that each has been affected in a unique way by its history. Callisto shows the most cratering and probably shows us the oldest surface. Ganymede shows large fault systems which look like features on the Earth, such as the San Andreas fault in California. Europa looks as if it might have a liquid layer under its ice, as there is little cratering visible but large fracture systems reminiscent of similar structures near the Earth's north pole.

Jupiter also has a faint ring, which was discovered by Voyager I by chance. The rocky particles in this ring may have originated on Io or from meteoritic or cometary debris. The ring is not visible from the Earth.

Magnetic field

Jupiter has a magnetic field which, at the height of the cloud layer, is more than 10 times that of the Earth. The interaction of this field with the solar wind causes an enormous toroidal system rather like the van Allen belts around the Earth. The moon, Io, lies within this field structure and is responsible for bursts of radio waves which are observed to come from Jupiter.

Jupiter has enormous thunder storms in its atmosphere and also aurorae.

The NASA spacecraft mission to Jupiter, appropriately called Galileo , was launched in 1989 and reached the planet in December 1995. After two years successful observing, NASA agreed to extend the mission by a further two years to obtain more data on Europa and Io.

The Shoemaker-Levy 9 comet impact

This sequence of six images shows the A impact of 16 July 1994. These images were taken at the German-Spanish 3.5 meter telescope on Calar Alto in southern Spain, using MAGIC, the near infrared camera of the Max-Planck-Institut fuer Astronomie in Heidelberg, Germany.
The bright object to the right of Jupiter is the innermost Galilean moon, Io, and the fainter oval structure in the southern hemisphere is the Great Red Spot. The polar caps appear bright at the wavelength of the observations, 2.3 um, which was selected to maximize contrast between the impact region and the jovian cloud deck. All six frames have a logarithmic relation between intrinsic brightness and colour, which mimics the behaviour of the human eye. Click here to see a more detail description of each image

Comet impact on Jupiter
This sequence of images shows the impact of 16 July 1994Image: Calar Alto Observatory/Max-Planck-Institut fuer Astronomie 

On July 7, 1992, during its closest approach to Jupiter, Comet Shoemaker-Levy 9 was torn into many pieces by Jupiter's gravity. The close approach to Jupiter also deflected the comet into a smaller orbit, which brought the fragments back two years later, when they collided with Jupiter. Twenty-one pieces of the comet disintegrated in Jupiter's upper atmosphere between 16 July and 22 July, 1994. Some of the fragments created huge fireballs, rising up to 3,000km above Jupiter's clouds and clearly visible with telescopes here on Earth. The impacts left dark rings in Jupiter's atmosphere, the largest of which were still visible in December 1994, 5 months after the collisions.


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