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 temperatures in Jupiter's atmosphere are very cold ranging from --130
Appearance.
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.
Satellites.
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. 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. The 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.
Produced by the Information Services Department of the Royal Greenwich Observatory.
