JUPITER

Image of Jupiter in true colours, taken by Hubble Space Telescope. It is possible to recognize the structures of the clouds, with crystals of frozen ammonia and carbon, sulphur and phosphor compounds, which confer their typical colours. (HST)

Jupiter is the fifth planet of the Solar System and the first as far as mass and size are concerned. For this reason, it was named after the king of the Olympus. It is one of the giant gaseous planets.  Jupiter has been known since ancient times too, being the fourth brightest object in the sky after the Sun, the Moon and Venus.

 
 

The diameter of Jupiter measures 142,800 Km, and its volume could contain more than 1,000 planets of the same size as the Earth. Its mass is equal to one million nine hundred thousand billion billion tons (1,9 1030 g), 318 times larger than that of the Earth, and greater than the sum of the masses of all the other planets.    The jovian gravitational attraction, due to the mass of the planet, has a remarkable effect on the other components of the Solar System, asteroids and comets included.  The density of the planet is equal to 1.33, rather low, like that of all giant planets.

One of the best pictures of Jupiter obtainable from the Earth, taken at the North Optical Telescope of the Canary Islands. (NOTSA)

 
 
 
 
 

Image of the satellite Io with Jupiter in the background, taken by the Galileo spacecraft. (NASA/JPL)

Jupiter was visited for the first time in 1973 by the Voyager 1 probe, and later by Voyager 2, by the Pioneer 10 and 11 probes, and by Ulysses. At the present time, the spacecraft Galileo is orbiting around the planet. The atmospheric probe that has been sent towards the interior of the planet is programmed to reach 150 Km below the outer layer of clouds, and to transmit data on the jovian atmosphere.  The mean distance between the planet and the Sun is 778,330,000 Km and the orbital period is 11.86 years. Jupiter's rotation is much faster; the day on this planet is shorter than on the others: 9h 50' 30". The resulting centrifugal force is so intense that confers to the planet a flat shape.

 
 

The atmosphere 
 

The aspect of the planet is characterized by parallel bands with different width and colour, which surround its surface. Actually, like all gaseous planets, Jupiter does not have a solid surface, but the gaseous material becomes denser and denser towards the centre of the planet. Therefore, what we see is not the physical surface, but the upper part of its atmosphere.
The coloured bands are systems of clouds pushed by strong winds along a direction parallel to the equator of the planet, at over 600 Km/h. The winds blow in opposite directions in two adjacent bands.

The different colours of the bands indicate the presence of different chemical compounds at the various latitudes of the jovian atmosphere, and maybe of chemical reactions between them: the most important components of the clouds are frozen crystals of ammonia and ammonium hydrosulphide. Besides, the colours turn from red to blue according to the height of the clouds. The aspect of the bands of clouds changes within hours or days.
The first data received from the atmospheric probe of the Galileo mission should indicate the lack of Oxygen with respect to the expected abundance. 
 

Image of the Great Red Spot and of the surrounding zone, taken by Voyager 1 in 1979, from a distance of 9 million Km. You can notice details of the clouds up to a dimension of 160 Km. (NASA/JPL)

Besides the bands there are protuberances, vortices and irregular spots, the largest of them being the "Great Red Spot". It is a huge atmospheric vortex, with an elliptic shape, 25,000 Km long and 12,000 Km wide. It is localized in the southern hemisphere of the planet, and was discovered by the astronomer Cassini in 1665. Other similar and smaller vortices, can be found along other bands of clouds.

The structure 
 

Jupiter is composed by an enormous body of liquid Hydrogen. Inside, at a high depth, in fact, the pressure is so high that the Hydrogen atoms break up into electrons and protons, and the gas turns to fluid metal. This is possible only at pressures that are over 4 million bars (1,013 bar = 1 atmosphere), such as those inside the planet.
In the centre there is probably a small nucleus of iron and silicates, which contains a mass 10-15 times larger than that of the Earth. 
The whole is surrounded by a dense atmosphere of Hydrogen, which constitutes most of the planet; it is composed mainly by Hydrogen and Helium, with traces of ammonia, methane and vapour.
The knowledge about the interior of Jupiter is impossible, as it is for all the other gaseous planets, and the little we know came mostly from indirect evidence.

The planet is composed by Hydrogen (75% of its mass), Helium (25%), with small traces of other elements. Such composition, like that of Saturn, is very similar to the composition of the primordial cloud from which it is thought that the Solar System originated.

Like Saturn, Jupiter emits a large quantity of energy, two and a half times the amount it receives from the Sun, therefore it must have some internal source of energy, unlike the other planets. Such energy is produced by the slow gravitational contraction of the planet, which transforms potential gravitational energy into radiative energy, thus producing a central temperature in the vicinity of 20,000 degrees. 
Such temperature is not sufficient to trigger the thermonuclear fusion reactions in the centre of Jupiter, but if it were at least 90 times bigger, the temperature and pressure in the interior would be high enough to trigger the nuclear fusion, and the planet would become a star.
 

Auroral emission of Jupiter. Io and Jupiter are bound by an invisible stream of charged particles, emitted by the satellite. They flow along the lines of the jovian magnetic field, and cause the aurora when they penetrate in its atmosphere. (HST)

The jovian magnetic field is very intense and has opposite direction compared to the terrestrial one: a compass on the planet would indicate the South instead of the North! Such field is due to the large mass of fluid Hydrogen which composes the planet, and which works as a gigantic electroconductive body.  In the polar regions of Jupiter some auroral emissions have been observed, similar to the terrestrial polar aurorae; they are probably due to charged particles that came from the satellite Io, that move in the jovian magnetic field emitting a radiation. The bands of radiation around the planet are much more intense than the terrestrial ones, and contain highly energetic charged particles.

 
 

The satellites and the rings 
 

Jupiter has 16 satellites: Adrastea, Amalthea, Ananke, Callisto, Carme, Elara, Europa, Ganymede, Himalia, Io, Leda, Lysithea, Metis, Pasiphae, Sinope and Thebe.
Four of them (Io, Europa, Ganymede and Callisto) were observed for the first time by Galileo with his telescope in 1610, and are known since then as the "galilean satellites". Galileo dedicated his discovery to Cosimo II Medici, and therefore they also bear the name of "medicean satellites". 
Due to the tidal forces between the planet and its satellites, Jupiter's rotation is slowing down and the satellites are slowly modifying their orbits.

Five of the satellites of Jupiter. Namely:  Amalthea, Io, Europa, Ganymede and Callisto. (Calvin J. HAmilton)

Jupiter, Io (left) and Europa (right), photographed in 1979 by the Voyager 1 spacecraft, from a distance of 20 million Km. (NASA/JPL)

 

A ring of Jupiter, 6,500 Km wide and less than 10 Km thick. It was photographed by Voyager 2 in 1979. The image is in false colours.  (Calvin J. Hamilton)

The planet also has a very thin system of rings, not visible from the Earth because it is not very bright (in fact, it has an albedo of 0,5).  It was observed for the first time by Voyager 1 in 1979. It spreads from a distance of 30,000 Km to 130,000 Km from the planet. It is quite uniform, composed by particles of dust with the size of less than 10 microns, and it probably originated thanks to the disintegration of micrometeorites coming from the jovian satellites.

 
 

The impact of the Shoemaker-Levy comet on Jupiter 
 

Images of the fragments of the Shoemaker-Levy comet in collision course with Jupiter, taken by Hubble Space Telescope in July 1994. (HST)

Mosaic of images showing the evolution of the impact region of the G fragment of the comet, from July 18th 1994 (bottom right) to July 23rd 1994 (top left). (HST)

 

Evolution of the impact regions of the D and G fragments of the comet. (HST)

Arch  Auroral emission after the impact of the K fragment. It is caused by the charged particles produced in the impact and trapped in the jovian magnetic field. (HST)

 
 

ANIMATIONS

The jovian atmosphere,  AVI, 3.4 Mb (NASA/JPL)

The Great Red Spot,  AVI, 1.8 Mb (NASA/JPL)

The jovian magnetic field,  AVI, 6 Mb (NASA/JPL)

Impact of the S-L comet on Jupiter,  MPEG, 651 Kb (STScI)