Jupiter

The Bringer of Jollity

  Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus or Saturn.

  Jupiter is the fifth planet in order of distance from the sun and the by far the largest. Jupiter is more than twice as massive as all the planets combined. Jupiter is 318 times Earth.

               orbit :     778.330.000 km from Sun (5.20 AU)
           diameter :    142.984 km equatorial
     mass, Earth=1 :    317.89

  Jupiter is the fourth brightest object in the sky, after the Sun, the Moon and Venus sometimes is Mars brighter. Jupiter Has been known since prehistoric times. Galileoīs discovery, in 1610, of Jupiterīs four large moons Io, Europa, Ganymede, and Callisto, now known as the Galilean moons, was the first discovery of a centre of motion not apparently centred on the Earth. It was a major point in favour of Copernicusīs heliocentric theory of the motions of the planets; Galileoīs outspoken support of the Copernican theory got him arrested by the Inquisition. He was forced to recant his beliefs and was imprisoned for the rest of his life. His final words were: "But it rotates after all".

  Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11. Then Voyager 1 and 2, and finally Ulysses. The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years.

  There is no solid surface on the gas planets, the gaseous material simply gets denser with depth. What we see when looking at these planets is the tops of clouds high in their atmospheres.

  Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) 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 has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

  Our knowledge of the interior of Jupiter, and other gas planets, is highly indirect and likely to remain so for some time. The data from the Galileoīs atmospheric probe goes down only about 150 kilometres below the cloud tops.

  The core of Jupiter is probably of rocky materiel amounting to something like 10 to 15 Earthmasses.

  Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements 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 ionised protons and electrons, like the interior of the Sun but a far lower temperature. At the temperature and pressure of Jupiterīs interior hydrogen is a liquid, not gas. It is an electrical conductor and the source of Jupiterīs magnetic field. This layer probably also contains some helium and traces of various "ices".

  Jupiterīs 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 mixture of ice and water. However the primarily results from the Galileo spacecraft show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the spacecraft's entry point 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 and least cloudy areas on Jupiter at that time.

  Data from the Glileo atmospheric probe also indicate that there is much less water than expected. The expectation was that Jupiterīs atmosphere would contain 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 the Sunīs. Also surprisingly was the temperature and the 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 wind blows in opposite directions in adjacted bands. Slight chemical and temperature and differences between these bands are responsible for the coloured bands that dominate the planetīs appearance. The light coloured 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 Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometres into the interior. Jupiterīs atmosphere was also found to be quite turbulent. This indicates that Jupiterīs winds are driven in large part by its internal heat rather than from solar input as on Earth

  The strange and beautiful colours seen in Jupiterīs clouds are probably the result of subtle chemical reactions of the trace elements in Jupiterīs atmosphere, perhaps involving sulphur whose compounds take on a wide variety of colours, but details are unknown.

  Jupiterīs colours correlate with the cloudīs altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we can see the lower layers through holes in the upper ones.

  Jupiterīs great red spot has been seen by Earthly observers for more than 300 years. Its discovery is usually attributed to Cassini, or Robert Hooke in the seventeenth century. The great red spot is an oval about 12.000 by 25.000 km, which is big enough to hold two Earth's. Other smaller but very similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the great red spot 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 can persist for so long time.

  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 is probably causes convection deep within Jupiterīs 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 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 nuclear heat source. Jupiter would have to be at least 80 times more massive to become a star.

  Jupiter ahs a huge magnetic field much stronger than Earthīs. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiterīs magnetosphere is far from special, it extends "only" a few million kilometres in the direction toward the Sun.) Jupiterīs moons therefor 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 Jupiterīs magnetic field This "radiation" is similar to, but much more intense than, that found within Earthīs Van Allen belts. It would be immediately fatal to an unprotected human being.
  The Galileo atmospheric probe has discovered a new intense radiation belt between Jupiterīs ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earthīs Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.

  Jupiter has rings like its neighbour Saturn, but much fainter and smaller. 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 were they. It was a major coup. They have since been imaged in the infrared from ground based telescopes and by Galileo.

  Jupiterīs rings are, unlike Saturnīs, very dark (albedo about .05). They are probably composed of very small grains of rocky material. Jupiterīs rings seem to contain no ice.

  Particles in Jupiterīs rings probably donīt stay there for long (due to atmospheric and magnetic drag). Galileo found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiterīs large gravitational field. The inner halo ring is broadened by interactions with Jupiterīs magnetic field.

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

  Jupiter is often the brightest object in the nightime sky. Only Venus is brighter when visible. The four Galilean moons are easily visible with binoculars; a few cloud bands and the great red spot can be seen with small telescopes.

  Jupiter has 16 known satellites, the four Galilean moons and 12 small ones.

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