Jupiter

Jupiter is the fifth planet from the sun. It is also the largest of all planets, including the Jovian planets. It contains more than twice the mass of all the other planets combined. Saturn, which is the second largest planet and comparable in radius with Jupiter holds less than one third of Jupiter's mass (Mass of Jupiter: 318 x Earth's, Saturn's mass: 95 x Earth).
Even through a small amateur telescope its four prominent companions, the Galilean moons or the Jovian moons Io, Europa, Callisto and Ganymede are visible, which are one of the things Jupiter is known for. Another feature is the great red spot orbiting below the equator of Jupiter. It orbits in the opposite direction of Jupiter and is large enough to contain the Earth. It also has a very faint ringsystem.
Jupiter is known to share its orbit with a few asteroids (though they are located at another place in the orbit), they're called the Trojans.
The planet is so large that its gravity pulls many nearby passing objects to itself, not only asteroids, but also comets. That is what happened in 1994, when the comet Shoemaker-Levy 9 struck Jupiter with mindboggling force. It had been broken up, into 21 fragments by Jupiter's tidal forces in the previous orbit. This fierce attack on Jupiter opened astronomers' eyes on what could happen to Earth.

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Exploring Jupiter
Jupiter's interior and atmosphere - Illustration 1

Jupiter's moons
- Io - Illustration 2
- Europa - Illustration 3
- Callisto - Illustration 4
- Ganymede - Illustration 5

Jupiter's interior and atmosphere

Jupiter is the largest planet in the known solarsystem. It is a gas giant, and as such, it does not have a solid surface. At the center of Jupiter there may be a rocky core, which might contain as much as 10-15 times more mass then Earth (6x10^26 Kg) and is about 50 000 K hot, surrounded by a layer ices of water, ammonia and methane. Above this layer is yet another layer, where the pressure is so large that hydrogen starts acting like a metal, hence the layer is composed of liquid metallic hydrogen. To enter such a state where hydrogen becomes a liquid metal, a pressure of 1 million bars and a temperature of 6 000 K is needed, which we (by comparison, the pressure at the surface of Earth's surface is about 1 000 bars, but the gases exerting this pressure are different). These conditions are found about 10 000 km below Jupiter's cloud tops. It should be noted that we cannot know for sure the exact conditions in Jupiter's interior, as much of it is veiled.
As we move further out from the planet's core, we will find gaseous material that decrease in density and pressure.
Jupiter consists of about 90% hydrogen and 10% helium, with traces of ammonia, water ices and methane.
Jupiter's appearance is characterized by colourful horizontal bands of clouds. One particular weather system is especially prominent: The Great Red Spot, located below Jupiter's equator. It is so large that it could engulf the whole Earth! Winds are much stronger on Jupiter than on Earth.

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Jupiter's moons

Jupiter has atleast 63 known moons. Many are small in size and others, like the largest moon in our known solarsystem, Ganymede are so large that if it would have been discovered orbiting the sun for itself, it would be considered as a planet. Among all these moons are four prominent moons, the Galilean moons or the Jovian moons, which are Io, Europa, Callisto and Ganymede. They are visible through a small amateur astronomer telescope and were discovered by Galileo Galilei on January 7, 1610. They orbit relatively quickly around the planet and changes in their location can be observed within hours or few days.

Io is the innermost and smallest of the jovian moons. It orbits at an average distance of 421 700 km from Jupiter. It is locked in a Laplace-resonant orbit with Europa and Ganymede in such a way that for each orbit Ganymede completes around Jupiter, Europa will complete two. While Europa has completed those two orbits, Io will have completed four orbits. This process, where the moons interact gravitationally, along with strong gravitational forces exerted by Jupiter is probably the cause of Io's volcanic surface. Io's "body" is bent and stretched about 100m in this process, which causes it to heat up. The surface is never completely solid (about 2000 K hot), but it is continuously resurfaced. Io is the most volcanic active body in the known solarsystem. It will never have a solid surface, but always be scarred with volcanoes and liquid sulphur, unless something dramatical happens, like significantly altered orbit.
Volcanoes have been detected to spew out gas and dust at heights of 400 km. But Io's orbit happens to lay where Jupiter's magnetic field is particularly strong. When material is ejected from the volcanoes it is to an extent carried on by Jupiter's magnetic field which also accelerates it to an amazing speed of 300km/s.
Some of the matter that is accelerated by Jupiter might actually reach Jupiter's poles, producing the most spectacular and powerful aurorae ("nothern lights"). The matter that isnt picked up by the magnetic field crystallizes to suplhurous flakes falls back as snow. Astronomers have even managed to find NaCl on Io, which is ordinary "table salt".
Studies show that Io has a magnetic field of its own, along with an atmosphere consisting of sulphur compunds.
Io reminds astronomers of how the Earth might have looked like at a primordial state.

Europa is closely associated with the possibility of finding life elsewhere in the solarsystem. It orbits Jupiter at a distance of 671 034 km and is slightly smaller than our moon in size. Its composition is similar to that of the terrestrial planets and has a rocky core. Europa has a thick layer of water ice that might be as thick as 100 km, though recent studies show it might be less than 20 km deep. The thickness of this crust is important to know in order to measure how much tidal heating Europa is getting, which in turn is important to know when calculating how much liquid water could exist. During the spacecraft Galileo's mission, impact craters were discovered which after a while were filled up again, by water from lower levels.
Europa's surface is extremely smooth, there are very few features higher than a few hundred meters that have been detected.
Remarkable streaks have been seen on Europa, suggesting cracks on the surface.

Callisto is the fourth galilean moon and the theird largest moon in our solarsystem (the largest are Ganymede and Saturn's Titan). Its size rivals that of Mercury. Callisto is among the heaviest cratered objects in the solarsystem. It also has a very thin atmosphere, which was discovered by studying its magnetic field. Like Europa, Callisto also has a significantly thick ice crust which harbors a salty ocean at the bottom.

Ganymede is larger than the planets Mercury and Pluto. It is also the largest moon in our solarsystem (diamater is 5 262 km). It orbits Jupiter at a distance of about 1.1 million kilometres and is the third of the four galilean moons. Like Callisto it is also heavily cratered, but there are features such as mountains, valleys, lavaflows. Some areas of the surface are particularly dark, which are really old. The lighter areas are newer, but still very old. Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice.
The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon.

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Exploring Jupiter

Jupiter has been visited by many spacecraft, like Voyager 1 and 2, Pioneer 10 and 11, Ulysses and Galileo, which have contributed to the majority of the knowledge we have about Jupiter and its moons. Though some studies have also been made by both earth-based and space-based telescopes orbiting the Earth, such as the Hubble Space Telescope, which in particular helped us view the giant comet collision with Shoemaker-Levy 9 in 1994.
NASA is planning on sending a new probe, Juno to Jupiter, to study it from a polar orbit. The probe is scheduled to be launched in 2010. In 2007 Jupiter will be briefly visited by the probe New Horizons for a gravity assist.
There is an interest for a mission to study the ice of three of the galilean moons, but the project has been cancelled.

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Moons: 63. Ganymede, Callisto, Europa and Io are the most known.
Average distance from sun:
778 412 020 km.
Equatorial Radius: 71 492 km.
Mass: 1.8987 x 10^27 kg, about 317 times the mass of the earth.
Density: 1.33 g/cm^3.
Escape Velocity: 59.540 km/s.
Length of day: 9.925 hours.
Length of year: 11.8565 Earth years.
Mean Orbit Velocity: 13.069.7 km/s.
Equatorial Inclination to Orbit: 3.12 degrees.
Effective Temperature: -148 °C.
Atmosphere: Hydrogen 90%, Helium 10%.
Misc: Has four moons that were discovered by Galileo Galilei. They are called the "Galilean moons", or the "Jovian moons".

Source: NASA.

Illustration 1: Jupiter's cloud bands are prominent in this 3D model of Jupiter. The Great Red Spot is also visible, along with Io's shadow.

































Illustration 2: Io, as seen 250 000 km away.






















Illustration 3: Europa, as seen 250 000 km away.






Illustration 4: Callisto, as seen 300 000 km away.



Illustration 5: Ganymede, as seen 300 000 km away.