
	Introduction The influence of Jupiter Mythology Astronomy

Jupiter is the fifth planet from the Sun and by far the largest. For example, it is 318 times the size of the Earth and more than twice the size of all the other planets combined.

Orbit: Diameter: Mass:		778,330,000 km (5.20 AU) from the Sun 142,984 km (equatorial) 1.900e27 kg

Astronomy

Jupiter is the biggest planet in our solar system and the fourth brightest object in the sky (after the Sun, the Moon and Venus; and occasionally Mars is also brighter). Jupiter's powerful gravity controls a group of 16 moons known as the Jovian system, since Jove is another name for Jupiter. Twelve of the moons are the size of small asteroids and four are bigger than Pluto. Galileo's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (often referred to as the Galilean moons) was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets (see Unravelling the Mystery). Galileo's outspoken support of the Copernican theory got him into serious trouble with the Catholic Church via the papal tribunal known as the Inquisition, which saw his support as a threat to Christian beliefs. Amazingly, it took the church until 1993 to formally recognize the validity of Galileo's work!

Together with Saturn, Uranus and Neptune, Jupiter is one of the so-called gaseous planets, consisting of a huge ball of hydrogen and helium gases. These same gases make up the Sun, but Jupiter would need to contain 80 times more gas before it would begin to shine like a star. Space ships could not land on Jupiter. The gaseous planets do not have solid surfaces, and their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of one atmosphere). What we see when we look at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level). Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot. It is estimated that at the core it is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz effect, the slow gravitational compression of the planet. This interior heat probably causes convection deep within Jupiter's 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.

(N.B. Jupiter does NOT produce energy by nuclear fusion like the Sun; it is too small and thus its interior is too cool to ignite nuclear reactions.)

A day on Jupiter is less than 10 hours long. In spite of being the largest of the planets, it spins faster than any of the others, a phenomenon that produces fierce storms with winds up to 500 km (300 miles) per hour, and huge bolts of lightning in the restless thunderclouds. Lightning bolts on Jupiter are up to 10 times more powerful than those here on Earth, and one hurricane, the Great Red Spot that was photographed by the Galileo spacecraft, has been raging for about 300 years. In 1995 the probe from the Galileo spacecraft plunged into Jupiter's atmosphere. The fierce winds blew the probe hundreds of miles sideways on its descent through clouds of ammonia ice crystals. The probe lasted only an hour recording data before it vaporized in the fierce atmosphere.

Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). Jupiter's magnetosphere is far from spherical, extending "only" a few million kilometers in the direction of the Sun. Jupiter has rings like Saturn's, but much fainter and smaller. Unlike Saturn's, Jupiter's rings are dark, with an albedo (which is the ratio of the light reflected by a planet to that received by it) of only about .05. The rings are thought to be composed of very small grains of rocky material and, unlike Saturn's rings, are thought to contain no ice.

If you would like to know more about the solar system, Starscine suggests the following websites: