Jupiter

1. The largest planet in the solar system, accounting for roughly seventy percent of the total planetary mass, and the fifth from the Sun. The largest and innermost of the gas giants, Jupiter makes an interesting sight through even a modest telescope. Orbits the Sun in 11.86 Earth years. Radiates over twice the energy received from the Sun due to gravitational contraction. Temperature at cloud decks about 143 degrees Centigrade below zero. Pronounced equatorial bulge; diameter through the equator is 88,846 miles (to within five miles, measured to the altitude where gas pressure equals that of Earth's air at sea level); diameter through the poles is 83,082 miles. Mass over 317 times that of Earth, volume over 1,321 times that of Earth. Composed mostly of gas, chiefly hydrogen with a considerable amount of helium; traces exist of ammonia, water vapour, methane, ethane, silicon compounds, carbon compounds and sulphur, among others. At greater depths the hydrogen enters molecular and then metallic states not found among gases on Earth; there may be a rock-metal core accounting for perhaps five percent of the planet's mass. The planet does not rotate on its axis as a solid body, but faster in the equatorial regions than around the poles, by about five minutes per rotation; the whole turns once on its axis in just under ten hours. The entire visible face of the planet consists of clouds in the upper reaches of a vast ocean of gas. Has a striped appearance with light zones of upwelling gas and dark, descending belts; there are numerous rotating storm systems, the largest and longest-lived of which is the Great Red Spot, a storm larger than the Earth. Jupiter's powerful magnetosphere, its trailing end still detectable at the orbit of Saturn, funnels considerable amounts of ionising radiation, carrying at its strongest one thousand times the lethal dose for the human body. Jupiter receives comparatively many asteroid and cometary impacts, recently including the string of impacts from the tidally disrupted comet Shoemaker-Levy 9 in 1994. Visited by Pioneer, Voyager, Galileo and Cassini probes. Recognised to have 63 satellites at last count; the largest of these, Io, Europa, Ganymede and Callisto, are collectively known as the Galilean satellites because they were discovered by Galileo Galilei in 1609-10. (This made them the first extraterrestrial moons discovered, which made for an epochal discovery.) Ganymede is the largest moon in the solar system and is larger than, though not as massive as, the planet Mercury. Europa is thought a possible abode for extraterrestrial life. 2. Jupiter's namesake is the big daddy of the gods of ancient Rome, equivalent to the Greek Zeus. Known in full as Jupiter Optimus Maximus Soter; Jupiter the best, the greatest and the Saviour. Popularly imagined sitting on a throne with a bunch of thunderbolts in his left hand. The patron deity of the Roman state, he ruled over laws and the social order. He was the father of Mars and hence mythologically the grandfather of the Roman founders Romulus and Remus.