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Planet Jupiter The Bringer of Jollity Diameter (km) Diameter (Earth = 1) Distance from Sun (x 106 ) Distance from Sun (AU) Escape Velocity (km/s) Mass (kg) Mass (Earth = 1) Density (g/cm3) Density (Earth = 1) Gravity (m/s2) Average Surface Temperature (oC) Rotational Period (days) Orbital Period (years) Number of Moons Space Craft Missions 142,980 11.209 778.3 5.203 59.5 1.899 E+27 317.833 1.2463 0.22578 23.1 -150 0.4101 11.86 28 Pioneer 10 & 11, Voyager 1 & 2, Galileo, Cassini The first of the gaseous giants and second of the superior planets, is the largest planet in our solar system, Jupiter. Jupiter is over 11 times the diameter of the Earth and has a mass 2.5 times that of all the other planets combined. This giant is composed almost entirely of hydrogen (82%) and helium gas (17%). There is no solid surface on Jupiter; instead, the surface consists of a dense atmosphere topped with a layer of colourful clouds about 100 kilometres thick. The clouds are made up of bands running parallel to the equator, ranging from white to dark reddish brown, their colour arising within the clouds from complex chemical reactions that are not fully understood by scientists. The bands are in constant motion, and create an active surface of red belts and light-coloured zones which rapidly rotate around the planet. Because Jupiter does not have a solid surface, it experiences differential rotation, similar to the Sun. The bands of cloud rotate at different speeds, but unlike the Sun, the bands also travel in opposing directions. Two bands of gas traveling in opposite directions can create intricate circular currents, also known as eddies, along their border which can become whirlwind storms. While storms are constantly forming and dissipating on Jupiter, the Great Red Spot is a huge storm that has been visible for hundreds of years. It is about three times the size of the Earth and is the most recognizable feature on Jupiter, visible even in amateur telescopes. The rotational rate of the equatorial region is just under ten hours, which is extremely fast for a planet as large as Jupiter. As a result, Jupiter is noticeably oblate, its equatorial diameter being 6.5% greater than its polar diameter. According to scientists, the interior of Jupiter is most likely composed of a small metallic core surrounded by liquid hydrogen. Currents within the layer of liquid hydrogen cause a strong magnetic field which shields Jupiter from the solar wind and focuses the particles to the poles, causing aurora similar to the aurora on the Earth. Had Jupiter been more massive during its formation, the internal pressure would have generated a high enough temperature to ignite nuclear reactions within the core, creating a sister star to the Sun. Instead, Jupiter’s core never ignited, but has been steadily cooling since the planet’s formation, and is presently around 25 000K, compared to a temperature of only about 110K at its surface. Four spacecraft have visited Jupiter, two Pioneer fly-bys in 1974 and two Voyager probes in 1979. The probes obtained high-resolution images of Jupiter’s gaseous surface and gave scientists valuable information about the planet and its intricate atmosphere of clouds. One discovery that these probes made was the detection of Jupiter’s thin and delicate ring structure, which is not visible from the Earth. The probes also detected many new satellites, and along with recent discoveries we now know of 52 moons in orbit around Jupiter, the most of any planet. Many of the satellites are very small and resemble asteroids (the most recently discovered moons are only a kilometre or two in diameter); however, four of its moons are among the largest and most interesting in the solar system and were discovered by Galileo Galilei in 1610. The Galilean satellites had never been seen in any detail until the Voyager spacecraft passed by them in 1979, increasing our knowledge of the four moons tremendously. Io, Europa, Ganymede and Callisto are approximately the same size as our Moon (Europa being slightly smaller) and are easily seen from the Earth with binoculars or a telescope as star-like points of light along Jupiter’s equatorial plane. Their alignment constantly changes on a nightly basis as the moons rapidly circle around Jupiter and dance from one side to the other in a matter of hours. Io is closest to Jupiter and is the most volcanically active body in the solar system with numerous vents ejecting molten sulphur. The thin atmosphere and low gravity of the moon cannot contain the material which can erupt hundreds of kilometres above the surface. The gravity of Jupiter and the three outer Galilean satellites pull Io in opposite directions and exert tidal forces which heat its interior, generating pressure which is released by the eruption of the internal molten materials. Europa is covered with a smooth tan coloured surface without mountains or craters, suggesting a youthful surface with recent activity. The surface appears to be a layer of frozen water with numerous cracks. Ganymede is larger than Mercury and is the largest satellite in the solar system. Ganymede and Callisto are both dark in colour, icy, and heavily cratered, Callisto being one of the most heavily cratered objects in the solar system.