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THE COMPLETE COSMOS Chapter 10: Jupiter Bigger Than the Other Planets Combined, a Turbulent Gas Giant with 16 Moons - a Voyage Through This Mini Solar System

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THE COMPLETE COSMOS Chapter 10: Jupiter Bigger Than the Other Planets Combined, a Turbulent Gas Giant with 16 Moons - a Voyage Through This Mini Solar System THE COMPLETE COSMOS Chapter 10: Jupiter Bigger than the other planets combined, a turbulent gas giant with 16 moons - a voyage through this mini Solar System. Outline Larger than all the other planets combined, Jupiter lies 780 million kilometers from the Sun, has an almost 12-year orbit, and the shortest planetary day. Eleven Earths are needed to span its diameter and 1,300 to fill its volume. Jupiter captures Comet Shoemaker-Levy 9. Its fragments impact the planet. The twin Voyager spacecraft pass the giant planet in 1979. Detailed images of Jupiter acquired by the Voyagers, including the Great Red Spot, a tempest three times larger than Earth. Ten years later, the Galileo probe parachutes into Jupiter's clouds and makes further discoveries before being crushed by atmospheric pressure. Jupiter's 30,000-degree Celsius core generates more heat than the planet receives from the Sun - and drives 500 kilometer-per-hour winds. Internal structure and its magnetic field. Galileo's orbital tour of Jupiter's 16 moons, focusing on the four majors - Io, the most volcanically-active body known; frozen Ganymede, largest moon in the Solar System with a surface temperature of minus 150 degrees; Callisto, Ganymede's crater- scarred twin; and Europa, icy world with a possible sub-surface ocean that may contain extra- terrestrial life. Jupiter and its moons are a mini solar system. Sub-chapters Largest Planet • Jupiter, a giant ball of gas, bigger than all the other planets combined. • Spinning like a top, Jupiter's short day, orbit and distance from the Sun. • Jupiter/Earth comparisons: 11 Earths are needed to span Jupiter's diameter, and 1,300 to fill its volume. • Jupiter's role as a "cosmic vacuum cleaner", Jovian gravity capturing comets and drawing them into the planet. • Brief overview of the Comet Shoemaker-Levy 9 story. Weather Bands and Storms • The two Voyager probes, launched in 1977, reach Jupiter two years later. • Whirling belts of weather, storms racing around the planet, and its most famous blemish, the Great Red Spot - an anti-cyclone much larger than Earth. Descent into Hell • Galileo's circuitous route to Jupiter. The probe's descent through the Jovian clouds - buffeted by 500-kilometre-per-hour winds (driven by heat from Jupiter's core), and eventually crushed by atmospheric pressure. • Jupiter's composition: 90 per cent hydrogen, the rest mainly helium. The planet's atmosphere - layer by layer - and its internal structure, down to the rocky core. • Generation of Jupiter's magnetic field in a layer of liquid metallic hydrogen. • Jupiter's magnetosphere - after the Sun, the biggest entity in the Solar System. • Auroral halos around the Jovian poles, created by the solar wind. Volcanic Io • Galileo's tour of Jupiter's four largest satellites. First stop, Io - flexed by the pull of Jupiter and neighboring moons. An extraordinary colored surface and erupting volcanoes. Ganymede and Callisto • Ganymede, the Solar System's largest moon with a thick icy crust. The magnetic field detected by Galileo, suggests an internal structure of slush, rock and ice. • Callisto, Ganymede's twin, but heavily cratered with a multi-ringed impact basin. Icy Europa * Europa, a possible water-world. Is a salty ocean hidden beneath its cracked, icy surface? * Tidal flexing may warm the waters, making Europa an incubator for life. With the surface ice so thick, any search for life must await a landing. Background The Great Red Spot A large, reddish oval feature called the Great Red Spot is usually visible in Jupiter's southern hemisphere - lying within the planet's south tropical zone. Since the spot was first observed in the mid-17th century, observers have reported many variations in its size and color. At its largest, the spot was so huge that three Earth's could have fitted side-by-side along its length. In the mid- 1970s, however, the spot faded from view. At the time of the Voyager flybys in 1979, the Great Red Spot was only slightly larger than Earth. The Great Red Spot is a very long-lived storm in the planet's ever changing atmosphere. Observations of cloud motions in and around the spot show that it spins anti-clockwise, completing a full revolution in about six days. Furthermore, winds to the north of the spot blow from the east, those south of the spot blow from the west. Consequently, the Great Red Spot is rather like a wheel spinning between two surfaces moving in opposite directions, producing a surprising stable wind pattern. The spot is a high pressure weather system that protrudes above the surrounding visible cloud tops. Jupiter's Volcanic Moon In 1979, the Voyager 1 spacecraft flew past Io, the innermost of Jupiter's four large moons, uncovering an extraordinary world. The surface displayed a broad range of colors from white, yellow and orange, to black, which scientists jokingly compared to a cheese and tomato pizza, or a rotten orange. A major clue to this puzzling appearance was found several days after Voyager's Jupiter flyby when a scientist noticed a large umbrella-like plume protruding from Io in one photograph - an erupting volcano. Careful examination of Io images obtained by both Voyagers 1 and 2 - and the more recent Galileo spacecraft - have shown that Io's surface is pocked with numerous volcanic vents from which eruptions occur. These vents appear as black spots typically ten to 50 kilometers in diameter, many with radiating lava flows. The plumes and fountains of material spewing from Io's volcanoes rise to heights of 70 to 300 kilometers above the surface. To reach these altitudes, material must be shot out of the vents at speeds of between 300 and 1000 meters per second. The plumes from Io's volcanoes contain particles of sulphur and the acrid gas sulphur dioxide. When this gas is erupted into the vacuum of space it crystallizes into white crystals of sulphur dioxide frost or snow. It is the sulphur ejected from Io's volcanoes that causes the brilliant colors. Although normally bright yellow, when heated and suddenly cooled, sulphur can assume a range of colors from orange to red and black. Io's volcanic vents are not located at the tops of volcanic peaks, as on Earth, but are more akin to terrestrial geysers, similar to those at Yellowstone Park in the USA. Both sulphur and sulphur dioxide will be molten at depths of a few kilometers, but as they are forced up to Io's surface, they are explosively converted from a liquid to a high pressure gas. The heating of Io's interior is a consequence of the continual squeezing and flexing of the moon by tidal stresses caused by the gravitational tug-of-war between Jupiter, on one side, and Europa and Ganymede on the other. Jupiter's Water-Worlds At least two of Jupiter's four largest moons are probably water-worlds, with hidden salty oceans beneath their icy surfaces. One of the moons, Europa, has long been speculated to have a sub- surface ocean of slushy or liquid water. Europa's water is kept warm by powerful gravitational tidal forces exerted by both Jupiter and the moons Io and Ganymede. Although nothing could survive on Europa's frozen crust, which looks like Arctic pack ice, scientists have postulated that water-based organisms, probably of a primitive microbial kind, could exist in Europa's hidden ocean. Measurements have shown that the liquid layer might be up to 100 kilometers deep, beneath an icy crust several kilometers thick. More surprising was the discovery in October, 1998, that another Jovian moon, Callisto, may also have a hidden salty sea. Callisto was previously thought to be a solid sphere of rock and ice. A liquid ocean on Callisto, however, seems to be the only explanation for data sent back by the Galileo spacecraft while in orbit around Jupiter. Observations by Galileo have confirmed that neither Europa nor Callisto have an appreciable internal magnetic field. But evidence from the spacecraft shows that both moons disturb Jupiter's magnetic field as they pass through it. The only way this could happen is if electrical currents flowing within the moons induced a magnetic field. This would require sub-surface layers on both moons that are very good conductors of electricity. Deep salty liquid oceans on both moons seem to be the only adequate explanation. Callisto is not subject to the same tidal forces as Europa. So flexing cannot be the reason that Callisto's sub-surface water - if it exists - doesn't freeze. Explanations could be warming from radiation, high pressures, eddy currents or the presence of some kind of "anti-freeze" in the form of salts or ammonia. There may also be a sub-surface ocean on Ganymede. But Gannymede has an appreciable internal magnetic field which would obscure any induction effect. So, for the time being, there is no way of proving that Ganymede has a layer of water beneath the surface. Links for Further Information Comprehensive Jupiter page, information on its atmosphere and internal structure, Jupiter's moons, facts, missions to the planet, recent news and images. http:// www.windows.umich.edu/cgi-bin/tour.cgi?link=/jupiter/jupiter.html Good Jupiter page, images of the planet and moons with text links. http://pds.jpl.nasa.gov/planets/welcome/jupiter.htm Voyager 1 mission page, including information on the mission, extensive images and FAQs. http://nssdc.gsfc.nasa.gov/ imgcat/html/mission_page/JP_Voyager_1_page1.html Voyager 2 mission page, as above. http://nssdc.gsfc.nasa.gov/imgcat/html/mission_page/JP_Voyager_2_page1.html JPL's Galileo homepage, including up-to-date information on Galileo's orbital tour, details of the mission and the craft, background and images of Jupiter and its moons, plus frequently asked questions.
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