Jupiter

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Note: You can click on each image below to view a larger version. Jupiter is the largest planet in the solar system, more than twice as massive as all the other planets combined. It is what is known as a "gas giant", as are Saturn, Uranus, and Neptune. This means that it has no solid surface as such. RIGHT: A photo-montage of Jupiter and its largest moons. Instead, it is just a huge ball of gas, which gets steadily denser the nearer you get to the centre. About 1000km below the cloud tops the gas becomes a sea of liquid hydrogen, which is thought to be 20,000km deep. Below this the pressure becomes so high that the liquid takes on the properties of metal, a strange form of matter known as liquid metallic hydrogen. The bulk of the planet is in this form, although there is probably a rocky core at the centre. Since all we can see from Earth is the cloud-tops, the planet's appearance is always changing. Overall, the cloud features consist of alternate light and dark bands, with any number of eddies, swirls and storms. LEFT: The famous Great Red Spot of Jupiter is simply a very large and long-lasting storm, thought to be somewhat like a hurricane. It was first observed over three hundred years ago, but since that was only when telescopes became good enough to see it properly it could be much older. It is roughly 12,000km wide and 25,000km long - for comparison, the diameter of the Earth is only 12,756km. Jupiter's four largest moons are known as the Galilean moons as they were first seen by the great Italian scientist Galileo in 1610, using an extremely small refractor telescope. Each one is a complex world in its own right. Io Europa Ganymede Callisto Io, the innermost of the Galilean moons, is a world of constant volcanic activity. The surface is coloured by deposits of Sulphur from the eruptions, and NASA's Voyager and Galileo space probes photographed plumes of material 100km high rising into space. Its surface is therefore being constantly reshaped. Europa is covered with a layer of ice, beneath which there is thought to be an ocean, possibly up to 50km deep. Consequently, the surface is smooth and flat, and the visible features appear to be mostly cracks in, or marks on, the layer of ice. Photographs taken by the Galileo probe in 1997 show a surface that looks very much like sea ice on Earth. Perhaps the most important question about Europa is whether there could be life in its hidden ocean, but we know too little about conditions beneath the surface to do more than speculate. Ganymede, at 5262.4km in diameter, is even larger than Mercury, and is the largest moon in the solar system. It is believed to have an icy surface, but unlike Europa shows a variety of terrain, including craters, grooves, ridges, and circular indentations which appear to be craters whose ice walls have slowly collapsed over the years. Callisto, the outermost of the Galilean moons, has an extremely heavily cratered surface, which is thought to have changed little, apart from the occasional impact, over the past 4 billion years. The next space probe to Jupiter will be Juno, due to launch in 2011. NASA had been considering a highly ambitious nuclear-powered probe called the Jupiter Icy Moons Orbiter, which would have gone into orbit around Callisto, Ganymede and Europa, but unfortunately funding for this ceased in NASA's 2006 budget. Jupiter is the second brightest planet, after Venus, and to the naked eye looks like a bright yellow star. With binoculars, it appears as a tiny yellow blob, with the Galilean moons visible as a row of tiny sparks strung out alongside it, and it is possible to follow their movements from day to day as they orbit Jupiter. A telescope is needed to see the patterns in the clouds, however.

		Last Updated: 1 Oct 07