The Star Garden
Io Europa Ganymede Callisto (to scale)
Io is the closest Galilean moon to Jupiter and has the third largest mass of the Galilean moons. It is the most geologically active object in the Solar System, with over four hundred active volcanoes and one hundred mountains, some taller than Everest. These emit plumes of sulphur dioxide which can reach five hundred kilometres, over five times the distance to space from the surface of the Earth. Io is mostly composed of silicate rock, its surface is covered with sulfur and sulfur dioxide frost and it has a molten core made of iron or iron sulfide.
Europa is the second Galilean moon from Jupiter, it is also the least massive of the Galilean moons and has a diameter similar to that of the Earth's Moon. It has a thin oxygen atmosphere and, like Io, it is mostly composed of silicate rock with an iron core. Europa is expected to contain a salt water ocean which extends for about one hundred kilometres below its frozen surface.
Ganymede, is the third Galilean moon from Jupiter, it is the largest and most massive moon in the Solar System. It is also larger than Mercury, although not as massive. Ganymede is primarily composed of silicate rock and frozen water but a saltwater ocean is thought to exist nearly two hundred kilometres below the surface, between two layers of ice. Ganymede has a thin oxygen atmosphere and a liquid iron core. It is the only moon in the Solar System to have a magnetosphere. The surface of Ganymede is covered in dark, cratered regions and lighter regions which are covered in ridges and grooves.
Ganymede
Callisto is the furthest of the Galilean moons, this means that it does not feel Jupiter's tidal effects as strongly and has no active volcanoes. It has the second largest mass of the Galilean satellites and is about the same size as Mercury. Callisto has a thin atmosphere composed of carbon dioxide and oxygen. The surface is composed of silicate rock, frozen water, carbon dioxide and other organic compounds, it is heavily cratered. Callisto is thought to have a small silicate core and an ocean of liquid water about one hundred kilometres below its surface.
NASA's Europa Jupiter System Mission, which is expected to launch in 2020, will look for evidence of life on the Galilean moons.
Saturn
Saturn is the sixth closest planet to the Sun and the second largest planet in the Solar System, after Jupiter. It takes over ten thousand Earth days to orbit the Sun, this is about thirty Earth years. One day on Saturn is just under eleven hours long. Saturn is visible from the Earth and like Mercury, Venus, Mars and Jupiter, it is named after a Roman God, the god of agriculture.
NASA's Pioneer 11 probe reached Saturn in 1979, followed by the Voyager probes in 1980 and 1981. Saturn is almost entirely composed of hydrogen, with some helium and trace amounts of higher elements. It has a small core of iron, nickel, silicon and oxygen compounds surrounded by liquid hydrogen and helium and produces a magnetosphere.
Saturn's rings are mostly composed of frozen water but also contain rock and dust. Another, virtually invisible, ring of dust was discovered using NASA's infra-red Spitzer Space Telescope in 2009.
Saturn has at least sixty moons and the largest of these is Titan. Titan is the second largest moon in the Solar System, after Jupiter's Ganymede and is about twice the diameter of the Earth's Moon. Titan is the only moon in the Solar System to possess a significant atmosphere and this is mostly composed of nitrogen with some methane and ethane clouds. Like Jupiter's Ganymede, Callisto and Europa, Titan is thought to have an ocean of liquid water beneath its surface. In 2005, the international Cassini-Huygens mission discovered liquid hydrocarbon lakes at its poles, the only surface liquid ever discovered outside of Earth. Titan's surface is similar to the Earth's, with sand dunes, shore lines and the effects of seasonal weather patterns.
Jupiter and Saturn
Jupiter
Jupiter is the fifth closest planet to the Sun and it is over twice as massive as all the other planets in the Solar System combined. It takes over four thousand Earth days to orbit the Sun, this is almost twelve Earth years. One day on Jupiter is just under ten hours. Jupiter is sometimes as bright as Mars, making it visible from Earth. As with Mercury, Venus and Mars, Jupiter is named after a Roman God, the god of the sky, known to the Greeks as Zeus.
NASA's Pioneer 10 probe flew past Jupiter in 1973, followed by Pioneer 11 in 1974 and the Voyager probes in 1979. Jupiter is thought to have a rocky outer core, about 75% of its mass is composed of hydrogen gas and the rest is mostly helium. Its red spot is a cyclone three times the size of Earth. Jupiter has a faint ring system and at least sixty moons, most of which are less than ten kilometres in diameter. The largest of Jupiter's moons are the Galilean satellites; Ganymede, Callisto, Io and Europa, these were discovered by Italian astronomer Galileo Galilei in 1610.
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