SOLAR SYSTEM/Jupiter, Saturn and Their Moons 283 Figure Linear zones of light and dark, rising and descending gases on Jupiter’s surface Reproduced from NASA Table Space craft to Jupiter Name Launch date Encounter date Nearest approach, km Pioneer 10 Pioneer 11 Voyager Voyager Galileo Mar 1972 Apr 1973 Sept 1977 20 Aug 1977 18 Oct 1989 131 400 46 400 150 000 714 000 Entry Dec 1973 Dec 1974 Mar 1979 July 1979 Dec 1995 Fly by Fly by Went on to Saturn Images of Jupiter and the Galileans Went on to Saturn Complemented Voyager Went on to Saturn, Uranus, Neptune Orbiter and entry probe Data were also obtained from Ulysses (1992) and Cassini (2001) Satellites The four Galilean satellites – Io, Europa, Ganymede, and Callisto – were observed by Galileo as long ago as 1610 (hence the name of the space-craft) Four small inner satellites were discovered between 1892 and 1979, and there are many small bodies moving round Jupiter beyond the path of the outermost Galilean, Callisto The total number of known satellites by April 2004 was 62 Data for all the satellites over km in diameter are given in Table Io Io is violently volcanic In March 1979, S Peale and his colleagues in America suggested that since Io’s orbit is not perfectly circular, the interior might be ‘flexed’ by the gravitational pulls of Jupiter, and also the other Galileans, sufficiently to produce active surface volcanoes Only a week later the first volcanic plume was detected on an image from the Voyager space-craft, and many dozens have since been identified, both from the space probes and with the Hubble Space Telescope Lava-flows and lava lakes are plentiful; the average temperature of the lavas is about 1600 C Many explosive eruptions are driven by sulphur dioxide gas emission; the surface is remarkably colourful, with yellow, orange, red, and black areas (Figure 2) The surface is ‘young’ and virtually without surviving impact craters One volcano, Loki, is the most powerful in the Solar System, emitting more heat than all the Earth’s active volcanoes combined Io is connected to Jupiter by a strong flux tube, and has a marked effect upon the Jovian radio emission; material sent out by the volcanoes is spread along the orbit, producing a torus Europa Europa is only slightly smaller than Io, and rather further from Jupiter, but the two satellites are very different Europa has a smooth, icy surface with very limited vertical relief and few impact craters, though one of these, Pwyll, shows bright rays extending outward and crossing all other features There are plains, chaotic areas, and low ridges, together with shallow pits Detailed views from space-craft (particularly Galileo) show what look remarkably like icebergs, and it is widely believed that an ocean of salty water lies below the visible surface, with the icebergs floating around (Figure 3) Fragmented blocks of ice seem to look very like the blocks in the Earth’s polar seas during a springtime thaw Europa does not have a strong internal magnetic field, but it orbits within Jupiter’s magnetosphere, and the instruments on Galileo have detected an induced magnetic field which produces significant effects linked with the rotational period of the planet Jupiter’s magnetic field at Europa changes direction every 512 h, and this indicates the existence of a layer of electrically conducting material, such as salty water, not far below the icy surface of Europa It seems that the ocean may lie at a depth of less than 100 km If it really does exist (and as yet there is no