<text><span class="style10">he Outer Planets (1 of 6)</span><span class="style7">The outer planets are very different from the inner planets. They are much further away from the Sun and, with the exception of Pluto, are much larger than the inner planets. Jupiter, Saturn, Uranus and Neptune are giant `gas' planets without solid surfaces.Much of our knowledge of the outer planets has been gained from the American space probes </span><span class="style26">Pioneer 10, Pioneer 11, Voyager 1</span><span class="style7"> and </span><span class="style26">Voyager 2</span><span class="style7">, the last of which visited Jupiter, Saturn, Uranus and Neptune in turn from 1979 to 1989.</span><span class="style10">Jupiter</span><span class="style7">Jupiter is the largest planet in the Solar System. It appears very bright to the naked eye and can outshine everything in the sky except the Sun, the Moon, Venus and (very occasionally) Mars.Through a telescope, several belts or bands can be seen in Jupiter's atmosphere. The planet's rapid rotation rate of 9 hours 55 minutes throws the equator outwards, producing a distinct `squashed' appearance. One of the most prominent features is the Great Red Spot, which may have been seen as long ago as 1664. Modern observations of Jupiter have been made by four spacecraft, </span><span class="style26">Pioneers 10 and 11</span><span class="style7"> and </span><span class="style26">Voyagers 1</span><span class="style7"> </span><span class="style26">and 2;</span><span class="style7"> pictures from these show that the Red Spot is a whirling storm in Jupiter's atmosphere.At the center of Jupiter is a rocky core. Above this are layers of metallic hydrogen (i.e. so cold that it is solid) and liquid hydrogen. Jupiter's upper atmosphere is mainly hydrogen and helium, but also contains small amounts of many different gases, including methane, ammonia, ethane, acetylene, water vapor, phosphine, carbon monoxide and germanium tetrahydride.Jupiter is known to have at least 16 satellites. Four of these, Io, Europa, Ganymede and Callisto, were seen by Galileo with his early telescope in 1610 and they are often called the </span><span class="style26">`Galilean Satellites'</span><span class="style7"> in his honor. The remaining satellites are small objects - the outermost group revolving around the planet in a retrograde direction. They are probably asteroids that were captured by Jupiter's immense gravitational pull. </span><span class="style26">Voyager 1</span><span class="style7"> also discovered a very faint ring around Jupiter (see below), which </span><span class="style26">Voyager 2</span><span class="style7"> was able to study in more detail.</span></text>
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<text><span class="style10">olcanic plumes on Io,</span><span class="style7"> one of Jupiter's moons. These various views were taken using different filters. Io is continuously rocked by voilent volcanic activity, and matter may be thrown up to 250 km (155 mi) above the surface.</span></text>
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<text>ΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE INNER PLANETSΓÇó THE HISTORY OF ASTRONOMYΓÇó SPACE EXPLORATION</text>
