<text><span class="style10">he Inner Planets (4 of 5)Mars</span><span class="style7">Mars is the fourth planet from the Sun. It is the most hospitable planet other than the Earth, having a thin carbon dioxide atmosphere. Early observers, including Schiaparelli, believed they saw canals and vegetation on Mars, but modern observations have shown that these do not exist.In 1965 the American </span><span class="style26">Mariner 4</span><span class="style7"> space craft flew by Mars and returned pictures showing a barren, cratered surface, massive extinct volcanoes and deep chasms. The largest volcano on Mars is Olympus Mons, which rises 22 km (14 mi) above the surrounding plains. Later probes, including </span><span class="style26">Mariner 9</span><span class="style7"> and </span><span class="style26">Vikings 1</span><span class="style7"> and </span><span class="style26">2</span><span class="style7"> (the latter two successfully sending back pictures from the planet's surface) revealed other features that may have been caused by running water at some time in the past history of the planet.Mars has two small satellites, Phobos and Deimos, which were mapped by </span><span class="style26">Vikings 1</span><span class="style7"> and </span><span class="style26">2</span><span class="style7"> in 1976. In 1989 Phobos was further studied by the Soviet </span><span class="style26">Phobos 2</span><span class="style7">.</span><span class="style10">The minor planets</span><span class="style7">The minor planets, sometimes known as </span><span class="style26">asteroids</span><span class="style7">, comprise several thousand objects, most of which orbit between Mars and Jupiter (the </span><span class="style26">asteroid belt</span><span class="style7">). The largest of the minor planets is Ceres, which has a diameter of 940 km (584 mi). Once thought to be the residue of a planet broken up by the gravitational pull of Jupiter, most astronomers now think that the minor planets represent a class of primitive objects which were `left over' during the formation of the Solar System, due to Jupiter's disruptive pull.</span></text>
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<text><span class="style10"> view of the surface of Mars</span><span class="style7">, processed by computer from images sent back by the Viking spacecraft and from topographic maps. In the left foreground is the giant Valles Marineris canyon system, which is over 3000 km (1860 mi) long and up to 8 km (5 mi) deep. In the background are the three Tharsis volcanoes. The canyon system was formed by a combination of geological faulting, landslides, and erosion by wind and water.</span></text>
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<text>ΓÇó TIMEΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE OUTER PLANETSΓÇó THE HISTORY OF ASTRONOMYΓÇó SPACE EXPLORATIONΓÇó THE EARTH'S STRUCTURE AND ATMOSPHERE</text>