Tharsis quadrangle Mount Everest and about as big in area as all of Europe. Tharsis contains a group of large volcanoes. Olympus Mons is the tallest. Tharsis is a land of great volcanoes. Olympus Mons is the tallest known volcano in the Solar System; it is 100 times larger than any volcano on Earth. Ascraeus Mons and Pavonis Mons are at least 200 miles across and are over six miles above the plateau that they sit on—and, the plateau is three to four miles above the zero altitude of Mars. Pavonis Mons, the middle in a line of three volcanoes, sits at
Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in