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Fourth planet from the sun. Mars is about 141 million miles (about 228 million kilometers) from the sun and revolves around the sun in 687 days. Mars is smaller than Earth, about 4,221 miles (about 6,796 kilometers) in diameter. A Martian day is only about 40 minutes longer than our own. The atmosphere of Mars is very thin and made up mostly of carbon dioxide and nitrogen with only traces of oxygen. No life has been found on Mars, though the possibility of some kind of life on the planet has not been ruled out.The temperatures on Mars range from -125 degrees Celsius in the Martian winter to 22 degrees Celsius in the Martian summer. Mars has polar ice caps which shrink in size during the Martian spring and summer. From data gathered by the Viking 1 and 2 probes, we know that the Martian surface is covered by rocks and a soil rich in an iron-laden clay which gives Mars a reddish-orange appearance. The surface of Mars has not only been affected by meteorite impacts, but also by volcanic and earthquakes. In fact, Mars has some of the largest volcanoes in the solar system; Olympus Mons is over 600 kilometers wide and 26 kilometers high! Valles Marineris, a gigantic canyon on Mars, is over 8 kilometers deep and 4500 kilometers long. Mars has two tiny moons, Deimos and Phobos. They are highly irregular in shape and are thought to be asteroids captured by Mars' gravitational pull.