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Mars
Mars
Mars (diameter 6,800km) is the fourth planet from the Sun. About a tenth of the mass of the Earth, Mars has a thin atmosphere rich in carbon dioxide. It is 95% carbon dioxide, nearly 3% nitrogen, and nearly 2% argon. Free oxygen makes up only 0.1%. The dry, volcanic rocky surface is made
of basalt, the same rock that
covers much of the Earth. When it weathers, the iron in it turns rusty red. Dust storms are common and may cover the entire planet for weeks. This makes the planet look red when seen in the night sky.
Mars has two small moons, Phobos and Deimos. Mars spins
on its axis at about the same
rate as the Earth and orbits
240 million km from the Sun.
Mars appears to have strange features on its surface called canals, which led people in the
past to wonder if there had ever been life on the planet. Some “canals” are, in fact, rift valleys, signs of the way the surface formed (see: Valles Marineris). Others may well be dried up canyons that were cut by rivers several hundred million years ago. But they are mainly optical illusions. Only ice now exists on Mars.
Crust
Iron core
Rocky mantle
 Mars—The structure of Mars. A thin crust surrounds a thick mantle, which, in turn, encloses a small core.
 Mars—A piece of Mars that landed in Antarctica as a meteorite. It gave vital clues to the nature of the red planet.
Mars is a geologically active world. Volcanoes grow to
spectacular heights on Mars: The largest yet spotted—Olympus
Mons—is 24km high and 500km across its base, three times the height and four times the breadth of the largest volcano on Earth. Mars has
polar ice caps like Earth, but on Mars they grow and shrink
dramatically each year as the seasons change, mainly due to sheets of frozen carbon dioxide.
Mars has no magnetic field. (See also: Mariner.)
 Mars—The Martian surface as seen from Viking Lander 2. The surface is mostly dust or sand with some large rocks. It looks not unlike a desert on Earth.
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