The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. The time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny satellites are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.

Mars appears bright and almost like a star in the night sky. Because of the movements of Earth and Mars around the sun,

Mars does not have active plate tectonics and is thought to never have them. Because Mars is so much smaller than Earth, it cooled quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of plate tectonics as is seen on Earth. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts.

The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. The Tharsis bulge includes many smaller volcanoes and stress fractures, in addition to the large volcanoes. Its presence affects the weather on Mars and may have changed the climate by changing the rotation of the planet. Valles Marineris is the most notable stress feature associated with the Tharsis bulge. It is a Great Rift Valley extending from the Tharsis region away to the east-southeast. It is about the same length as the distance from New York to California. This canyon system reaches widths of 440 miles and depths of 4 miles.

At certain times of the year like spring and summer when Mars is closest to the sun, great dust storms appear as yellow clouds. The largest of these storms can cover the globe of Mars and last for months. At other times white clouds of water vapor are visible. Scientists now believe that the canals people observed on Mars during the 19th century are actually another optical illusion, caused by the tendency to draw connections between irregular patches in a fuzzy image.

Mars appears to move backward in the sky for a short time. As Mars and Earth orbit the sun, the distance between them varies from about 47 million miles at their closest point to about 233 million miles when the planets are on opposite sides of the sun. This change in distance causes the size of Mars to vary by

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