Mars receives its name from the Roman god of war, it is the fourth from the Sun and the seventh in terms of mass. Mars has two small satellites with craters, Phobos and Deimos, which some astronomers consider to be asteroids captured by the planet very early in its history. Phobos measures about 21 km in diameter and Deimos only about 12 kilometers.
At first glance, Mars is a reddish object of varying brightness. When it is closer to Earth (55 million kilometers), it is, after Venus, the brightest object in the night sky. It can be seen more easily when it is in opposition and when it is close to Earth. The concurrence of both circumstances occurs every 15 years, when the planet reaches its closest approach to the Sun.
The surface of Mars has bright reddish regions, due to oxidation or corrosion of its surface. It also has dark areas, formed by rocks similar to terrestrial basalt, whose surface has been eroded and oxidized.
Because of the inclination of its axis and the eccentricity of its orbit, the summers are short and hot and the winters are long and cold. Huge shiny caps, apparently formed by frost or ice, indicate the polar regions. Its seasonal cycle has been followed for almost two centuries. In the Martian autumn bright clouds form on the corresponding pole. A thin layer of carbon dioxide is deposited on the polar cap during autumn and winter.
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