Solar System -
Mars is the next planet from the Sun after the Earth, so is the fourth planet. It takes 687 days to go around the sun, and a Martian day is 24.6 hours. Mars' orbit is much more eccentric than Earth's, varying between 206,620,000 km and 249,230,000 km. Mars is close to spherical, with a mean diameter of about 6,792 km; just over half that of Earth, but being much less dense, it weighs only about 10% of Earth's mass. Even when observed with the naked eye, its red color is obvious, and is due to the fine particles of iron oxide dust on its surface. It has a very thin atmosphere which varies in atmospheric pressure as the carbon dioxide alternately freezes and thaws. The highest it reaches is about 10.8 millibars; just over 1% of the Earth's pressure. The main constituent gases in its atmosphere are carbon dioxide 95.3%, nitrogen 2.7%, argon 1.6% (with traces of other noble gases), oxygen 0.13%, carbon monoxide 0.07% and water vapor 0.03%. Temperatures range from a bracing -140ºC (-250ºF) up to a balmy 20ºC (68ºF), but it is very rare that the temperature rises above freezing.
Mars is a rocky, terrestrial style planet with similar impact craters to the Moon and old volcanoes, valleys, and other geological features similar to Earth. Mars is not geologically active. Olympus Mons, an old, extinct shield volcano, is the highest known mountain on any planet. It is about 27 kilometers high, and covers an area about the size of the state of Arizona in the US. The Borealis basin, is a large basin covering 40% of the planet's surface. It was probably formed by a collision with an object between 1,600 - 2,700 km across at a velocity of about 8 km/second. The crater is elliptical and about 10,500 km by 8,500 km; the largest crater yet found in the Solar System. Finally, Valles Marineris is a canyon that dwarfs the Grand Canyon, being 4,000 km long, and nearly 7 km deep, compared to the Grand Canyon at about 450 km long and nearly 2 km deep.
Mars has two small, irregular moons, Phobos and Deimos, discovered in 1877, which are probably captured asteroids. In 1878, Henry Madan in England suggested calling Mars' moons Phobos & Deimos. His great neice, Venetia Burney, was the person who suggested the name Pluto in 1930 for the then newly discovered planet! Phobos largest dimension is 27 km (16.75 miles), and many of its craters are named for characters in Gulliver's Travels. Diemos largest dimension is 15 km (9.4 miles), and two craters are named for Jonathan Swift, author of Gulliver's Travels, and Voltaire.
Until Mariner 4 in 1965, many thought that there might be liquid water on the planet's surface, and, indeed, there are features that look like shorelines, gorges, riverbeds and islands. Mars had a much denser atmosphere in the past that may have permitted liquid water to exist on the surface. It is long gone, however, and the Martian "canals" so beloved of Percival Lowell are consigned to myth. There are large quantities of ice below the surface near the south pole.