Mars is a medium-sized Terrestrial Planet being one-<1> the size of the Earth. Mars has captured the interest of astronomers and the public like no other planet since it is the only one where we can observe changes in its surface. These changes include variations in coloration due to dust storms and seasonal changes in the <2>. Mars also has some very impressive geologic features including the 4000 km long canyon Valles Marineris and the largest volcano in the solar system <3>. This volcano is much larger than those on the Earth since Mars doesn't have <4> and the volcanoes stay in the same place. Mars has been visited by the <5> probes in 1976 and the Pathfinder probe in 1996.
Because of its smaller size, Mars cooled more rapidly than the Earth. Although there is substantial evidence of volcanic activity on the surface of Mars in the past, the amount has certainly decreased and it is unknown whether there will be any in the future.
The small size of Mars has also allowed most of the atmosphere to escape. All that is left is a very thin atmosphere of mostly <6>. The current low level of atmospheric pressure is insufficient for liquid water to exist on the surface. However, there is substantial evidence for the existence of liquid water on the surface of Mars in the past suggesting that long ago the atmospheric pressure was <7>.
Mars has two small irregularly-shaped moons that are most likely captured <8>.
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