Mars is the fourth closest planet to the Sun and the seventh largest planet in the solar system. Mars is often referred to as the “Red Planet.” Except for Earth, Mars has the most highly varied terrain of any terrestrial planet, including permanent ice caps at both poles, which are comprised mostly of solid carbon dioxide (dry ice). Mars is easily visible with the unaided eye. Its apparent brightness varies greatly, according to its relative position to Earth. Mars has two small moons, which are named Phobos and Deimos.