Mars Computer artwork of the planet Mars left partially illuminated by the Sun upper right Mars is the fourth planet from the Sun the next one out after Earth around million kilometres further out at around million kilometres from the Sun It is around half the diameter of Earth being about kilometres across Mars is one of the rocky planets of the inner solar system Its surface terrain is mostly cratered though there are rift valleys and volcanic mountains It has a thin atmosphere of mostly carbon dioxide Frozen water exists beneath the ice caps of frozen carbon dioxide in the polar regions but Mars distance from the Sun makes it too cold for liquid water