Mars. Artwork of the planet Mars showing features on its surface. At far left is the volcano Olympus Mons. Standing 27 kilometres high, it is the largest volcano in the Solar System. To its right are the three Tharsis Ridge volcanoes. From top to bottom, these are Arsia Mons, Pavonis Mons and Ascraeus Mons. To the right of these three volcanoes, the huge valley system Valles Marineris is seen running horizontally. The north polar ice cap is seen at upper left. Mars is the fourth planet out from the Sun. It orbits some 228 million kilometres from the Sun, taking just under 687 days to complete one orbit.