Mars and Phobos. Illustration of the planet Mars and Phobos, one of its moons, seen from orbit. The enormous Valles Marineris canyon system is visible on the surface of Mars. This is the largest canyon system known in the solar system. The entire canyon system is some 4000 kilometres long, 200 kilometres wide and up to 7 kilometres deep. Phobos is irregular in shape, measuring 19 by 21 by 27 kilometres. It orbits 9400 kilometres above Mars, taking just over 7.6 hours to complete an orbit. Mars, the fourth planet from the Sun, is an arid desert world with a thin atmosphere of carbon dioxide. It has a diameter of around 6800 kilometres.