The second smallest planet in the solar system,Mars is half the size of Earth . it's somethings called the red planet because of its rusty colouring.A vast canyon called Valles Mariner is stretches a quarter of the way around this frozen desert world.It formed long ago when the crust of the young planet split open. Elsewhere are dusty plains strewn with boulders and giants, extinct volcanoes, including Olympus Mons - the Solar system's largest volcano.