Animated model of the internal structure of Mars. Mars is the second smallest and the outermost of the rocky terrestrial planets. It has two polar ice caps, consisting of solid carbon dioxide dry ice frozen out of the atmosphere. Like the other rocky planets it has a crust of rock, in Mars磗 case some 50 kilometres thick, overlying a thick mantle layer dark red composed of silicate minerals. The mantle surrounds the planet磗 core orange, composed of iron, nickel and sulphur. Unlike Earth, Mars has no magnetic field, which on Earth is generated by a dynamo of circulating liquid iron and nickel in the outer core. Mars磗 core is thought to be partially fluid, and indeed the surface rocks show evidence of having been magnetised billions of years ago, but the dynamo has long since stopped. The lack of a protective magnetic field has exposed Mars to the solar wind and radiation, stripping much of its atmosphere. Mars orbits the Sun at an average distance of around 228 million kilometres, some 1.5 times the Earth_Sun distance.