Mars, optical image. Surface features including the south polar ice cap can be seen. Mars is the fourth planet from the Sun and is a rocky desert world. Its diameter is around half Earth's. It has a thin carbon dioxide atmosphere, which forms ice at the poles. The orange colour is due to the iron oxides in the rocks. Imaged from Chilescope Observatory, Cerro Pachon, Atacama, Chile, on 17th September 2018.