Mars, artwork. Mars is the fourth planet from the Sun, and it is a rocky desert world. It orbits the Sun in around 689 days at an average distance of around 227 million kilometres. Its diameter is around half Earth's, and the surface area is about the same as Earth's land surface area. It has a thin carbon dioxide atmosphere, which forms ice (not seen) at the poles. The orange colour is due to the iron oxides in the rocks.