Atmospheric oxidation and the creation of modern Mars

Like Earth, Mars was formed about 4.5 billion years ago, but its early surface was very different than today's. Mars' surface then had high rates of meteorite and asteroid impacts from the period known as the Late Heavy Bombardment. But whereas today's Mars is cold, arid, has two icy poles, with an oxidizing atmosphere—where, for example, iron-rich materials develop rust—early Mars was characterized by icy highlands, episodic warmth and a reducing atmosphere.