Anoxic chemical weathering under a reducing greenhouse on early Mars

Reduced greenhouse gases such as methane (CH4) and hydrogen (H2) might be the only tenable solution to explain warming of the ancient Martian climate, but direct geological evidence that a reduced atmosphere actually existed on Mars has been lacking. Here we report widespread, strong Fe loss in chemically weathered bedrock sections in the Mawrth Vallis region and other 3–4-billion-year-old terrains on Mars. The separation of Fe from Al in Martian palaeosols, which is comparable to trends observed in palaeosols before the Great Oxidation Event on Earth, suggests that the ancient Martian surface was chemically weathered under a reducing greenhouse atmosphere. Although for different reasons than on Earth, Mars underwent an oxidation event of its own in the late Noachian that forever changed the geological path of the planet. A comparative analysis of weathered bedrock in the Mawrth Vallis region of Mars and on Hainan Island, China, provides geological evidence for a reduced greenhouse atmosphere on early Mars, as there was on early Earth.