Warming early Mars with CO₂ and H₂

The presence of valleys on ancient terrains of Mars suggests that liquid water flowed on the martian surface 3.8 Gyr ago or before. The above-freezing temperatures required to explain valley formation could have been transient, in response to the frequent large meteorite impacts on early Mars, or they could have been caused by long-lived greenhouse warming. Climate models that consider only the greenhouse gases carbon dioxide and water have been unable to recreate warm surface conditions, given the lower solar luminosity at that time. Here we use a one-dimensional climate model to demonstrate that an atmosphere containing 1.3-4 bar of CO₂ and water, in addition to 5-20% H₂, could have raised the mean surface temperature of early Mars above the freezing point of water. Vigorous volcanic outgassing from a highly reduced early martian mantle is expected to provide sufficient atmospheric H₂ and CO ₂-the latter from the photochemical oxidation of outgassed CH ₄ and CO-to form a CO₂ and H₂ greenhouse. Such a dense early martian atmosphere is consistent with independent estimates of surface pressure based on cratering data.