Mass of water in the surface reservoir of Mars (condensed and vapour) as a function of the oxidation state of the mantle relative to the iron–wüstite buffer (IW), with higher values of ΔIW indicating increasing partial pressure of oxygen in the atmosphere. The top line marks the modern Earth ocean mass, while the bottom line marks a 500 metre water layer on Mars. For the nominal value of ΔIW – 2, Mars loses its entire surface and atmosphere water reservoir, but the water amount increases rapidly from IW-1 and upwards.