Mass of water in the surface (condensed and vapour), mantle, and core reservoirs after 100 Myr of evolution for Earth (filled circles) and Mars (asterisks) as a function of the partition coefficient of water between metal melt and silicate melt. The measurement is given for H₂ + O to reflect the H₂O equivalent mass of the dissolved H in the core and the mantle. The best match to the surface water reservoir of Earth is found for high values of D_H = 5 and D_H = 8. Mars, on the other hand, loses all its surface water unless D_H = 3 or less. The core reservoir mass falls with decreasing D_H, particularly for Mars that has a smaller core than Earth. This hydrogen was outgassed as H₂ from the martian mantle and subsequently lost to hydrodynamical escape. The mantles of Earth and Mars store significant H₂ reservoirs that can later be outgassed as H₂ and H₂O in volcanism and after impacts. The symbols are displaced slightly horizontally to better distinguish them.