Finally, it would appear inescapable that most of the water on Earth arrived as part of the accretion process and the presence of that water influenced primary differentiation. Accretion of hydrous materials and subsequent outgassing led to the formation of a dense ‘steam’ atmosphere, which in turn served as a thermal blanket allowing the long-term persistence of a terrestrial magma ocean. The higher pressure of water in the atmosphere would have allowed the presence of liquid water at the Earth’s surface, and a significant amount of water would have been able to dissolve in the silicate liquid of the magma ocean. Cooling and crystallisation of the magma ocean would have provided a further source of water for the atmosphere and hydrosphere, and a mantle reservoir of stable hydrous phases.