Schematic illustration of the internal structure of a rocky planet as assumed in this paper, with an atmosphere containing a mixture of captured hydrogen and gaseous species derived from a molten mantle that forms a global magma ocean below the planet atmosphere. In the centre of the planet, an iron/nickel core is present. The composition of the magma ocean is assumed to be fixed, while the composition of the atmosphere is at thermodynamic equilibrium with that of the magma ocean.