Evolution of the EUV flux (top panel, a), escape rate (b), envelope mass (c), and planet radius (d) for a planet with M_p = 2.5M_⊕, an initial envelope mass fraction of 1%, and different initial values of H₂O/H₂ on an orbit with a main-sequence stellar irradiance equal to the terrestrial value. The solid (dashed) lines represent results without (with) mass fractionation between H and O. The mass fractionation results are omitted from panel b for clarity.