Growth of the helium ocean in Saturn. The calculations come from the results of Saturn’s evolution using the SR2018 phase diagram with T_offset = 350 K (Sect. 3.1.2). Top panel: Helium mass fraction at different ages. The black dashed line corresponds to the protosolar value at Y = 0.27. At t = 2 000 Myr, the helium ocean is already there and starts growing. Bottom panel: temperature-pressure profiles of Saturn’s envelope at these ages. Thick portions of the profiles indicate the helium ocean region. Phase curves from the SR2018 phase diagram with T_offset = 350 K are shown with grey lines for Y = 0.33, 0.27, 0.18, 0.1 and 0.05. Grey-shaded areas correspond to the de-mixing regions.