Fig. 3: Phase diagram of bcc water ices.

Solid-solid phase boundaries in pure ice are indicated by black lines. Corresponding phase boundaries in NaCl-bearing ice (NaCl·RH₂O) ice are shown at 1600 K and 300 K. Grey areas indicate where pure H₂O is superionic, with darker areas corresponds to regions with higher H-diffusion coefficients. The phase boundaries in pure H₂O ice are drawn based on our DFT-MD simulations^19,24 and on both experimental⁶⁴ and theoretical works^18,65 that take into account quantum effects at 300 K. The melting line of pure ice as obtained in our single-phase simulations is shown by the red dashed line. Experimental melting points from Schwager et al.⁶⁶. and Queyroux et al.²⁶. are shown as red circles and red empty squares respectively. Black empty squares correspond to the experimental determination of first-order transition between superionic ice VII″ and ice VII′²⁶, and black dots refer to a solid-solid transition⁶⁶, likely corresponding to the same transition. Blue dashed lines show adiabatic profiles in the H₂O layer of hypothetical ocean exoplanets with habitable pressure-temperature conditions in the surface ocean and no or thin atmosphere (see Supplementary Information). The different profiles are anchored at surface temperatures of 273 K (temperature of crystallization of pure ice Ih), 300 K and 350 K (representative temperature for an optimal growth of thermophile and hyperthermophile organisms⁶⁷) respectively. Circled numbers 1, 2, 3 and 4 indicate approximate pressures at the bottom of the high-pressure ice mantle for 1, 2, 3, and 4 \({M}_{\oplus }\) planets (i.e., ∼1.3, 1.5, 1.7, 1.9 \({R}_{\oplus }\)) with 50 wt% \({{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}\) and Earth-like cores as calculated in Sotin et al. (2007) for a surface temperature of 300 K.