Figure 1.

Schematic of the thermodynamic + extraction + structure model to simulate the build‐up of Europa's ocean from exsolved volatiles. After each heating step before differentiation, Gibbs energy minimization is carried out, resulting in an equilibrium assemblage in each cell (left figure). A portion of the fluid phase(s) is then extracted according to a specified rule (see Section 2.2), joins the ocean reservoir, and no longer affects the chemistry of the deep interior. Fe $\pm$ S is extracted from the bulk composition from the deep interior once the interior reaches the Fe‐FeS eutectic temperature (Section 2.3). Finally, Europa's structure is resolved (Section 2.4), here assuming a 30 km ice shell, requiring a temperature of 270.8 K at the ice‐ocean interface.