On the chemical evolution of Europa’s hydrosphere during the accretion phase

Europa, one of Jupiter's Galilean moons and a primary target of the upcoming JUICE and Europa Clipper missions, is one of the most promising candidates for habitability in the Solar System, largely due to the presence of a subsurface ocean. However, Europa's origin and evolution remain poorly constrained, particularly with respect to the volatile inventory of its hydrosphere. A key aspect of Europa-shared with Saturn's moon Enceladus-is the potential for water-rock interactions between the subsurface ocean and the underlying rocky mantle. These interactions likely influence both the current chemical composition and physical properties of the ocean and are thought to have shaped its long-term evolution. In addition, the composition of the primordial atmosphere in contact with the ocean shortly after accretion is also expected to have significantly influenced the chemistry and pH of the ocean.

This study models the chemical evolution of Europa's hydrosphere during accretion, assuming that it was formed by the delivery of ice-rich planetesimals and solids. As Europa accumulates mass through accretion of surrounding material and bombardment by impactors, both its size and the volume of its hydrosphere increase. The surface temperature is calculated from a combination of impact heating and thermal input from the circumjovian disk. Rocky and icy impactors striking the liquid surface result in rock precipitation into the ocean, which is treated as a Stokes flow. At each time step, we compute the composition of the primordial atmosphere and the evolving ocean, taking into account water-rock interactions with volatile-rich water. We also consider the ongoing interactions between the ocean and the underlying rocky mantle, which progressively incorporates precipitated material and changes its water content over time. Chemical equilibria associated with water-rock interactions are calculated using PHREEQC, while atmosphere-ocean exchange is modeled following the framework of Amsler Moulanier et al. (2025)

Our results provide a comprehensive view of the evolving chemical composition of Europa's ocean throughout the accretion phase. In particular, we highlight the key role of water-rock interactions and the composition of the primordial atmosphere in controlling the chemical evolution of the ocean, including fundamental properties such as pH and salinity.