Accreted volatiles influence low-temperature rock equilibria on Europa

The subsurface ocean of Europa has been identified as a high-priority target in the search for habitable environments beyond Earth. Determining the chemical composition of Europa's ocean, a result of water-rock interaction in Europa's interior, is essential to understanding the moon's geological history and whether it can support life. In this study, we used thermochemical computer modelling code (CHIM-XPT) to explore the range of potential ocean compositions that could result from water-rock interactions at the ocean-silicate interface immediately following the formation of the ocean. We modelled a range of plausible silicate starting materials (based on CV, L and LL chondritic material) and endmember accreted ice compositions (pure H 2 O vs. cometary) that could have contributed to the fluid inventory of Europa. These models assume a water/layer of its current size and do not incorporate compositional changes resulting from further processes (e.g., ice sheet formation) or additional sources of material (e.g., micrometeorite impacts). Our results show that the initial composition of volatiles had a stronger influence on the resulting ocean composition than the initial composition of the silicate, which produced variation in secondary mineral assemblages and differences in ocean chemistry that could be detectable by future missions. In particular, the absence of Fe-oxides in secondary mineral assemblages for cometary melt systems indicates limited Fe-oxidation , which could impact the generation of H 2 . We also show that, under all modelled scenarios, water-rock interactions would release redox elements into the ocean, albeit at varying concentrations, supporting proposals that Europa's ocean could be a habitable environment. Significantly, the ocean composition derived from water-rock reactions involving initial fluids with a cometary melt composition had relatively higher concentrations of Mg (by one to five orders of magnitude) and lower concentrations of Fe (by more than two orders of magnitude, except at W/Rs of 3–10) compared to those involving pure H 2 O initial fluids. This difference in Fe and Mg concentration found between the pure H 2 O and cometary melt systems could potentially be used as a diagnostic tool to identify the source material for Europa's volatile inventory. • We modelled potential ocean compositions for Europa using a range of plausible endmember starting compositions for the silicate mantle and initial fluid. • The initial fluid composition had a stronger effect on ocean composition than the composition of the silicate mantle. • The initial fluid composition led to distinct ocean compositions that future missions to Europa could detect.