A Metamorphic Origin for Europa's Ocean

Europa likely contains an iron-rich metal core. For it to have formed, temperatures within Europa reached $\gtrsim$ 1250 K. Going up to that temperature, accreted chondritic minerals - for example, carbonates and phyllosilicates - would partially devolatilize. Here, we compute the amounts and compositions of exsolved volatiles. We find that volatiles released from the interior would have carried solutes, redox-sensitive species, and could have generated a carbonic ocean in excess of Europa's present-day hydrosphere, and potentially an early ${\text{CO}}_2$ atmosphere. No late delivery of cometary water was necessary. Contrasting with prior work, ${\text{CO}}_2$ could be the most abundant solute in the ocean, followed by ${\text{Ca}}^{2+}$ , ${\mathrm{SO}}_4^{2-}$ , and ${\mathrm{HCO}}_3^{-}$ . However, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved S/Cl ratio, such that Cl $>$ S at the shallowest depths, consistent with recently inferred endogenous chlorides at Europa's surface. Gypsum would form a 3-10 km thick sedimentary layer at the seafloor.