The Role of Minerals in the (Bio)-Geochemical Cycles of the Ocean Worlds of the Solar System

Abstract

The missions of exploration to the outer Solar System launched in the late 20th century revealed that the Solar System harbours multiple ocean worlds. Water, an essential solvent for life, might exist in liquid phase beneath the ice crusts of these planetary bodies, sustained by primordial heat and the energy dissipated from gravitational interactions, both from the parental giant planet they orbit and from resonance with nearby moons. The effect of this heat would be further enhanced by the presence of dissolved salts and antifreeze compounds that would prevent the complete freezing of the subsurface oceans. In addition, layers of insulating minerals in the crust and/or ocean, such as clathrate hydrates, would help to retain the heat, preventing its loss to space and creating favourable conditions for habitability. The key evidence supporting the existence of these subsurface oceans in the outer Solar System has been provided by several discoveries from NASA-led space missions. The Galileo spacecraft detected a self-induced magnetic signal on Jupiter’s moon Europa, suggesting the presence of a salty ocean interacting with the giant planet’s magnetic field. The Cassini spacecraft identified kilometer-high plumes of water vapour containing hydrogen, carbon dioxide, methane, and other long-chain hydrocarbons, along with sodium salts such as chlorides, carbonates, and phosphates, emanating from the southern pole of Saturn’s moon Enceladus. In addition, infrared and ultraviolet spectrometers on-board the Galileo and Dawn spacecrafts detected salts on Europa and the dwarf planet Ceres. On Europa, sulphates, along with halite and carbonates, were primarily identified, while on Ceres, calcium-magnesium, sodium, and ammonium carbonates, as well as sodium and ammonium chlorides, were found. These deposits were associated with tectonic, impact, or cryovolcanic structures, suggesting an endogenous origin, having been transferred to the surface from the ocean or from subsurface reservoirs within the crust. Phyllosilicate, minerals formed by aqueous alteration and which constitute a significant part of the composition of Ceres’ water-ice crust, along with its core of hydrated silicates, also support this hypothesis. These large-scale hydrothermal water-rock interactions can only be explained by the existence of a global subsurface ocean. These findings have expanded the concept of the habitable zone, previously restricted to inner solar system planets with water on the planetary surface, to now include the possibility of deep habitats beneath ice crusts.