The Habitability of Ocean Worlds: A Chemical Systems Perspective

The icy moons of the outer solar system are some of the most promising locations to search for life beyond Earth. Tidal heating can generate global, liquid water oceans beneath the surface ice of these moons, and heat their rocky interiors to create hydrothermal activity. Hydrothermal reactions may provide a supply of organic compounds - the building blocks of life as we know it - and reduced volatile species to the ocean. When paired with oxidants, which could be created as radiation breaks apart water molecules in the surface ice and in the ocean, these hydrothermal reductants could provide a source of chemical energy to chemotrophic life on ocean worlds. Modeling these types of chemical systems on these moons is key to better understanding whether their oceans could be habitable. The first part of this dissertation examines how these different chemical systems could operate on Saturn's icy moon Enceladus. Applying observational constraints from the Cassini mission, we model the production of radiolytic oxidants and abiotic redox chemistry in the ocean and seafloor, and estimate the amount of chemical energy that could result from this redox budget. From these estimates, we constrain the size of the microbial biosphere that could be sustained in Enceladus' ocean. We then dive deeper into the interior, developing a model of hydrothermal geochemistry on Enceladus, and exploring whether hydrothermal reactions could be the source of the complex organic compounds detected by Cassini. In the last part of the dissertation, we move to Jupiter's icy moon Europa, which will be studied by two flyby missions in the coming years. By considering a broad possible parameter space, we develop a forward modeling approach to predict the influence of fundamental geochemical properties on the composition of hydrothermal fluids on Europa, and demonstrate how these predictions could be used to constrain the Europa system with future observations. Together, this dissertation offers a comprehensive view of how different chemical systems on ocean worlds could create habitable environments, preparing for a new generation of space missions that will search for life beyond Earth.