Modeling serpentinization: Applied to the early evolution of Enceladus and Mimas

Abstract

Serpentinization is an exothermic reaction which involves the metamorphic transition of ultramafic rocks, and liquid water, into hydrated serpentine minerals. It has been suggested that serpentinization may be a fundamental process in the development of icy planetary objects, prompting a reaction runaway, in which heat from serpentinization in a localized region of the body could result, in principle, in the melting of ice throughout the body. We use a 1-dimensional adaptive-grid thermal evolution code suited for small and medium sized icy bodies of the Solar System, which we apply to Enceladus and Mimas, the satellites of Saturn. The code is used to investigate the multiphase flow of water through a porous rocky medium, thus giving us a detailed look on the internal distribution of mass and energy. We consider heating by serpentinization reactions, accounting for the phase transition of the chemically altered rock and the water lost in the process; short-lived and long-lived radionuclides; gravitational potential energy; and the introduction of ammonia, to modify the thermodynamical properties of water–ammonia solution. We test several cases with different initial compositions. Our results show, that in a short term radioactive heating scenario, Enceladus will fully differentiate into a rocky inner core, topped by a very thin icy crust. Considering only long-term radioactive heating, in the absence of any short-term radionuclides, serpentinization in Enceladus can be triggered, providing ammonia is present in the water. However, long-term triggered serpentinization is confined only to the inner core. We conclude that serpentinization may have been triggered in Enceladus during its early evolution, and not in Mimas, leading to the dichotomy between them. Based on the assumptions that went into our model, we may conclude that the formation time of Saturn’s satellites was around 3Myr after the formation of Calcium–Aluminum inclusions.