Geochemical Cycling in the Subsurface Environment of Enceladus

Hamp, Rachael Elizabeth (2022). Geochemical Cycling in the Subsurface Environment of Enceladus. PhD thesis The Open University.



Enceladus, an icy moon of Saturn, is a potentially habitable environment. Its South Polar Region hosts active plumes that eject material from the subsurface into space. Plume material was analysed by the Cassini spacecraft, which confirmed the presence of a global subsurface ocean, active hydrothermal activity, and the presence of bio-essential elements (carbon, nitrogen, and hydrogen). Data from the plumes provided a snapshot of the subsurface chemical environment, however, could not fully constrain the composition of the silicate interior or specific ocean chemistry.

In this thesis, hypothetical geochemical cycles occurring in the subsurface of Enceladus were constrained. This was achieved through the design of a simulant to represent the silicate interior of Enceladus, based on the chemical composition of a CI carbonaceous chondrite. A modern-day ocean composition was determined by thermochemical modelling using CHIM-XPT, where hypothetical starting compositions for the ocean interact with the silicate interior to produce a modern-day fluid. This fluid is then both cooled and frozen to study changes in the chemical composition as water ascends from the water-rock interface on Enceladus. The fluid composition generated through thermochemical modelling was then critically compared to Cassini’s plume data. Finally, experimental simulations of the water-rock interface were conducted, using the silicate simulant and the modern day ocean composition previously defined through this work.

This study found that a CI chondrite and cometary ice composition are suitable analogues for the silicate interior and icy exterior of a proto-Enceladus. The results from experimental and modelling simulations found the ocean is likely dominated by salts and carbonates, with a pH of 8.5-9.5. Geochemical cycles for carbon, silica and sulfur occurring in a modern Enceladus have been defined and the availability of different chemical species that could be utilised by life have been assessed, which enhanced the prospect of Enceladus as a habitable environment.

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