Ongoing hydrothermal activity in the chondritic core of Enceladus inferred from nano-silica particles and laboratory experiments
Abstract:Enceladus is considered to possess a subsurface ocean interacting with the rock components [1,2]; yet, the chemical conditions of the interactions are poorly constrained. Cassini’s discovery of nano-silica particles derived from Enceladus implies the presence of high-temperature water–rock interactions . However, the lack of N2 in the plumes  may question the presence of high-temperature conditions, because formation of N2 from NH3 is expected from equilibrium calculations . Here, we report results from hydrothermal experiments to further constrain the conditions of water–rock interactions. To sustain the formation of nano-silica in Enceladus, the composition of the rocky core would need to be similar to that of carbonaceous chondrites, rather than highly-differentiated ultramafic rocks. We suggest that a change in pH of fluids upon mixing with seawater requires high reaction temperatures (≥~150°C) to sustain formation of nano-silica in Enceladus. Nano-silica particles in the ocean would readily dissolve if high-temperature reactions ceased, supporting the occurrence of present-day hydrothermal activity. Our results also show that formation of N2 from NH3 is kinetically inhibited even under high-temperature conditions. Under the conditions required for water–rock interactions to explain the observations, oxidization of ferrous iron and H2 production would proceed efficiently, which may provide a habitable environment for chemoautotrophic life on Enceladus.
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