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 [3]. However, the lack of N₂ in the plumes [4] may question the presence of high-temperature conditions, because formation of N₂ from NH₃ is expected from equilibrium calculations [5]. 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.

 [1] Postberg et al., Nature 459, 1098 (2009). [2] Iess et al., Science 344, 78 (2014) [3] Hsu et al., DPS meeting 45, #416 (2013); Hsu et al., Workshop on the Habitability of Icy Worlds, 4042 (2014). [4] Hansen et al., Geophs. Res. Lett. 38, L11202 (2011). [5] Matson et al., Icarus 187, 569 (2007).