A high pH ocean on Enceladus as evidence of serpentinization: Setting the stage for life?

Abstract:

The Cassini mission has revealed Enceladus to be one of the most dynamic bodies in the solar system, with a cryovolcanic plume that emanates from a subsurface ocean of liquid water [1,2]. The chemical composition of solids and gases in the plume have been determined by the Cassini CDA [2] and INMS instruments [3], respectively. We have shown that measurements of carbonate salts in the plume particles and carbon dioxide in the plume gas can be coupled using a geochemical model of carbonate speciation in Enceladus’ ocean to estimate the pH of the ocean [4]. This leads to a self-consistent pH of 11-12, which is similar to those observed (9-12.5) in low-temperature serpentinization environments on Earth, such as the Lost City hydrothermal field. A high pH is also consistent with theoretical simulations of water-rock interactions that lead to the formation of serpentine [5]. Therefore, the high pH implies serpentinization. This is astrobiologically significant because serpentinization also produces hydrogen gas [4], which allows abiotic organic synthesis that could lead to an origin of life. Moreover, hydrogen could serve as an energy source to support possible microbial life, such as methanogens [6]. These possibilities that arise from the geochemistry of serpentinization are interesting given that Enceladus’ plume contains organic molecules, methane, and possibly hydrogen [3]. The emerging picture of Enceladus’ ocean is a habitable serpentinization environment, although questions remain concerning the timing of serpentinization, and the sources of methane and organic molecules in the plume. [1] Porco et al. (2006), Science 311, 1393. [2] Postberg et al. (2009), Nature 459, 1098. [3] Waite et al. (2009), Nature 460, 487. [4] Glein et al. (2015), GCA 162, 202. [5] Zolotov (2007), GRL 34, L23203. [6] McKay et al. (2008), Astrobiology 8, 909.