The Carbonate Geochemistry of Enceladus' Ocean

Abstract

The plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and salt‐rich ice grains. It is found that self‐consistency between the data sets can be achieved with a derived range of 10−4.6 to 10−3.2 for the activity of CO2 in Enceladus' ocean. This range is compatible with long‐term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO–FeO–SiO2–CO2–H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2‐rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability.

In Situ Formation of Monohydrocalcite in Alkaline Saline Lakes of the Valley of Gobi Lakes: Prediction for Mg, Ca, and Total Dissolved Carbonate Concentrations in Enceladus’ Ocean and Alkaline-Carbonate Ocean Worlds

“…The multiple ocean worlds in the outer solar system are icy bodies with subsurface liquid oceans beneath their icy crusts [1,2]. The water chemistries of subsurface oceans on icy bodies that formed from ultramafic (e.g., chondritic) rocks beyond the CO 2 snowline [3] should be saline, carbonate-rich, and have high pH [4][5][6][7][8], suggesting Na ± , Cl − , CO 3 2− , and HCO 3 − as major dissolved species. Such icy bodies include Saturn's moon Enceladus in the present day, icy dwarf planets (e.g., Ceres and Pluto), and the parent bodies of D-type asteroids in the past [1,2].…”

Returning Samples From Enceladus for Life Detection

“…These experiments constrain hydrothermal fluid temperatures to above 50 • C and pH up to 10.5, provided that silica precipitates upon mixing into the cold (≈0 • C) ocean with a pH between 8.5 and 10.5 (Hsu et al, 2015;Sekine et al, 2015). Modeling of the plume composition suggests that Enceladus' ocean pH is in this range (Glein et al, 2018;Glein and Waite, 2020). This supports the potential hydrothermal formation pathway for silica nanoparticles.…”

“…This is slightly lower than Earth's oceans. The pH has been inferred to be in the broad range 8-12 (Glein et al, 2015;Sekine et al, 2015), with the current best estimate at 8.5-9 (Glein and Waite, 2020). Except for the pH 12 endmember, the temperatures, pressures, salinity, and pH inferred for Enceladus' ocean are in ranges that not only do not come close to pushing the limits of life as we know it (e.g., Takai et al, 2008;Inagaki et al, 2015), but also closely match those of known ecosystems on Earth (Kelley et al, 2005;Imachi et al, 2020).…”

“…On geological timescales, H 2 and CO 2 should equilibrate with H 2 O and CH 4 to reach concentrations different from those measured (McCollom, 2016;Waite et al, 2017). This may suggest that the plume H 2 and CO 2 originate in distinct locations of water-rock interaction in a core of heterogeneous composition (Glein and Waite, 2020). It is unclear whether this points to a geologically young ocean or instead to incomplete chemical homogenization of the core due to limited pervasiveness of fluid circulation (Macdonald and Fyfe, 1985;Cable et al, 2020).…”

“…• Bubble scrubbing in which bubbles of exsolved gases collect overlying organic material as they rise, selectively enriching the plume with compound classes that better bind to the bubble's surface (Porco et al, 2017); • Interactions with ascent conduits such as condensation on cold walls or at bottlenecks where pressure is increased (Khawaja et al, 2019); • Formation of a buoyant film of hydrophobic organic compounds at the top of the water table (Postberg et al, 2018a); • Nucleation of grains around ejected salt and organic particles (Khawaja et al, 2019); • Flash freezing of ocean water droplets whose outer layers vaporize into vacuum, cooling the rest (e.g., Glein and Waite, 2020).…”