The role of hydrothermal sulfate reduction in the sulfur cycles within Europa: Laboratory experiments on sulfate reduction at 100 MPa

1,2Shuya Tan,1,3Yasuhito Sekine,4Takazo Shibuya,2Chihiro Miyamoto,2Yoshio Takahashi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114222]
1Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan
2Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
3Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa, Japan
4Super-cutting-edge Grand and Advanced Research (SUGAR) Program, Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
Copyright Elsevier

There are several lines of evidence for the subsurface ocean within Europa; however, its oceanic chemistry and geochemical cycles are largely unknown. The recent observations by large telescopes show that exogenic sulfur ions and SO2 are implanted from Io and accumulate as sulfuric acids in Europa’s trailing hemisphere. This suggests that a large amount of sulfate could have been supplied into the ocean over geological timescales. The telescope observations also suggest that chloride salts appear on chaotic terrains of Europa, suggesting that the primary oceanic anion may be chloride despite a supply of sulfate into the ocean. These observations imply the presence of possible sinks of exogenic sulfate within the ocean. Here, we report the results of laboratory experiments on hydrothermal sulfate reduction under the pressure conditions that correspond to Europa’s seafloor. Using a Dickson-type experimental system, we obtain the reaction rate of sulfate reduction at a pressure of 100 MPa and temperature of 280 °C for various pH levels (pH 2–7). We find strong pH dependence and little pressure dependence of the reaction rate. Sulfate reduction proceeds effectively at fluid pH < 6, whereas it is kinetically inhibited at fluid pH ~7. These results suggest that, if hydrothermal fluid pH is <6, hydrothermal sulfate reduction within Europa can be a sink of exogenic sulfate within the ocean in addition to precipitation of sulfate salts. Such acidic fluid pH may be achieved if hydrothermal activity is hosted by basaltic rocks. We suggest the importance of the thermal evolution of the rocky interior for both the ocean chemistry and sulfur cycles of Europa.

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