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

Abstract 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