^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.

^1,2Xunyu Zhang,³Minggang Xie,^4,1,2,5Zhiyong Xiao
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114214]
¹State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China
²CNSA Macau Center for Space Exploration and Science, Macau, PR China
³College of Science, Guilin University of Technology, Guilin 541006, PR China
⁴School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai 519082, PR China
⁵CAS Center for Excellence in Comparative Planetology, Hefei 230026, PR China
Copyright Elsevier

The South Pole-Aitken (SPA) basin is the largest impact structure on the Moon and is believed to have excavated the orthopyroxene (Opx)-rich lower crust and/or upper mantle materials. For the complex craters outside of the SPA transient cavity, the origin of the Opx-rich central peaks is possibly from either the Opx-rich materials of the SPA ejecta deposits or the unexcavated lower crust and/or upper mantle. To estimate the thickness of the Opx-rich materials of the SPA ejecta deposits, this study investigated large complex craters (dozens of kilometers) that have penetrated the Opx-rich materials and exposed deeper mafic-poor crustal material based on spectra extracted from small fresh craters (sub-kilometer scale). The amount of foreign material introduced to these large complex craters by other lunar impact events was estimated to guarantee the least influence on the compositional analysis. The study results suggest that a 56 km-diameter crater at ~640 km northwest of the SPA center is large enough to penetrate the Opx-rich materials of the SPA ejecta deposits, which are thinner than ~4.7 km at this location. This result also indicates that the intense bombardment history of the large craters and basins outside of the SPA transient cavity excavated and redistributed a large amount of crustal material across the basin, possibly resulting in the heterogeneous distribution of mafic-rich and mafic-poor materials on the SPA surface.