¹Lingxi Zhang,^1,2Xiaohui Fu,^1,2Zongcheng Ling,¹Erbin Shi,¹Haijun Cao
Journal Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2023JE008157]
¹Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China
²CAS Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei, China
Published by arrangement with John Wiley & Sons

Akaganeite and jarosite were detected in two mudstone drill samples from Vera Rubin ridge (VRR), Gale crater by Chemistry & Mineralogy X-Ray Diffraction (CheMin). The co-occurrence of these two minerals is quite rare in both terrestrial and Martian aqueous environments. In order to confine the chemical conditions of paragenetic akaganeite and jarosite, and provide insight into late-stage diagenetic alterations at VRR, we synthesized akaganeite and jarosite with varying SO42− concentrations and initial pH levels. Synthetic samples were characterized using Field Emission Scanning Electron Microscopy, X-ray powder diffraction and Raman spectroscopy. Our study reveals that akaganeite and jarosite exist in equilibrium in the solution with 0.011–0.028 M SO42− with respect to 0.6 M Cl− and an initial pH of 1.3–2.2. In combination with the CheMin detection results, the chemistry and pH values of the fluids at VRR can be further constrained. Considering the absence of goethite and the relative higher portion of akaganeite than jarosite in the drill samples, the pH values should be 1.4–2 and the S/Cl molar ratio should be within the range of 0.018–0.042. Based on our laboratory results, we hypothesize that the presence of akaganeite and jarosite at VRR represents an individual episode of acidic groundwater activity. During the late-stage diagenetic process at VRR, upwelled acidic groundwater dissolved the local chlorides to form the Cl−-dominated fluids. Subsequent evaporation further concentrated the acid saline fluids and therefore resulted in an extremely acidic environment (1.4 ≤ pH＜2 with S/Cl molar ratio of 0.018–0.042), which produced akaganeite and jarosite.

^1,2Ery C. Hughes,²Katherine de Kleer,²John Eiler,³Francis Nimmo,⁴Kathleen Mandt,⁵Amy E. Hofmann
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008086]
¹Te Pū Ao, GNS Science, National Isotope Centre and Avalon, Lower Hutt, Aotearoa New Zealand
²Division of Geological and Planetary Science, Caltech, Pasadena, CA, USA
³Earth & Planetary Sciences Department, University of California Santa Cruz, Santa Cruz, CA, USA
⁴NASA Goddard Space Flight Center, Greenbelt, MD, USA
⁵Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

Stable isotope fractionation of sulfur offers a window into Io’s tidal heating history, which is difficult to constrain because Io’s dynamic atmosphere and high resurfacing rates leave it with a young surface. We constructed a numerical model to describe the fluxes in Io’s sulfur cycle using literature constraints on rates and isotopic fractionations of relevant processes. Combining our numerical model with measurements of the 34S/32S ratio in Io’s atmosphere, we constrain the rates for the processes that move sulfur between reservoirs and model the evolution of sulfur isotopes over time. Gravitational stratification of SO2 in the upper atmosphere, leading to a decrease in 34S/32S with increasing altitude, is the main cause of sulfur isotopic fractionation associated with loss to space. Efficient recycling of the atmospheric escape residue into the interior is required to explain the 34S/32S enrichment magnitude measured in the modern atmosphere. We hypothesize this recycling occurs by SO2 surface frost burial and SO2 reaction with crustal rocks, which founder into the mantle and/or mix with mantle-derived magmas as they ascend. Therefore, we predict that magmatic SO2 plumes vented from the mantle to the atmosphere will have lower 34S/32S than the ambient atmosphere, yet are still significantly enriched compared to solar-system average sulfur. Observations of atmospheric variations in 34S/32S with time and/or location could reveal the average mantle melting rate and hence whether the current tidal heating rate is anomalous compared to Io’s long-term average. Our modeling suggests that tides have heated Io for >1.6 Gyr if Io today is representative of past Io.