LPSC_final

LPSC_final

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  GEOCHEMICAL CONSTRAINTS ONEUROPA’S OCEAN COMPOSITION AND POSSIBLE SIGNATURES OF HYDROTHERMAL ACTIVITY. W. G. Levine1 , M.A. Leitner2 , S. D. Vance1 , 1 Jet Propul- sion Laboratory, Caltech, Pasadena, CA 91109, wLevine@caltech.edu, 2 Department of Geological Sciences, Cornell University, Ithaca, NY 14853. Introduction: We assess Europa’s ocean compo- sition, considering both its present density and estimat- ed sources and sinks for water. Precipitated minerals and ion concentrations in resulting fluids from differ- ent bulk composition rocks provide a check on previ- ously published work, and an agnostic starting point for more detailed investigations of Europa’s composi- tion, chemical evolution, and present activity in prepa- ration for future exploration missions. Candidate Materials: CI and CV chondrites are among the most primordial materials in the solar sys- tem, and are often assumed as the starting seafloor material for models of Europa’s ocean chemistry [1-4]. In contrast, studies accounting for Europa’s moment of inertia conclude that an ordinary chondrite is a better fit for the accreting material [5]. Geochemical Inputs: We determined model accre- tion mineralogies and elemental ratios from the major compositions of seven candidate materials (CI, CM, CV, H, L, and LL chondrites, and P-type asteroids) [6]. Lists of 10-14 commonly-found minerals were com- piled for each rock type. We imposed constraints on major elemental ratios of Mg/Si, Ca/Si, Mg/Ca, Fe/Mg, and Fe/S using Excel Solver’s minimization software. The P-type chondrite was modeled as 31% CI, 49% CM, and 20% Tagish Lake chondrite [7]. Ocean Compositions: To assess pathways for Europa’s bulk geochemical evolution, we applied the same bulk silicate Earth model for ocean composition [3] to each model bulk rock material for Europa. First, the bulk silicate earth model used assumes a differentiated Europa and is comprised of (BSE mass), in descending order, the ocean (100km), upper conti- nential crust, lower continental crust, and oceanic crust. Extraction parameters are applied based on Earth’s major geochemical reservoirs to determine the mineralogy that can react with hydrothermal fluid [3]. We consider potential mantle and ocean thicknesses and their candiate accretionary materials to compare with corresponding planetary moments of inertia [5]. Finally, water flux estimates for Europan history, de- termined from radiolysis rates [8], cometary impact rates, and Europa’s currently estimated ocean thickness [3] were used to put more geophysical constraints on starting materials. Given these constraints, we suggest that P-type asteroids provide a good match to available geochemical and geophysical constraints. Hydrothermal Alteration: We model the altera- tion of our mineralogical suites using Geochemist’s Workbench (GWB) [10]. For all models, the water- rock ratio (W/R) was set to 1, temperature was 275o C, and the initial pH was 4.2 [11], consistent with a mod- ern moderately oxidized ocean [12] . Results: Europan ocean compositions were com- puted from a bulk silicate earth model with an Fe core. Figure 2 shows that the compositions are within rough- ly 10% of previous estimates [3] for CI, CV, and H. Thus, we extend the method to L, LL, and P type chondrites (also pictured). Carbonaceous chondrite models of Europa have higher final pH than ordinary chondrite models, but all values are acidic. pH increases with the amount of aqueous alteration in the rock’s history. Precipitated materials were grouped by major min- eralogical class. Figure 3 shows silicate production, divided into serpentines and other silicates. Serpen- tines, mostly antigorite, were found only in rocks with higher iron content (C-, H, and P). For ordinary sili- cates, talc production increases sharply once rock Figure 1. Candidate Rock Mod- els. Basic mineralogy and the dif- ferences from theoret- ical values of elemental ratios for all seven used initial rocks in Geochem- ist’s Work- bench simu- lations. Figure 2. Elemental Abundances in Europan Ocean from Total BSE Extractions. Values, in g kg-1 of water on a logarithmic scale. The numbers 1-8 correspond to Cl, Na, Mg, S, Ca, K, Br, and B, respectively.
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  models reach 20%Fe by weight, in line with previous studies [11]. The models precipitated similar amounts of total silicates. Figure 4 shows metal oxides, sulfides, and car- bonates. Sulfides precipitated the most from C-type rocks as pyrite or troilite. By contrast, oxides precipi- tated the most from ordinary chondrites, typically as magnetite and FeO. Carbonates, most commonly cal- cite, only precipitated from carbonaceous, LL, and P models. Ordinary chondrites have elevated concentra- tions of Ca2+ and CaCl+ in the product fluids, as shown in Figure 5. If Europa formed from ordinary chondrite material, its present-day ocean could have higher lev- els of calcium ions by a factor of 5. Determining such relatively small differences requires a tight handle on ocean pH [12]. Conclusions: In places such as Earth’s Lost City vent field, life catalyzes reactions between coexisting oxidized and reduced species. At Europa, carbona- ceous chondrite bulk materials, by precipitating sul- fides and dark-colored metallic oxides, may create hydrothermal chemistry resembling Earth’s black smoker vents, whereas ordinary chondrite models, by precipitating more talc, create systems more analogous to Earth’s white smokers. Species produced at hydrothermal systems can be transported to the surface of Europa by ocean currents and surface eruptions. Decreasing carbonate precipita- tion from carbonacoues to ordinary chondrites creates differences in global calcium ion levels among the starting materials. Future missions to Europa may de- tect this and other minerals and could give insight about Europan accreting material. Future Work: We can analyze alternative scenari- os for Europa’s hydrothermal alteration by varying W/R, pH, and temperature. For example, an early ocean on Europa would probably have been more re- ducing, with an alkaline pH as high as 12 [12]. The amount of available rock for reaction would have var- ied through time as the seafloor cooled [13]. We can also consider Europa in the context of the other Galilean satellites, which have related, but differ- ent geological histories. Such comparisons will be val- uable for investigations of water loss and other pro- cesses, which may be conducted by future missions such as JUICE and NASA’s Europa Mission. Another major question not adequately considered in this or other models of Europa’s ocean chemistry is the degree of alteration in Europa’s mantle and sea- floor crust, by analogy to mantle convection, continen- tal processing, hotspots on Earth, which can be inves- tigated using increasingly sophisticated thermodynam- ically consistent models [14] Acknowledgments: This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Govern- ment sponsorship is acknowledged. This work was supported by the NASA Astrobiology Institute Icy Worlds Project References:[1] Kargel, J. S. (1991) Icarus, 94, 368–390. [2] Kargel, J. S. et al. (2000) Icarus., 148, 226-265.. [3] Zolotov, M. S. and Shock, E. L. (2001) Jounral of Geophyical Research. 106. E12. [4] McKinnon, W. B. and Zolensky, M. E. (2004) 3(4). 879-897. [5] Kuskov, O. L. and Kronrod, V. A. (2005) Icarus. 177. 550-569. [6] Lodders, K. and Fegley, B. (1998) The Planetary Scientist’s Companion. [7] Hiroi, T. et. al. (2004) LPS XXXV, Abstract #1616. [8] John- son, R. E. et. al. (2004) Jupiter: The planet, satellites, and magnetosphere. 485-512. [9] Zanhle, K. J. et. Al. (2003) Icarus. 163, 263-289. [10] Bethke, C. (2008) Geochemical and Biogeochemical Reaction Modeling. [11] Gavin, P. and Vance, S. D. (2012) LPS XXXIII, 1683-1684. [12] Zolotov, M. S. and Kargel, J. S. (2009) Europa. 431-446. [13] Vance, S. D. et. al. (2007) Astrobiology. 7(6). 987-1005. [14] Stixrude, L. and C. Lithgow-Bertelloni (2011) Geophys. J. Int. (2011) 184, 1180–1213. Figure 3. Precipitated Silicates. Amount of silicate formed (Log[g]) for each rock model sorted into ser- pentines and other silicates. Figure 4. Oxides, Sulfates, and Carbonates. Shows the amount of other mineral classes precipitated (g/kg of reac- tant rock on a log scale) for each of the simulated chon- drite rock models. Figure 5. Resulting Fluid Composition. Log[M] of five ions of interest that were commonly found in the final fluids for all simulated rock models.