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Lunar and Planetary Science XXXII (2001) 2138.pdf

The System Sulfuric - -: Europa’s Ocean Properties Related to Thermal State. J. S. Kargel1, J.W. Head, III2, D. L. Hogenboom3, K.K. Khurana4, and G. Marion5; 1US Geological Survey, Flagstaff, AZ 86001, e-mail [email protected] , 2Department of Geological Sciences, Brown University, Providence, RI, email [email protected]; 3Dept. of Physics, Lafayette College, Easton, PA 18042, e-mail: [email protected] , 4Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, email [email protected] ; 5Desert Research Institute, Reno, NV, email [email protected]

Overview. Europa’s aqueous evolution is mod- MPa, the eutectic composition shifts toward a more water- eled in the system H2O-H2SO4-MgSO4. For a thin ice crust, rich composition, but MgSO4 remains unabundant in the the ocean is thick, warm (~267 K) and dominated by MgSO4; ternary eutectic. As we show below, at higher temperatures for a thick ice crust, the ocean layer is thin, cold (as low as MgSO4 is highly soluble in . 211 K), and dominated by H2SO4. In Figures 1 and 2 we show solid-liquid phase dia- Introduction: Theoretical and laboratory studies grams at 0.1 MPa for the binary system H2O-H2SO4 and the of chondritic leachates [1-4] predict a sulfate brine ternary H2O-H2SO4-MgSO4, respectively. The binary system crust/ocean and allow a hydrated sulfate surface on Europa H2O - MgSO4 is well known at 0.1 MPa [1], and a significant and other icy satellites. Interpretations of Galileo NIMS data body of phase equilibrium and volumetric data also exists for support these models, indicating that Europa has both icy this system at pressures up to 400 MPa [8]. The ternary sys- terrains and nonice terrains apparently surfaced by some type tem has had relatively little study. Figure 2 was compiled of hydrated sulfate. Good alternative spectral matches in- from knowledge of (1) the H2O - MgSO4 and H2O - H2SO4 clude magnesium/ contaminated by or binaries, (2) our determination of a very low of sulfuric acid contaminated by sulfur [5,6]. Both classes of MgSO4 in the eutectic H2O - H2SO4 liquid, and (3) a signifi- substances can be derived from an initial carbonaceous cant body of ternary liquidus and cotectic data in a paper by chondrite composition of Europa. These materials are not D’Ans and Freund [9]. The latter gives a set of data primar- mutually exclusive, and so a third possibility is that both sets ily in the high-temperature parts of the fields of hepta- and of materials are present. This paper explores the chemical hexahydrates of in the ternary system. differentiation due to progressive freezing of a model ocean Collectively, the data are sufficient to give a fair idea of the enriched in both magnesium sulfate and sulfuric acid, which ternary phase relations at 0.1 MPa in the water-rich (>60% we believe is a natural consequence of expected processes. H2O) part of the system, which is most relevant to icy satel- Initial composition: Magnesium sulfate in this lites. There remain significant uncertainties in important model is produced by low-temperature leaching of chondritic ternary phase relations, particularly as the ternary eutectic material, whereas sulfuric acid is generated by high- composition and temperature is approached. Nevertheless, we feel that Figure 2 is reliable enough to provide useful temperature devolatilization and venting of SO2 into the ocean, followed by low-temperature aqueous chemistry. insights and produce quantitative models of Europan Sulfuric acid also would be produced by radiolysis of surface ocean/crust evolution (if this is indeed the appropriate system and ice containing Iogenic sulfur impurities implanted to model). However, further lab work is needed and is un- from the Jovian magnetosphere. derway at Lafayette College to refine our knowledge of The amount of endogenic sulfuric acid is likely to phase equilibria and crystallization behavior this system. greatly exceed that produced by radiolysis, if SO2 volcanism Model results: In Figure 3 we show some re- on Io and the rate of S and O loss from Io is any hint at what sults of an idealized model of cooling and fractional crystal- might be occurring on Europa’s seafloor. An estimated lization an initial ocean. These results suggest that a Euro- minimum Europan inventory of SO2 is calculated from the pan ocean could evolve through a wide range of pressure- amount of SO2 Io has lost over geologic time (assuming re- and-temperature-dependent compositions, starting with a cent loss rates have been characteristic of the past); that warm magnesium sulfate-dominated composition and evolv- amount is then scaled to Europa’s smaller silicate mass. This ing to a cold sulfuric acid dominated ocean. Spaun and 20 calculation suggests a minimum of 2.04 x 10 kg of H2SO4 Head [4] have previously modeled the fractional crystalliza- after reaction (5) of reference [3] has taken place. The low- tion of an ocean in the binary system H2O - MgSO4, and temperature chondritic leaching model suggests 3.72 x 1021 Kargel [1] and Kargel et al. [3] have presented various satel- kg of highly soluble sulfate salts (MgSO4 + Na2SO4, ap- lite models based mainly on fractional melting in the system 21 proximated here as just MgSO4), and 8.08 x 10 kg of H2O. H2O - MgSO4 - Na2SO4. Notable in these systems, and quite This initial composition corresponds to 67.32% H2O + unlike that explored here, is the absence of any solute that 30.98% MgSO4 + 1.70% H2SO4. The total mass of the strongly depresses the of ice. In this sense, crustal/oceanic system, when including the mass of salts H2SO4 plays a role more similar to that of ammonia or mul- having low (e.g., Ca-Mg carbonates and Ca sul- ticomponent chlorides, all of which are very effective freez- fate, which would initially reside beneath the ocean), ex- ing-point depressants in aqueous systems. It will be an im- ceeds what Europa’s moment of inertia allows, but it is not portant next step to consider the role of chlorides and Na2SO4 far different (see Model D’ in Figure 3 of [3]). added into the ternary system considered here. Phase relations: Key phase relations at pressures Systematic changes in the ocean’s mass, density, vis- from 0.1 MPa to 200 MPa are presented in preliminary form cosity, electrical conductivity, and other properties, and sys- in a companion abstract by Hogenboom et al. [7]. In that tematic growth of the floating crust and subsurface oceanic abstract we show that the ternary eutectic liquid contains layers would accompany this cooling. In future work, we will explore (1) possible constraints on this evolution very little dissolved MgSO4 (<1%) and about 35.5% H2SO4 at pressures close to 0.1 MPa. As pressure increases to 200 from magnetometer data, and (2) geological implications. Lunar and Planetary Science XXXII (2001) 2138.pdf

EUROPA’S OCEANIC CHEMISTRY: J.S. Kargel et al.

References: [1] Kargel, J.S., Icarus, v. 94, p. 368-390. [2] Fanale, F.P., et al. 1999. Icarus v. 139, p. 179- 188. [3] Kargel, J.S., et al. 2000, Icarus v. 148, p. 226-265. [4] Spaun N.A. and J.W. Head, 2001, Geophys. Res. Lett. (in press). [5] McCord, T.B. et al. 1999, Jour. Geophys. Res., v. 104, p. 11,827-11,851. [6] Carlson, R. et al. 1999. Science v. 286, p. 97-99. [7] Hogenboom, D.L. et al. 2001 (abstract, this volume). [8] Hogenboom, D.L, et al. 1995, Icarus, v. 115, p. 258-277. [9] D’Ans, J. and Freund, H.-E. 1960, Kali und Steinsalz, v. 3, p. 31-33. [10] Gable, C.M. et al. 1950, Jour. Amer. Chem. Soc., v. 72, p. 1445-1448. [11] Hornung, E.W. et al. 1956, Jour. Amer. Chem. Soc., v. 78, p. 5747- 5751.

Figure. 1 (below). Phase diagram of system H2O-H2SO4. Data of [10], reinterpreted by [11]. Figure.2 (upper right). Ternary phase diagram of system

H2O-H2SO4-MgSO4. (0.1 MPa, preliminary). Figure 3 (lower right). Model of Europa’s oceanic chemical evolution and ice crust thickness variation with oceanic temperature. (Preliminary, based on Fig. 2 and

Model D’ of reference [3] with SO2 added].