Composition and stability of salts on the surface of Europa and their oceanic origin

Abstract

Abstract. We present theoretical models of the composition, the relative abundances, and the stability of hydrated salts on the surface and in the icy shell of Jupiter's satellite Europa and discuss whether those salts have an oceanic origin. The evaluations were done with thermodynamic calculations of (1) salt dehydration equilibria at the conditions of the surface of Europa and its icy shell, (2) chemical equilibria involving solids and water vapor in the Na-K-Mg-Ca-S-C1-H20 system at surface temperatures and variable partial pressures of water vapor, and (3) changes in aquatic chemistry and sequences of salt precipitation from freezing oceanic water, using cosmochemical, mass balance, and physical-chemical constraints on the elemental and ionic composition of the ocean. Mass balance calculations of total or partial extraction of elements into an ocean from a carbonaceous chonddhte type mantle show that magnesium and sulfate rather than chloride and sodium could be the most abundant solutes in the ocean. Freezing oceanic water of this composition leads to brines that differ in composition from the original water and to deposition of ice and highly hydrated sulfates of Mg, Na, and Ca as well as alkali chlorides. After freezing is complete, highly hydrated salts remain stable in ice-bearing surface matedhals and throughout the icy shell. For hypothetical surface salt lag deposits, formed through sublimation/sputtedhng of ice and dehydration of salts, we predict hydration stratification with depth, approaching the highest hydration states in ice-beadhng matedhals in the lowest parts of the deposits. We discuss the effects of fast disequilibrium freezing and variable dehydration rates of salts on the predicted mineral assemblages at the surface. All of our models, which are independent of observations, predict the predominance of Mg and Na sulfates in surface salts, in agreement with spectroscopic models for the nonicy surface matedhal in the near infrared spectral region. IntroductionGalileo spacecraft reflectance spectroscopy in the near-infrared spectral region tentatively indicates the presence of hydrated saks in a non-H20 ice component on the surface of Jupiter's moon Europa [McCord et al., 1998a[McCord et al., , 1998b[McCord et al., , 1999. It was shown that magnesium and sodium sulfates (MgSO4.7H20, epsomite; MgSO4.Na2SO4.4H20, bloedite; Na2SO4.10H20, mirabilite), sodium carbonate (Na2CO3.10H20, natron), and mixtures of these salts provide good fits for the near-infrared spectra for the nonicy material Here we undertake evaluations of this type from a theoretical basis. In this paper we present internally consistent geochemical and thermodynamic models that describe leaching of aqueous components from a silicate mantle, aquatic chemistry of the ocean, freezing of oceanic water during upwelling in fractures in the icy crust and on the surface, and dehydration of salts on the surface of Europa. The results are compared with observational models for the composition and distribution of the nonicy surface mater...

Chemistry of Frozen Sodium–magnesium–sulfate–chloride Brines: Implications for Surface Expression of Europa’s Ocean Composition

“…Previous theoretical and experimental simulations (Fanale et al 2001;Zolotov & Shock 2001) strongly suggest that the composition of surface ices on Europa results from ocean materials; other studies based on observational data (Carlson et al 2009;Brown & Hand 2013;Dalton et al 2013) argue that certain surface species (e.g., MgSO 4 •7H 2 O and H 2 SO 4 •nH 2 O) are radiation products. Given the significance of sulfate as an electron acceptor for prokaryotes, Europa's potential for habitability is directly linked to whether these oxidized sulfur species have been delivered from, or could be delivered to, the ocean beneath.…”

“…3. NaCl has also been invoked as one of the dominant nonice components on Europa's surface (Zolotov & Shock 2001;Brown & Hand 2013;Hand & Carlson 2015). Our study (and previous geochemical modeling, summarized in Table 3) suggests that such composition could conceivably be achieved if (1) the ocean is very rich in sodium and chloride, and poor in magnesium and sulfate (as in Experiment 1) or (2) the ocean has comparable sodium-magnesium-sulfate-chloride concentrations and the freezing rate is rapid enough to yield detectable amount of NaCl (as in Experiment 2).…”

Pressure-induced ion pairing in MgSO4 solutions: Implications for the oceans of icy worlds

“…The salts of planetary oceans are acquired chiefly from sub-seafloor weathering reactions (Zolotov and Shock, 2001). About 75 % of the salts produced by low-temperature aqueous alteration of chondrite under oxidising conditions are magnesium sulphate hydrates (Hogenboom et al, 1995 et al, 2012), Europa (Spohn and Schubert, 2003;Hand and Chyba, 2007;Valenti et al, 2012), Ganymede (Vance et al, 2014;Saur et al, 2015), and Callisto (Kuskov and Kronrod, 2005) with approximate depths of the subsurface ocean boundaries in km.…”

“…Here, we studied the pressure-dependent variation in contact ion pairing in MgSO 4 solutions at 21 °C over a large range in pressure across the LDW-HDW transition until the sample solidified upon compression. We used MgSO 4 solutions because they are thought to be important components of planetary oceans (Zolotov and Shock, 2001) and because Raman scattering from the symmetric stretching vibration of sulphate (ν 1 -SO 4 2-) provides direct information about the fraction of contact ion pairs.…”

Exospheric signatures of alkali abundances in Europa's regolith

“…For comparison, Zolotov and Shock [2001] derived a range of oceanic Na/K ratios R O between 14 and 19 from chemical and thermodynamic models of oceanic waters resulting from chondritic rock/water equilibrium. However, as emphasized by Johnson et al [2002], this ratio should be smaller than the actual Na/K surface ratio R D because of fractional crystallization of oceanic water during upwelling in the icy shell.…”