nai

Project Progress

The composition of Europa’s subsurface ocean is a critical determinant of its habitability. However, our current understanding of the ocean composition is limited to its expression on the surface. We have begun to experimentally investigate the composition of mixed sodium–magnesium–sulfate–chloride solutions when frozen to 100 K, simulating conditions that likely occur as ocean fluids are emplaced onto Europa’s surface. Our goal is to systematically examine how chemical composition of Europa’s surface reflects subsurface ocean chemistry and composition, and thus enable constraints to be placed on ocean composition through the observation of surface materials.

Frozen Na⁺-Mg²⁺-Cl^--SO₄^2- brines were investigated using a confocal dispersive Raman spectrometer (Horiba Jobin-Yvon LabRam HR) with cryogenic optical stage (Linkam LTS 350). Ten microliter droplets of solution were placed inside the cryostage on a glass microscope slide, and cooled to the temperature of interest. In some cases, samples were then annealed at a higher temperature before recooling.

Figure 1 shows the results of experiments examining the freezing behavior of a solution composed of a 1:1 mixture of saturated solutions of NaCl and MgSO₄. The molar concentrations of the ions in this solution are given in Table 1 (sat. experiment). The onset of freezing, indicated by the appearance of crystals, occurred at 211 K.

Figure 1 compares the spectrum of the sat. solution with the spectra of control solutions composed of saturated solutions of MgSO₄, Na₂SO₄, NaCl, and MgCl₂. We identify the peak at 990 cm^-1 as the υ1 symmetric stretching mode of the sulfate anion. This position is diagnostic for the formation of mirabilite, Na₂SO₄•10H₂O [2]. Other features of interest include the sharp peaks around 100–200 cm⁻¹ and especially at ∼3420 cm⁻¹. These likely indicate presence of both MgCl2 (black dotted line) and NaCl (magenta dotted line). The feature at 3420 cm⁻¹ is indicative of hydrohalite (NaCl•2H₂O) [3]. MgCl₂ is likely also hydrated.

The sat. solution discussed above contains an excess of Na⁺, perhaps biasing the results towards the formation of mirabilite. To test this possibility, we performed an experiment with 1M each of Na⁺, Mg²⁺, Cl− and SO₄^-2 frozen to 100 K. In this solution, stoichiometry of the salts would yield an excess of Mg²⁺ and SO₄^-2 in the solution. The results are shown in Figure 2.

Again, the frequency of the υ1 sulfate stretch at 990 cm^-1 indicates the formation of mirabilite. Peaks at 3420 and 3536 cm⁻¹ (magenta dotted lines) point to the formation of hydrohalite. The sample was subsequently warmed to its melting temperature (263 K) allowing the sample to equilibratebefore refreezing it at 100 K. After thermal cycling virtually all of the remaining hydrohalite had melted and did not recrystallize upon subsequent cooling, suggesting that this species is not to be expected at thermodynamic equilibrium.

Figure 3 shows the results of an experiment using the “oxidized-salty” composition defined in Table 1. This solution more closely mimics the possible composition if the Europan ocean. The same behavior is observed with this solution as with the sat. solution and the equimolar solution. This solution was highly acidic (pH ~1), so a 0.1 M H₂SO₄ solution was frozen to 233 K for comparison. The υ1 sulfate stretch peak is again found at 990 cm^-1, indicating the formation of mirabilite.

Our results indicate that solutions containing Na⁺, Mg²⁺, Cl^-, SO₄^2- form Na₂SO₄ and MgCl₂ preferentially on freezing. Epsomite (MgSO₄.7H₂O) cannot form directly upon freezing of an Na-rich ocean unless the SO₄^2- concentration is more than half that of Na+. Alternatively, Radiolytic processing, with implantation of sulfur ions originating from Io’ s volcanic eruptions, then transforms MgCl₂ into epsomite as suggested by Brown & Hand (2013) [4].

If NaCl is present on the surface, as suggested by Hand & Carlson (2015) or Na⁺, Mg²⁺, Cl^-, and SO₄^2- concentrations are comparable, and the freezing rate is rapid.

This work and its implications are discussed in a publication in press at the time this report was written [6]. Future work will examine the effects of vacuum and radiolytic processing.

References: [1]Marion, G. M., et al. (2005) GeCoA, 69, 259.
[2] Hamilton, A., and Menzies, R. I. (2010) JRSp, 41, 1014.
[3] Samson, I. M., and Walker, R. T. (2000) Can. Mineral., 38, 35.
[4] Brown, M. E., & Hand, K. P. (2013) AJ, 145, 110.
[5] Hand, K. P., and Carlson, R. W. (2015) GeoRL, 42, 3174.
[6] Vu, T.H., et al (2016). ApJLett, 816:L26 (6pp).