Water Activity of Europa's Ocean

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2643.pdf

Water Activity of Europa’s Ocean: Temporal Variability and Implications. E. M. Spiers1 and B. E. Schmidt1, 1Georgia Institute of Technology, Earth and Atmospheric Science (311 Ferst Dr, Atlanta, GA; [email protected] ).

Introduction: Europa, a satellite of Jupiter, has proven to be a geologically active body [1][2][3] with evidence for harboring a global, subsurface ocean [4][5]. Eu- ropa’s scientific relevance is partially due to the habitability implications of this liquid water ocean, which is maintained by tidal heating. Tidal heat inputs to Eu- ropa’s interior have varied with the evolution of the La- place resonance, possibly becoming oscillatory at times [6][7]. This possible thermal oscillation can drive oscil- lations in other systems within Europa’s interior. One such factor that should be affected by the thermal state of Europa’s interior is the thickness of the ice shell [8][9][10]. As seen on Earth, when ocean water freezes, brine rejection out of the ice will occur [11]. Con- Figure 1: Variability in Europa's ocean salinity due to varia- versely, when ice sheets melt, fresh water is injected tions in ice shell thickness. In the ice shell model used, shell into the water body. On Europa, the overlying ice shell thicknesses vary from less than 1km to nearly 60km. Four may have varied by tens of kilometers over its evolu- different salt abundances are considered: a purely NaCl salt tion, leading to significant brine rejection into the ocean composition, a purely MgSO4 salt, and a Europan estimate during periods of freezing, and injection of fresh water consisting of combined NaCl and MgSO4 abundances as to the ocean during periods of melting. These could lead well as 0.014M of NaSO4, derived from bulk composition [11]. The final salt composition is an Earth ocean analog. to non-negligible variability of Europa’s ocean salinity. related to salinity of a solution, where increased salinity In this work we examine the variability in geochemical decreases water activity. However, not all salts affect processes of Europa’s rocky mantle due to thermally- the potential for hydration in the same way [14]. Some driven changes in water activity through ice shell thick- ions strengthen hydrophobic interaction, allowing for ness, and the implications of that variability on habita- higher levels of hydration potential for biological mole- bility. cules. In contrast, ions that are chaotropic will reduce Variation in Ocean Salinity: Current estimates of Eu- the potential for hydration, creating environments that ropa’s mantle heat production, H(t), after formation of are weakly hydrated. The composition, and by associa- the Laplace resonance incorporate both radiogenic and tion the water activity, of Europa’s ocean will evolve as tidal heating [6][7]. The model for heat production the thermal state evolves over time. The previously cal- where tidal dissipation is restricted to the silicate layer culated variable ocean salinities, are converted to com- is incorporated here [6]. The ice shell thickness, � , !"# position specific water activity using experimentally de- will vary with the mantle heat production and is mod- rived tables [15][16]. eled by assuming a single layer, conductive ice shell. Implications for Geochemistry: Water activity of Eu- The salinity and composition of the Europan ocean are ropa’s ocean has implications for water-rock reactions, unknown, so a range of estimates are considered for in- as a low water activity will have a reduced potential for itial salt abundances. These include an ocean of purely hydration reactions. If the water activity of the ocean halite (NaCl), purely magnesium sulfate (MgSO4), an and/or pore fluids within the mantle is variable, then the estimate based on Europan bulk composition [12], and rates of hydration reaction will vary as well, placing an Earth seawater equivalent. The change in ocean sa- limits on production of biologically relevant chemistry linity in weight percent, S, is calculated by measuring due to these reactions, such as hydrogen, methane, or the change in liquid water mass, � , due to freezing, $%& carbon dioxide. relative to mass of available salt, � , Figure 1. The '()* The reaction rates for low temperature serpentinization mass of available salts is calculated through abundances depend on the composition of the mantle. Due to the in- of individual ions per kg of water. ability to directly measure the Europan seafloor compo- Water Activity of Europa’s Ocean: The water activity sition with currently available resources, its assumed of a solution describes the degree of water available for that the composition of the seafloor is similar to that of hydration of materials, with a common water activity Earth’s, composed primarily of olivine (70%) and py- living limit for all life forms on Earth, including Archea, roxene (7%) [17]. The reaction equation for low Bacteria, and Eukarya [13]. Water activity is closely 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2643.pdf

temperature serpentinization due to the hydration of fay- for fluid access to previously unexposed mineral surface alite, and end member of olivine is listed in Equation 1. area [23] [24].

3�� + 2�� → 2�� + 3�� + 2� (1) The rate of reaction is competed against a model of the Rates of serpentinization are evaluated using laboratory rate of fracture opening due to cracking [17, 25] and in studies [18] [19] [20]. For lower water activity levels, turn, the rate of new fayalite exposure, Figure 2. As Eu- the rate of reaction is faster, and for higher water activity ropa approaches a colder thermal equilibria, the extent levels the rate of reaction is slower [18]. The water ac- of cracking will increase further into the mantle [17, 25]. Initial Results and Implications: For high water activities (lower salinities), the reaction proceeds faster than new rock is exposed via cracking for hydration. This limits the overall release of hydrogen as the reaction be- comes supply limited in olivine. As Europa enters an oscillatory thermal period, the release of byproducts mimics this oscillation, creating a stepped release. For low water activities (higher salinities), the system is entirely reaction rate limited. While the overall production of hydrogen increases with an increase of fluid accessible olivine, the fluid accessible area does not be- come supply limited as in the low water activity case. This produces a more constant, yet lower production of hydrogen over time, and implies that release of hydrogen due to serpentinization of fayalite is dependent on the water activity of pore fluids. Other similar rock-water hydration reactions are likely to be similarly influ- enced. The stepped release of hydrogen during periods of thermal oscillation may have broad consequences on redox state of the ocean, and consequently habitability of Europa’s interior. References: [1] Schmidt, B. E. et al. (2011). Nature, 479(7374), 502–505. [2] Soderlund, K. M. et al. (2020). Space Science Reviews, 216(5), 80. Zahnle, K. et al. (2003). Icarus, 163(2), 263–289. [4] Ki- velson, M. G. et al. (1997). Science, 276(5316), 1239–1241. [5] Ki- velson, M. G. et al. (2000). Science, 289(5483), 1340–1343. [6] Hussmann, H., & Spohn, T. (2004). Icarus, 171(2), 391–410. [7] Moore, W. B., & Hussmann, H. (2009). In Europa (pp. 369–380). Figure 2: Reaction of due to hydration of Europa's mantle for Tuscon, AZ: University of Arizona Press. [8] Hussmann, H., Spohn, various ocean salinities. Mol of each species are scaled to to- T., & Wieczerkowski, K. (2002). Icarus, 156(1), 143–151. [9] tal mol of rock that is fluid accessible due to cracking, show- McKinnon, W. B. (1999). Geophysical Research Letters, 26(7), ing the abundances of each species at a given time. Salinities 951–954. [10] Soderlund, K. M. et al. (2014). Nature Geoscience, 7(1), 16–19. [11] Buffo, J. J., Schmidt, B. E., Huber, C., & Walker, are converted into a composition dependent water activity C. C. (2020). Journal of Geophysical Research: Planets, 125(10), that defines the rate of reaction. Pore fluids are likely to have [12] Zolotov, M. Y., & Shock, E. L. (2001). JGR E: Planets, higher salinities than bulk ocean, so salinities that are double 106(E12), 32815–32827. [13] Stevenson, A. et al. (2015). The ISME and triple the bulk ocean composition estimates are included. journal, 9(6), 1333-1351. [14] Stevens, A. H., & Cockell, C. S. (2020). Frontiers in Microbiology, 11, 1478. [15] Rard, J. A., & Mil- tivity for pore fluids is likely to be higher than for the ler, D. G. (1981). Journal of Chemical and Engineering Data, 26(1), bulk ocean [21] due to dissolution of ions within the 38-43. [16] Fontana, A. J. (2007). In Water Activity in Foods: Fun- fluid accessible mantle. The overall rate of reaction for damental & Applications (pp. 391 - 393). [17] Vance, S. D., Hand, each product is defined by stoichiometry and the water- K. P., & Pappalardo, R. T. (2016). Geophysical Research Letters, 43(10), 4871–4879. [18] Lamadrid, H. M. et al. (2017). Nature Com- activity dependent rate, Equations 2 – 3, where �+ is the munications, 8(1), 16107. [19] Wegner, W. W., & Ernst, W. G. time-variable water activity. (1983). American Journal of Science, 283, 151-180. [20] McCollom, [ ] [ ] [ ] [ ] � = − !" = "# = ! = ! (2) T. M. et al. (2016). Geochimica et Cosmochimica Acta, 181, 175- 200. [21] Zolotov, M. Y., & Kargel, J. S. (2009). In Europa (pp. log( �) = −7.51 + 39 ∙ log(�) − 3797 ∙ log(�) (3) 431–458). Tuscon, AZ: University of Arizona Press. [22] Klein, F., It is assumed that the reaction can be run to completion & Le Roux, V. (2020).. Geology. [23] Farough, A. et al. (2016). Ge- ochemistry, Geophysics, Geosystems, 17(1), 44-55. [24] Tutolo, B. as serpentinization is a volume expanding reaction [22] M. et al. (2016). Geology, 44(2), 103-106. [25] Vance, S. D. et al. which can initiate additional fracturing surrounding the (2007). Hydrothermal Systems in Small Ocean Planets. Astrobiol- reacted products, increasing the porosity and allowing ogy, 7(6), 987–1005.