Experimental Simulation of Europan Seafloor Hydrothermal Systems

Introduction: Jupiter’s moon Europa is proposed to host a global liquid water ocean that is in contact with a silicate interior (Sotin et al., 2009). Similar to Earth’s oceans, water-rock reactions at Europa’s seafloor may produce habitable environments. In particular, the output of high-temperature altered fluids at hydrothermal vents would create chemical disequilibria upon mixing with seawater that chemotrophic life could exploit (McCollom, 1999). Microbes could also directly catalyse water-rock reactions in shallow, low-temperature crustal alteration environments (Nakamura and Takai, 2015). The amount of energy available to chemotrophic life in europan seafloor systems depends on the composition of the ocean and seafloor rock. However, little is known about Europa’s subsurface environment because the ice shell prevents direct observation or analysis. Previous studies of water-rock reactions at Europa’s seafloor have thus relied on analogues, e.g. terrestrial basalt or carbonaceous chondrites (e.g. Zolotov and Shock, 2001). However, these models are typically computer-based, possibly lacking important physical and kinetic effects (e.g. Neubeck et al., 2014) that could influence the potential for water-rock reaction environments to support life. There is thus a need for more experimental work to validate the results of computer models. In response to this, we will present the results of experimental water-rock reaction studies at elevated temperatures and pressures using bespoke ocean and rock simulants relevant to Europa’s past and current day. The results of these experiments will be used to inform further investigations into Europa’s continuous habitability, in particular how energy availability has changed over time. Simulant design: To simulate europan seafloor environments, we created ocean and rock simulants relevant to different points in Europa’s history based on models by Sym et al. (2024). Their models produced an ocean and seafloor rock composition at two different points in time, proto (i.e. soon after initial ocean formation) and current-day. However, compromises had to be made during manufacture in terms of practicality and component availability. The modelled seafloor rock is of identical chemical composition between the two times, albeit slightly different mineralogically. Since the overarching goal of this work is to constrain the energy available to life, it was decided to prioritise matching the chemical composition over mineralogical when making compromises. This results in the rock simulant for the proto and current-day times being identical, where components were added in the proportions outlined in Table 1. It is important to note that, although prioritising the chemical composition, all phases (except iron silicate glass) were also present in the modelled rock.Table 1 – Rock simulant composition The chemical composition of the ocean was distinct between the two modelled times, thus two ocean simulants were necessary (Table 2).Table 2 – Ocean simulant compositions*∑CO32- = CO32- + HCO3- + H2CO3 Simulant manufacture: ~1 kg of the seafloor rock simulant was created. Component rocks and minerals were crushed and sieved into the grain size fraction of 75-250 µm. The powders were then added to a mixing dish and mixed thoroughly to homogenise (figure 1).Figure 1 – Rock and mineral powders in mixing dish (left) and mixed simulant (right)The two ocean simulants were made anaerobically, since the models called for (essentially) no free oxygen in the fluid. Salts (e.g. KCl, NaSH, Na2CO3) and an ammonia solution were added to de-oxygenated water to achieve the concentration of species in solution in Table 2. Water-rock reactions: Water-rock reactions at Europa’s seafloor were simulated using two different Parr reactors (Olsson-Francis et al., 2020); one in a static configuration (figure 2) and one in a flow-through configuration (figure 3). Both reactor configurations were operated at 20 MPa, the closest the flow-through reactor can get to europan seafloor pressures (~84–205 MPa; Kargel et al., 2000), and were used to simulate both modelled times (proto and current-day).Figure 2 – Static Parr reactor in heating jacketThe static reactor simulates the production of high-temperature hydrothermal fluids beneath Europa’s seafloor. This was performed at 350°C, which is around the maximum temperature range for hydrothermal vent reaction zones on Earth (McCollom, 1999) but still below water’s critical point. The resulting rock, fluid, and headspace were sampled at the end of the experiment.Figure 3 – Flow-through Parr reactor set upThe flow-through reactor simulates shallower crustal alteration environments. This was performed at 75°C to allow for sufficient reaction progress on laboratory timescales but is still below the maximum temperature limit for known life (122°C; Takai et al., 2008). The output fluid was sampled daily, and the rock and headspace sampled at the end of the experiment.The fluid composition was measured by IC and ICP, and headspace gases were investigated by GC. Rock samples from the reactors were examined by SEM, Raman, and XRD. Future work: The static reactor simulations identified possible compositions of high-temperature hydrothermal fluids at Europa’s seafloor. This will be used to calculate the energy available to life upon the mixing of these fluids with fresh seawater at hydrothermal vents. The flow-through simulations identified possible reactions occurring during low-temperature shallow alteration of Europa’s seafloor. This will be used to constrain the energy available to microbes via direct catalysis. Comparing these results for the two different points in Europa’s history will also provide insight into how energy availability may have changed over time. Acknowledgements: This project is funded by the Science and Technology Facilities Council. References:Kargel et al. (2000) Icarus, 148, 226–265McCollom T. M. (1999) J. Geophys. Res., 104, 30729–30742.Nakamura K. and Takai K. (2015) in Subseafloor Biosphere Linked to Hydrothermal Systems: TAIGA Concept, pp. 11–30.Neubeck A. et al. (2014) Planetary and Space Science, 96, 51–61.Olsson-Francis K. et al. (2020) Journal of Microbiological Methods, 172, 105883.Sotin C. et al. (2009) in Europa, pp. 85–117.Sym L. S. J. et al. (2024) Composition and Habitability of Europa’s Ocean Over Time, in Astrobiology Science Conference, 5–10 May 2024Takai K. et al. (2008) Proc. Natl. Acad. Sci. U.S.A., 105, 10949–10954Zolotov and Shock (2001) J. Geophys. Res., 106, 32815–32827