Fuel Cell Simulations of Hydrothermal Vent Systems on Europa

44th Lunar and Planetary Science Conference (2013) 2200.pdf

FUEL CELL SIMULATIONS OF HYDROTHERMAL VENT SYSTEMS ON EUROPA. L. M. Barge1 , M. J. Russell1, I. Kanik1. (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasade- na, CA 91109, [email protected]).

Introduction: Numerous investigations [1-11] two solutions as the applied potential. We simulated a have considered whether icy moons in the outer solar hydrothermal system on an icy world in a membrane system host active hydrothermal systems similar to fuel cell experiment where the semipermeable mineral those found in Earth’s oceans, with implications for precipitate – representing the wall of a hydrothermal sustaining a low level of biological activity over geo- chimney compartment - is deposited on a synthetic logical timescales. Europa’s global ocean [12] is about membrane interface between simulated “ocean” and 100 kilometers deep and is in direct contact with its “hydrothermal” solutions (Fig. 1). The use of a synthet- silicate interior, and modeling of Europa’s history sug- ic membrane allows for more detailed characterization gests that hydrothermal activity may have been active of the ion / voltage / redox gradients produced, than for billions of years [11]. The rock/water interface on previous experiments where membranes are formed as Europa is significant since the circulation of seawater self-assembling chemical garden structures [15]. Ex- within a fractured permeable silicate layer would drive periments were conducted under generalized condi- production of chemical disequilibria - specifically, the tions that are relevant to the oceans of icy worlds with introduction of reduced components (such as H2, CH4, a rock/water interface: anoxic, slightly acidic ocean ± HS-) into the more oxidizing seawater (containing containing dissolved Fe+2 and an alkaline, sulfidic hy- - - species such as NO3 / NO2 and dissolved CO2) [10, drothermal fluid. In preliminary experiments we have 13]. On Earth, serpentinization-driven hydrothermal investigated the voltage and pH gradients mediated by systems exhaling a reduced, alkaline fluid into a con- the self-assembling FeS membranes, and used this trasting ocean produce precipitated mineral chimneys membrane as a working electrode for cyclic voltamme- at the interface, which act to mediate the redox, pH and try studies, to characterize the electrochemical proper- chemical gradients of the system and inhibit the dissi- ties of hydrothermal precipitates as a function of their pation of this free energy into the larger ocean [10]. In conditions of formation. terrestrial hydrothermal systems the interface between a reductive ocean crust and a more oxidized ocean can produce electrochemical energy approaching one volt. On the early Earth this electrochemical energy would have existed in tension between electron donors H2 and - - CH4 and acceptors NO3 / NO2 along with CO2 as well + - as between H3O and OH that could have driven carbon fixation and biosynthesis in mineral precipitates – the initial steep steps to metabolism [14]. If similar hydrothermal systems exist on icy moons, the formation of hydrothermal precipitates may extend the lifetime of active redox and pH potentials at the sea- floor, possibly allowing this chemical energy source to be present for the lifetime of the liquid carbonic ocean. Our objective is to test whether these inorganic “geochemical fuel cells” could produce enough energy from ambient pH / Eh gradients to drive prebiotic chemistry.

Experimental: Electrochemically active hydrothermal systems can form purely through water-rock chemistry in which seawater circulating through ocean crust reacts exothermically to produce a reduced, alkaline hydrothermal fluid [10]. The hydrothermal system can be visualized as a membrane fuel cell, with the Figure 1: Simulating hydrothermal vents on Europa as exterior (ocean) surface of the membrane as the cath- fuel cells. A) Interfacing “ocean’ and “hydrothermal” ode and the interior (hydrothermal) surface as the an- solutiosn across a semipermeable membrane. B) Iron ode, and the natural Eh and pH gradients between the 44th Lunar and Planetary Science Conference (2013) 2200.pdf

sulfide precipitate membrane precipitates, generating drothermal systems could lead to organic and eventual- membrane potential and mediating pH / Eh gradients. ly prebiotic chemistry. We will experimentally simulate some of the inorganic chemical reactions that might Results: Iron sulfide membranes precipitated im- lead to organic chemistry – and eventually, the emer- mediately on the membrane template, and generated gence of biochemistry – in alkaline (serpentinite- membrane potentials up to 0.6 V. The presence of oth- hosted) hydrothermal vent environments. These studies er components such as phosphates or silicates in the will help inform future missions to Europa and other ocean and hydrothermal fluid, as well as temperature, icy moons by demonstrating the possible organic / re- also affected membrane potential. The membranes dox chemistry that could occur in hydrothermal vent were also analyzed with Environmental Scanning Elec- systems, and help to narrow down geochemical condi- tron Microscopy (ESEM), and were found to be ~30 tions that would present a “best case scenario” for the microns thick and composed of plated crystals of iron emergence of life in the outer solar system. sulfides. These self-assembling membranes maintained their membrane potential for some days, and though References: [1] Collins, G.C., et al. (2000) the ion gradients between reservoirs slowly decayed JGR,105, 1709–1716. [2] Collins, G.C., et al. (2003). toward equilibrium, the ion conduction through the LPSC XXXIV, #1430. [3] Thomson, R.E. and Delaney, membrane was gradual enough that energy gradients J.R. (2001) JGR, 106, 12,355–12,365. [4] Goodman, could be maintained. Micro-scale fractures were ob- J.C. and Lenferink, E. (2012) Icarus, 221, 2, 970-983. served in the membranes, allowing specific channels [5] Goodman, J.C. and Lenferink, E. (2009) EOS AGU through which ion flow could be focused. (Fall Suppl.) 90, P31B-1251. [6] Goodman, J.C. et al. (2003) LPSC XXXIV, #1834. [7] Lowell, R.P. and DuBose, M. (2005) GRL 32. [8] Vance, S. et al. (2007) Astrobiology, 7, 987. [9] Glein, C. R., et al. (2008) Icarus 197: 157–163. [10] Russell, M. J. and Kanik, I. (2010) J. Cosmol., 5, 1008-1039. [11] Travis, B. J., et al. (2012) Icarus 218: 1006–1019. [12] Cassen, P., et al. (1979) GRL 6, 731–734. [13] Raulin, F., et al. (2010) Space Sci Rev. 153: 511–535. [14] Nakamura, R., (2010) Angew. Chem. Inter. Ed. 49, 7692-7694. [15] Barge, L.M., et al. (2012) Langmuir, 28, 3714– 3721.

Figure 2: ESEM images of iron sulfide membranes produced in simulated hydrothermal systems.

Conclusions: Future work will include studying the effects of applied voltage to the fuel cell experiment, and specifically investigate whether the simulated hydrothermal precipitates can act as electrodes to drive reduction of inorganic carbon. The reduction of dissolved oceanic CO2 or oxidation of hydrothermal me- thane by inorganic catalysts in electrically charged hy-