Self-Assembling Ice Membranes on Europa: Brinicle Properties, Field Examples, and Possible Energetic Systems in Icy Ocean Worlds
Steven D. Vance, Laura M. Barge, Silvana S.S. Cardoso, Julyan H.E. Cartwright
Self-Assembling Ice Membranes on Europa: Brinicle Properties, Field Examples, and Possible Energetic Systems in Icy Ocean Worlds Steven D. Vance, Laura M. Barge, Silvana S.S. Cardoso, and Julyan H.E. Cartwright NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK. Instituto Andaluz de Ciencias de la Tierra, CSIC–Universidad de Granada, Granada, Spain. Instituto Carlos I de Física Teórica y Computacional, Universidad de Granada, Granada, Spain.
Abstract
Brinicles are self-assembling tubular ice membrane structures, centimeters to meters in length, found beneath sea ice in the polar regions of Earth. We discuss how the properties of brinicles make them of possible importance for chemistry in cold environments—including that of life’s emergence—and we consider their formation in icy ocean worlds. We argue that the non-ice composition of the ice on Europa and Enceladus will vary spatially due to thermodynamic and mechanical properties that serve to separate and fractionate brines and solid materials. The specifics of the composition and dynamics of both the ice and the ocean in these worlds remain poorly constrained. We demonstrate through calculations using FREZCHEM that sulfate likely fractionates out of accreting ice in Europa and Enceladus, and thus that an exogenous origin of sulfate observed on Europa’s surface need not preclude additional endogenous sulfate in uropa’s ocean. We suggest that, like hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal places where ecosystems of organisms might be found. Key Words: Europa—Brinicles—Icy ocean worlds—Ice membranes—Chemical gardens. Astrobiology 19, xxx–xxx.
1. Introduction P HYSICAL AND CHEMICAL GRADIENTS are essential for life: they provide the setting where life can thrive between redox potentials, using this ambient energy to promote biosynthesis. Certain communities of organisms known today feed directly off environmental disequilibria such as the gradients of redox substrates that exist at sediment-water interfaces on Earth. Life in such systems produces chemical and electrochemical gradients in the environment, which can themselves be biosignatures (Shock and Canovas, 2010). The ocean-seafloor interface is the most well-known example ( e.g.,
Kelley et al., et al., et al., et al., et al., et al. et al., et al., et al., e.g., hydrothermal injection rate), osmotic force, and gradients in composition between the ocean and hydrothermal fluids (Barge et al., et al., et al., et al., et al., et al., et al., et al., et al., et al., et al., et al.,
2. Properties of Ice Membranes
The formation of ice-membrane structures has some interesting energetic implications, judging by the analogous processes in chemical precipitation systems that generate pH gradients and ion potentials. In a chemical-garden structure, the precipitation of insoluble components at the fluid interface produces a Nernst potential between the two solutions that is determined by he remaining concentration gradients of ions, including OH - and H O + , across the membrane (Barge et al., et al., et al., et al., et al., et al., et al., + , Cl - , Ca , Mg , SO , and other dissolved salts across the membrane wall. Gradients in pH might also develop as the brines produced in the ice shelf change pH as they become concentrated relative to seawater (Trinks, 2005). Chemical-garden membranes generate potential differences by being more permeable to some ions than others, and ice membranes might generate potential differences by functioning as proton exchange membranes. It is known that H O + permeates for a greater distance into ice than OH - (Dash and Wettlaufer, 2003; Moon et al., et al., (Nakamura et al., et al., et al., O + ) diffusion from one solution to another but maintaining the other chemical and redox gradients across the wall. The surfaces of the ice membrane might in this way be able to drive biochemistry by donating or accepting protons, or producing a proton-motive force at certain locations if the chemical conditions were right.
3. Brinicle Structures as Habitats
Chemical-garden structures are a general phenomenon emerging from fluid interfaces in far-from-equilibrium chemical systems, and they can occur at many different scales (Barge et al., e.g., cement tubes; Cardoso et al., e.g., hydrothermal chimneys at alkaline vents or black smokers [Ludwig et al., et al., et al., et al., et al., et al., et al., et al., is produced and delivered as a fuel for a whole ecosystem that lives in that vent (Lang et al., et al.,
4. Brinicles in Icy Ocean Worlds
Here we discuss the physical magnitude of liquid water expulsions in icy worlds, and the possible occurrence of brinicle structures that persist over geologically significant time scales. In sea ice on Earth, brine expulsions occur on a seasonal basis as insolation decreases with the onset of polar winter. Multiple meters of ice are generated in a matter of weeks (Bartels-Rausch et al., et al., et al., et al., et al., et al.,
Galileo last imaged them (Schmidt et al., et al., yr timescale of solid-state convection (Sotin et al., et al., et al., et al., et al., et al., Č adek et al., et al., et al., T c , is less than the eutectic temperature, and very close to the surface for brine mixtures with eutectic temperatures above T c . In Fig. 4, we reproduce the M2005 Europa equilibrium freezing calculation for a conductively cooling 20 km thick ice shell (Fig. 4 from that work) using FREZCHEM v15.1. The initial and final mass of water is fixed at 1 kg, as per M2005. Also as per M2005, we fix the temperature at the bottom of the ice to 263 K, so our calculations do not display intermediate fractionation for less concentrated solutions that initially increases the concentration of all brines within the ice. Whereas M2005 considered only the Mg and SO dominated briny ocean from the work of Kargel et al. (2000; Kargel2000), here we also investigate the bulk silicate Earth (BSE) K1 and K2 models for Europa from Zolotov and Shock (2001; ZS2001) and the standard composition of seawater (Millero et al., , with similar relative proportions of Na, Ca, and Cl such that the resulting brine compositions are identical except for the presence of K in ZS2001. Sulfate fractionates out of the ice in all calculations, regardless of the composition of the ocean. Because fluid brines are only stable in the portion of the ice that is above the eutectic temperature, salts transported toward the surface are likely to be in the solid phase and enriched in chlorides even if drawn from a sulfate-rich ocean. This fractionation at the ice-ocean interface is observed in multiyear sea ice on Earth (Gjessing et al., et al., content (Cragin et al., occurs mainly as an exogenic product of irradiated sulfur from Io (Brown and Hand, 2013) and that sulfates appear to be absent from many regions of Europa where they had previously been inferred from lower-resolution Galileo spacecraft spectra (Fischer et al., , et al., et al., et al., et al., × slower flow speeds (~cm s -1 ) predicted by Jansen (2016) may permit the formation of melt lenses under the ice (Zhu et al., . A Fluid Mechanical Model Gravity on Europa is 1/7 th g Earth ( g Europa = 1.3 m/s at the surface), and on Enceladus it is 1/90 th g Earth ( g Enceladus = 0.113 m/s ). Can we predict the relative lengths and thicknesses of brinicles on these bodies, neglecting the range of possible compositions and considering only the Earth-composition analog? Let us begin by considering a minimal model of the radius of fluid-jet precipitated tubes. An approximate 1D analytical solution to the Navier-Stokes equations in the laminar flow regime leads to a parallel-velocity flow model for the radius and flow rate of a cylindrical jet of fluid that forms the template for the growth of a tube precipitated about itself (Cardoso and Cartwright, 2017) !" ∆%&’( )* 𝑄 , = 𝑅 /0
11 + 4 " " : − = + & >?>@ " "
62 − :B = − 1=C (1) Here R’ is the radius of the tube growing in an environment of radius R C, where this depends on the radius of external recirculation in the environment (see Cardoso and Cartwright, 2017), Δ ρ i is the density difference between the brine and the external fluid, µ i and µ e are the internal and external fluid viscosities, and dP/dz is the longitudinal pressure gradient. We have compared the prediction of this equation with the results of laboratory experiments on growing brinicles by Martin (1974) (Fig. 5). As seen, most points lie within experimental error along the Poiseuille flow solution, which is contained within Eq. 1 in the limit of large external fluid viscosity. This simple model provides a good starting point for understanding brinicle growth: for brinicles, the inner flow is affected only by the presence of the wall, so that effectively the outside fluid is solid and Poiseuille flow applies. This is in contradistinction to the growth of chemical gardens in the laboratory, where a similar comparison of Eq. 1 with data shows that the tube wall in that case behaves as a liquid, rather than a solid (Cardoso and Cartwright, 2017). Among other things, then, Eq. 1 tells us that the volumetric flow rate determines the diameter of the brinicle ownflow tube. Alongside this argument for the tube diameter, let us also consider the wall thickness. The thickness of the growing tube wall should scale with √ ( D t ), but in a chemical garden the diffusivity D is a chemical diffusivity, while in a brinicle it is a thermal diffusivity, typically 100 × larger. Therefore, a brinicle should increase in thickness faster than a laboratory chemical garden. We can check this prediction with chemical garden data from Stone et al. (2005): t ~ 100 min, thickness ~1–2 mm; and brinicle data from Martin (1974): t ~ 6–10 min, thickness ~5 mm. Hence, the radial diffusivity is, for chemical gardens, approximately D ~ 0.001 /(100 . 60) = 1.7 × -9 m /s and for brinicles D ~ 0.005 /(6 . 60) = 7 × -8 m /s. These data then confirm our scaling prediction. So brinicles grow by thermal diffusion, ~100 × faster than the chemical diffusion in chemical gardens. This contrasting growth speed underpins the difference in scales between chemical gardens and brinicles and the pure Poiseuille flow behavior we find in the brinicle compared to the “liquid-wall” behavior in the chemical gardens (Cardoso and Cartwright, 2017). Let us now apply these arguments to brinicles on Europa and Enceladus. Firstly, the wall thickness scales as √ ( D t ), where D is the thermal diffusivity; that is, this aspect is independent of gravity. Secondly, from Eq. 1, for a given brinicle radius, the flow rate will be lower in lower gravity, and consequently the brinicle will grow more slowly. Hence, a brinicle of a given length will be older in lower gravity, and its walls will be correspondingly thicker. We see from Eq. 1 that the precise way in which this occurs depends on the interplay between pressure-driven and buoyancy-driven flow in the brinicle; that is, to what extent brine flow is forced versus buoyancy-driven. Altering g alters one part of the equation—the buoyancy-driven part—but does not affect pressure-driven flow. The maximum effect of lower gravity comes when the pressure-driven component is negligible. In that case, we can affirm that for the same rate of flow, a brinicle would be √7 * ~ 1.6 imes larger on Europa and √90 * ~ 3 times larger on Enceladus (Fig. 5). This is likely so for brinicles developing beneath a growing floating ice sheet as depicted in Fig. 2, where brine accumulates at the freezing interface at the base of the ice. Thus, brine outflow at the base of the ice will occur at a rate determined by the buoyancy-driven flux at the outflow point. Compared with a similar concentration of outflowing brine on Earth, a brine outflow on Europa, with smaller g, will move more slowly. At the point that the outflow enters the ocean, a strong temperature gradient is established, but the difference between the outflowing fluid temperature and that of the surrounding ocean should diminish in the growing chimney with distance away from the outflow point, to the place where the down-flowing fluid melts chimney material at the same rate that the ice forms. Entrainment of freshwater at this point will mean that the outflow becomes less negatively buoyant and begins to spread laterally. To consider the above flow rates in the context of possible fluid sources, we adopt the volume of a putative near-surface brine lens on Europa (Schmidt et al., et al., 𝜏=V/Q (2) Supposing the lens to be a cylinder with a radius R = 10 km and depth D = 100 m, the entire volume of the chaos feature ( V = π R D ) could be delivered to the ocean through a brinicle with R ≤
25 cm ( R c = 10 m) in less than 10 kyr. By contrast, prior studies suggest that the minimum ransit time through the ice is greater than 10 kyr (Sotin et al., et al.,
6. Conclusions
Brinicles provide a plausible setting for geochemical gradients amenable to life at the ice-ocean interface (Russell et al., et al., et al., et al., et al.,
Acknowledgments
The authors thank Norm Sleep and two anonymous reviewers for helpful input, and Mohit Melwani Daswani for additional input on the nearly final manuscript. Brine calculations benefitted from work by JPL interns Nina Bothamy and Amira Elsenousy. S.D.V. thanks Bruce Bills and Baptiste Journaux for many stimulating discussions relevant to this work. Research by L.M.B. and S.D.V was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. L.M.B. and S.D.V. were supported by the NASA Astrobiology Institute (NAI) Icy Worlds project (13-NAI7-0024). S.S.S.C. acknowledges the financial support of the UK Leverhulme Trust project RPG-2015-002. J.H.E.C. acknowledges the Spanish MINECO grant FIS2016-77692-C2-2-P. No competing financial interests exist. © 2018. All rights reserved.
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Abbreviations Used
Kargel2000 = Kargel et al. (2000) M2005 = Marion et al. (2005) Millero2008 = Millero et al. (2008) ZS2001 = Zolotov and Shock (2001) IG . . Brinicles under sea ice near McMurdo Station of the US Antarctic Survey. Images courtesy of Rob Robbins. FIG . . Process of brinicle formation. The brine, concentrated by osmotic salt rejection as the ice sheet freezes ( a ), siphons successively ( b – d ) through a network of brine channels and injects downward into the surrounding ocean ( e ). From Cartwright et al. (2013). FIG . . Seawater freezes on contact with this supercooled brine, forming a hollow tube. Comparison of surface, ice, and ocean conditions on Earth, Europa, and Enceladus, with relative scaling of the brinicles, and absolute scaling relative to the ice for Earth. FIG . . Equilibrium brine content of a 20 km thick ice shell on Europa in the absence of solid-state convection, after Marion et al. (2005). Pressure and temperature in the thermally conductive ice are shown on the right (dashes and solid, respectively). Magnesium- and sulfate-dominated compositions (bottom left) from Kargel et al. (2000) and Zolotov and Shock (2001) produce identical equilibrium brine contents within the ice. For these, and for the sodium- and chlorine-dominated ocean compositions from Zolotov and Shock (2001; middle left) and the standard composition of seawater (Millero et al., . FIG . . Variation of tube radius ( y axis) with flow rate for brinicles (Eq. 1; Cardoso and Cartwright, 2017). Left : nondimensional flow relative to the circulation length scale R c (Eq. 1). Martin’s (1974) experimental data for R c = 10 cm, ∆ρ i = 150 kg m -3 , µ i = 1.03 cP, µ i /µ e = 1, g = 9.81 m s -2 are shown with the prediction from Poiseuille flow (Eq. 1) with dP/dz = 0 and µ e → ∞ . Right : dimensionalized flow for Earth, Europa, and Enceladus ( R c = 10 m, ∆ρ i = 150 kg m -3 , µ i = 1.03 cP, µ i /µ e = 1, g Earth = 9.81 m s -2 , g Europa = 1.3 m s -2 ; g Enceladus = 0.113 m s -2 ). The dashed line depicts the drainage time for a hypothetical chaos lens on Europa modeled as a cylinder that is 100 m deep and 20 km in diameter ( 𝜏=V/Q)𝜏=V/Q)