Tectonic systems in the shells of icy worlds can support redox gradients for life.

Ice shell tectonics on Europa are one example by which surface materials can be cycled back into the ice shell of an ocean world, providing a mechanism to maintain chemical gradients over geologic timescales. One of the initial lines of evidence for Europa as a possible extant ocean world came from initial imaging showing a paucity of surface craters. This implied that the water ice shell is resurfaced, or renewed by emplacement of fresh material from below the surface, at a faster rate than craters can accumulate. While there appears to be good evidence for extensional ice tectonics with dilational band features akin to mid-ocean ridges on Earth, contraction features (like ocean plate subduction zones) required for mass balance were less obvious. In this work, the authors identified regions of subduction, or subsumption, of surface ice materials back into Europa’s shell, satisfying mass balance and demonstrating circulation of ice between the surface and ice shell interior on < 100 Ma timescales. Brittle ice plate tectonics at Europa are so far a unique example analogous to terrestrial plate tectonics in our solar system. This geologically-driven overturning and circulation of icy material could prove critical to life, where the entrainment and distribution of exogenous compounds and radiolytically-produced surface oxidants may support redox disequilibria over long time scales.

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Kattenhorn, Simon A and Prockter, Louise M

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Extant ocean worlds in our solar system, such as Europa, accreted with similar initial chondritic compositions as Earth and have subsequently differentiated.

Evidence-2: Extant ocean worlds in our solar system, such as Europa, accreted with similar initial chondritic compositions as Earth and have subsequently differentiated For the nearest extant ocean world, Europa, gravity measurements indicate a differentiated body with metallic core, rocky mantle, and an outer water layer of 80-170 km thickness split between liquid ocean and surface ice shell. For the required mass and density distribution in such a three-layer interior model, L/LL-chondrite composition with Fe-FeS core and silicate mantle below the water layer is proposed. Earth and most rocky bodies in the solar system are thought to have a chondritic composition, which matches solar composition (excluding some of the more volatile elements like H and He).

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Schubert, G and Sohl, F and Hussmann, H

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Some ocean worlds have pressure regimes permitting liquid water oceans in direct contact with silicate seafloors; others have high pressure ices with liquid or brine layers below which could support abiotic serpentinizing or hydrothermal seafloor alteration reactions.

Liquid water in direct contact with crustal material, such as Earth’s seafloor, is generally considered important to support abiotic organic molecule synthesis at hydrothermal systems or hydration reactions (serpentinization). Europa and Enceladus have pressure regimes permitting such a scenario, but it was previously thought that high pressure forms of ice on other bodies like Ganymede and Titan may insulate the seafloor from the ocean and preclude these reactions. However, pressure melt and dense brines situated between these high pressure ices and the seafloor may in fact still support organic synthesis. Modeling supported by geophysical measurements from previous NASA probes (e.g. Cassini, Galileo) constrains potential layering of ice, water, and silicates in several putative ocean worlds.

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Vance, Steven D and Panning, Mark P and Stähler, Simon and Cammarano, Fabio and Bills, Bruce G and Tobie, Gabriel and Kamata, Shunichi and Kedar, Sharon and Sotin, Christophe and Pike, William T and others

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Plume material from Enceladus’ ocean contains H2, demonstrating ongoing hydrothermal alteration in an extraterrestrial sub-ice ocean.

Enceladus’ icy crust contains several fissures which vent water and have been sampled directly by Cassini flybys. These flybys detected SIO2 nanoparticles and H2 gas, which the authors present as strong evidence for present day hydrothermal activity in Enceladus’ ocean. They found H2 abundances high enough to make methanogenesis thermodynamically favorable, a methane generating metabolic strategy which may be particularly relevant to origins of life chemistry. A continuous source of reduced compounds at Enceladus may provide the gradient required for early metabolisms as is hypothesized for the Archean ocean on Earth.

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Waite, J Hunter and Glein, Christopher R and Perryman, Rebecca S and Teolis, Ben D and Magee, Brian A and Miller, Greg and Grimes, Jacob and Perry, Mark E and Miller, Kelly E and Bouquet, Alexis and others

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Hydrothermal systems on Earth produce reduced compounds such as H2 which supports abiotic production of CH4, and these processes are hypothesized as the early inorganic model for the evolution of ancient metabolism on Earth.

Hydrothermal systems on Earth come in two main types: the high temperature 350°C black smokers driven by mantle heat at spreading zones, and cooler 50-90 systems driven by serpentinization reactions in ultramafic seafloor minerals. These systems both produce reduced compounds such as H2 which supports abiotic production of CH4. This process is hypothesized as the early inorganic model for the evolution of anaerobic methanogenesis, thought to be one of, or the most, ancient metabolism on Earth.

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Martin, William and Baross, John and Kelley, Deborah and Russell, Michael J

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