Evidence

At the serpentinite-hosted Lost City hydrothermal field, a microbial and macrofaunal communities excel in an ecosystem in which geological, chemical, and biologically processes are intimately linked.

Lost City is a hydrothermal field 15 km away from the Mid-Atlantic Ridge at ~800 m depth and is driven by serpentinization reactions – the highly exothermic metamorphic process of oxidizing and hydrating rocks. These reactions produce alkaline fluids rich in hydrogen and methane at temperatures up to 90°C, and, upon mixing with seawater, carbonate precipitates out of solution creating tens of meters tall carbonate chimneys. The hydrogen produced by the serpentinization reactions is a major energy source for microbes living here. Other than several distinct microbial communities, several species of fauna are present that live nearby or in the vents themselves.

Authors

Kelley DS,Karson JA,Früh-Green GL,Yoerger DR,Shank TM,Butterfield DA,Hayes JM,Schrenk MO,Olson EJ,Proskurowski G,Jakuba M,Bradley A,Larson B,Ludwig K,Glickson D,Buckman K,Bradley AS,Brazelton WJ,Roe K,Elend MJ,Delacour A,Bernasconi SM,Lilley MD,Baross JA,Summons RE,Sylva SP

Abstract

The serpentinite-hosted Lost City hydrothermal field is a remarkable submarine ecosystem in which geological, chemical, and biological processes are intimately interlinked. Reactions between seawater and upper mantle peridotite produce methane- and hydrogen-rich fluids, with temperatures ranging from <40 degrees to 90 degrees C at pH 9 to 11, and carbonate chimneys 30 to 60 meters tall. A low diversity of microorganisms related to methane-cycling Archaea thrive in the warm porous interiors of the edifices. Macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the Mid-Atlantic Ridge, but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems.

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Evidence

Owing to the water-rock reactions that provide an abundance of redox reactions, seafloor hydrothermal vents support diverse ecosystems with enormous biomass and productivity compared with elsewhere in the deep oceans.

On Earth, nearly a third of the heat lost from midocean ridges is from the convective circulation of nearby seawater. These interactions, occurring at nearly 400°C, result in a substantial chemical flux. Redox reactions are key to supporting chemosynthesis and hydrothermal vents support a number of them. High-temperature water-rock interactions forms reduced gases that dissolve in hydrothermal fluids and microbial reoxidation of reduced sulfur (H2S) at the seafloor releases stored energy and drives biochemical reactions. Hydrothermal systems are cyclic and microbial blooms have been observed at the beginning of the magmatic cycle. This suggests that microbial communities in the upper oceanic crust are abundant and poised to exploit the chemical energy carried by hydrothermal fluids. Further from the spreading zones, serpentinization reactions that can occur at much lower temperatures produces molecular hydrogen which is readily metabolizable by a variety of microbes. Despite the high temperatures near seafloor spreading centers, a diverse community of hyperthermophiles, mesophilic microbes, and macrofauna including tube worms and vesciomyid clams thrive through a number of evolutionary strategies for harnessing the chemistry nearby.

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Evidence

Serpentinization reactions in cracks in the rocky mantle may drive hydrothermal activity that could provide Europa’s and Enceladus’ seafloor heat fluxes on the same order as present-day radiogenic heat at Earth’s surface.

As the rocky mantle of icy ocean worlds cool, thermal microcracks may develop at their surface that continually penetrate further into the mantle with attending seawater. The mantle is likely heated by radiogenic decay of elements in the mantle and tidal heating. Additionally, serpentinization reactions – the low-temperature metamorphic process of oxidizing and hydrating rocks is highly exothermic and may be a source of nutrients and metabolites – may occur deep in cracks in the mantle. For Europa and Enceladus, this may provide heat flow to the seafloor similar to Earth’s radiogenic heat at the surface. Such reactions may be sustained for longer periods in icy ocean world crusts because the continuation of cracking further into the crust will expose fresh rock to be reacted on the order of 1 km/Gyr and serpentinization reactions may occur at lower temperatures there. The slow deepening of cracks over time may provide a steady source of thermal and chemical energy that may be sufficient to maintain stable ecosystems at the seafloor.

Authors

Vance S,Harnmeijer J,Kimura J,Hussmann H,Demartin B,Brown JM

Abstract

We examine means for driving hydrothermal activity in extraterrestrial oceans on planets and satellites of less than one Earth mass, with implications for sustaining a low level of biological activity over geological timescales. Assuming ocean planets have olivine-dominated lithospheres, a model for cooling-induced thermal cracking shows how variation in planet size and internal thermal energy may drive variation in the dominant type of hydrothermal system-for example, high or low temperature system or chemically driven system. As radiogenic heating diminishes over time, progressive exposure of new rock continues to the current epoch. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. We show that the time-varying hydrostatic head of a tidally forced ice shell may drive hydrothermal fluid flow through the seafloor, which can generate moderate but potentially important heat through viscous interaction with the matrix of porous seafloor rock. Considering all presently known potential ocean planets-Mars, a number of icy satellites, Pluto, and other trans-neptunian objects-and applying Earth-like material properties and cooling rates, we find depths of circulation are more than an order of magnitude greater than in Earth. In Europa and Enceladus, tidal flexing may drive hydrothermal circulation and, in Europa, may generate heat on the same order as present-day radiogenic heat flux at Earth's surface. In all objects, progressive serpentinization generates heat on a globally averaged basis at a fraction of a percent of present-day radiogenic heating and hydrogen is produced at rates between 10(9) and 10(10) molecules cm(2) s(1).

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Evidence

An abundance of molecular hydrogen detected in the subsurface global-ocean derived plumes on Enceladus signals a thermodynamic disequilibrium that hints at the possibility of methanogenesis at the seafloor.

At the south pole of Enceladus, a small, icy ocean moon of Saturn, a near-continuous stream of water vapor and gas erupts from cracks through the ice shell. In its final observations of Enceladus, Cassini flew through the plumes nearly perpendicular to the cracks where plumes are erupting. Measurements of plume material by the Ion Neutral Mass Spectrometer (INMS) – an instrument meant to detect the chemical, elemental, and isotopic composition of the Titan’s atmosphere, Saturn’s magnetosphere, and the ring system – detected a relative abundance of molecular hydrogen (H2), likely sourced from hydrothermal reactions in the (likely hydrated) rocky core beneath the global ocean. The dissipation of gravitational tides in the rocky core could create permeable fracture pathways and thermal gradients that drive hydrothermal circulation that produces the detected H2. This relative abundance of H2 and previously measured the abundance of CO2 and methane (CH4) means that the methanogenesis reaction could provide a source of chemical energy to support the synthesis of disequilibrium organic materials that the metabolic processes of life require.

Authors

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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Evidence

The estimated molecular hydrogen production at Enceladus’ ocean floor may be high enough to support methanogenic life.

Enceladus, a small, icy ocean moon of Saturn, emits a near-continuous stream of water vapor out fractures in the ice shell at the south pole. During Cassini’s final flyby, the Ion Neutral Mass Spectrometer (INMS) was set to measure the chemical, elemental, and isotopic composition Enceladus’ plumes. INMS detected silica-rich dust particles, molecular hydrogen, and methane. Methanogenic life, such as the archaeon M. okinawensis, may be able to produce methane at conditions similar to that of the Enceladus ocean floor. Furthermore, low temperature water-rock reactions at the ocean floor may produce enough molecular hydrogen to support autotrophic, hydrogenotrophic methanogenic life that could account for the methane detected by Cassini.

Authors

Taubner RS,Pappenreiter P,Zwicker J,Smrzka D,Pruckner C,Kolar P,Bernacchi S,Seifert AH,Krajete A,Bach W,Peckmann J,Paulik C,Firneis MG,Schleper C,Rittmann SKR

Abstract

The detection of silica-rich dust particles, as an indication for ongoing hydrothermal activity, and the presence of water and organic molecules in the plume of Enceladus, have made Saturn's icy moon a hot spot in the search for potential extraterrestrial life. Methanogenic archaea are among the organisms that could potentially thrive under the predicted conditions on Enceladus, considering that both molecular hydrogen (H) and methane (CH) have been detected in the plume. Here we show that a methanogenic archaeon, Methanothermococcus okinawensis, can produce CH under physicochemical conditions extrapolated for Enceladus. Up to 72% carbon dioxide to CH conversion is reached at 50 bar in the presence of potential inhibitors. Furthermore, kinetic and thermodynamic computations of low-temperature serpentinization indicate that there may be sufficient H gas production to serve as a substrate for CH production on Enceladus. We conclude that some of the CH detected in the plume of Enceladus might, in principle, be produced by methanogens.

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