Theoretical Considerations on the Characteristic Timescales of Hydrogen Generation by Serpentinization Reactions on Enceladus

Daval, Damien; Choblet, G.; Sotin, C.; Guyot, François

doi:10.1029/2021je006995

Cited by 11 publications

(13 citation statements)

References 85 publications

(165 reference statements)

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“…Because the required extents of rock exposure to ocean water are minimal, it appears that physical access of rock to water should not preclude equilibrium. Therefore, the actual equilibration period for P and its controlling metals should be much less than the complete serpentinization duration of Enceladus’s core (∼10 8 y) ( 54 , 55 ). However, phosphates and other minerals in rock would still need to dissolve at geologically sufficient rates for species concentrations in the ocean to reach equilibrium values.…”

Section: Resultsmentioning

confidence: 99%

Abundant phosphorus expected for possible life in Enceladus’s ocean

Hao

Glein

Huang

et al. 2022

Proc. Natl. Acad. Sci. U.S.A.

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Saturn’s moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean–seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO 4 2− ), and total dissolved inorganic P could reach 10 −7 to 10 −2 mol/kg H 2 O, generally increasing with lower pH and higher dissolved CO 2 , but also depending upon dissolved ammonia and silica. Levels are much higher than <10 −10 mol/kg H 2 O from previous estimates and close to or higher than ∼10 −6 mol/kg H 2 O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H 2 O within 10 5 y, much less than the likely age of Enceladus’s ocean (10 8 to 10 9 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.

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Section: Resultsmentioning

confidence: 99%

Abundant phosphorus expected for possible life in Enceladus’s ocean

Hao

Glein

Huang

et al. 2022

Proc. Natl. Acad. Sci. U.S.A.

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“…Many estimates do not reach the rates required to explain the observed flux of space plume H 2 (Figure S4 in Supporting Information S1). Daval et al (2022) suggest that serpentinization may only be able to last on Enceladus for ∼10 Myr. Other mechanisms, such as radiolysis (Bouquet et al, 2017;Ray et al, 2021), or yet-unknown processes may contribute to ongoing H 2 production on Enceladus.…”

Section: Extant Biosphere May Not Contradict Existing Observations An...mentioning

confidence: 95%

“…This provides only an instantaneous habitability assessment informed by the in situ chemical disequilibrium (Cockell et al, 2021;Higgins et al, 2021), but biospheres can only be sustainable for long periods when there is a continuous supply of energy and nutrients (Cockell et al, 2024). Such production is expected to be ongoing on Enceladus, although the exact internal production rates and mechanisms remain unclear (e.g., Daval et al, 2022;Vance et al, 2016;Zandanel et al, 2021). In consequence, modeling efforts of the total biosphere within a notionally inhabited Enceladus produce wide-ranging estimates spanning several orders of magnitude (e.g., Affholder et al, 2022;Porco et al, 2017;Ray et al, 2021;Steel et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

Quantifying Uncertainty in Sustainable Biomass and Production of Biotic Carbon in Enceladus' Notional Methanogenic Biosphere

Higgins,

Chen,

Glein

et al. 2024

JGR Planets

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Beneath Enceladus' ice crust lies an ocean which might host habitable conditions. Here, the scale and productivity of a notional Enceladean methanogenic biosphere are computed as a function of the core‐to‐ocean flux of hydrogen and the ratio between abiotic and biotic methane in Enceladus' space plume. Habitats with an ocean‐top pH range of 8–9 have up to 40%–60% probability of being energy‐limited. Those at pH > 9 are increasingly uninhabitable, and those <8.5 are increasingly likely to host exponential growth, possibly leading to compositional inconsistencies between the ocean and Cassini gas observations. In those cases, energy‐based habitability models cannot infer an inhabited Enceladus consistent with both Earth life and Cassini measurements without including additional microbial growth limiters such as nutrient limitation, toxicity, or spatial constraints. If methanogens are isolated to a 350 K seafloor habitat and consume 10 mol s−1 of H2, the most probable biomass is 103, 103.7 kg C with ocean‐top pH 8,9, respectively. Biomass production consistent with space plume fluxes is 104–106 kgC yr−1—milligrams of cellular carbon per kilogram of H2O ejected—but requires that >50% of the space plume methane is biotic. Alternative scenarios are presented, and biomass is generally lower when habitat temperature is higher. Ocean biomass density cannot yet be reliably estimated owing to uncertainties in the scale and physicochemical properties of Enceladus' putative habitats. Evaluating abiotic to biotic ratios in plume methane and organic material could help identify false negative results from life detection missions and constrain the scale of an underlying biosphere.

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“…Let us consider a widely studied example in our solar system: Enceladus. A combination of theoretical models, laboratory experiments, and observational data from the Cassini mission appear to support the conclusion that the timescale for serpentinization on Enceladus is ∼100 Myr (Zandanel et al 2021;Daval et al 2022). Of the noteworthy byproducts of serpentinization, which has been posited as "life's mother engine" (Russell et al 2013), molecular hydrogen (H 2 ) stands out since it can be employed in ancient chemoautotrophic pathways such as hydrogenotrophic methanogenesis and acetogenesis (Cockell 2020).…”

Section: Habitability Interval In Obhs Is Transientmentioning

confidence: 96%

A Bayesian Analysis of Technological Intelligence in Land and Oceans

Lingam

Balbi

Mahajan

2023

ApJ

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Current research indicates that (sub)surface ocean worlds essentially devoid of subaerial landmasses (e.g., continents) are common in the Milky Way and that these worlds could host habitable conditions, thence raising the possibility that life and technological intelligence (TI) may arise in such aquatic settings. It is known, however, that TI on Earth (i.e., humans) arose on land. Motivated by these considerations, we present a Bayesian framework to assess the prospects for the emergence of TIs in land- and ocean-based habitats (LBHs and OBHs). If all factors are equally conducive for TIs to arise in LBHs and OBHs, we demonstrate that the evolution of TIs in LBHs (which includes humans) might have very low odds of roughly 1 in 103 to 1 in 104, thus outwardly contradicting the Copernican principle. Hence, we elucidate three avenues whereby the Copernican principle can be preserved: (i) the emergence rate of TIs is much lower in OBHs, (ii) the habitability interval for TIs is much shorter in OBHs, and (iii) only a small fraction of worlds with OBHs comprise appropriate conditions for effectuating TIs. We also briefly discuss methods for empirically falsifying our predictions and comment on the feasibility of supporting TIs in aerial environments.

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Theoretical Considerations on the Characteristic Timescales of Hydrogen Generation by Serpentinization Reactions on Enceladus

Cited by 11 publications

References 85 publications

Abundant phosphorus expected for possible life in Enceladus’s ocean

Abundant phosphorus expected for possible life in Enceladus’s ocean

Quantifying Uncertainty in Sustainable Biomass and Production of Biotic Carbon in Enceladus' Notional Methanogenic Biosphere

A Bayesian Analysis of Technological Intelligence in Land and Oceans

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