Powering prolonged hydrothermal activity inside Enceladus

Choblet, G.; Tobie, G.; Sotin, C.; Běhounková, M.; Čadek, O.; Postberg, F.; Souček, Ondřej

doi:10.1038/s41550-017-0289-8

Cited by 185 publications

(346 citation statements)

References 51 publications

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“…Core dissipation thus mainly depends on internal structure through the factor Im(k • n )/|k • n + 1| 2 . Tidal heating reaches several tens of GW if the unconsolidated core is modelled as a very soft viscoelastic material [Choblet et al, 2017] (Roberts [2015] studied before the enhancement of tidal heating in a fluffy core, but without global ocean and with the shear modulus of the core larger than the shear modulus of ice). The complex shear modulus of the homogeneous core is parameterized in terms of the elastic shear modulus µ ce and a nondimensional parameter δ (zero if the core is elastic, otherwise positive): For Maxwell rheology, δ is related to the core viscosity η c by δ = µ ce /(ωη c ), but the above expression is generally valid for any linear rheological model.…”

Section: Dissipation In the Corementioning

confidence: 99%

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

Beuthe

2019

Icarus

103

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Tidal heating is the prime suspect behind Enceladus's south polar heating anomaly and global subsurface ocean. No model of internal tidal dissipation, however, can explain at the same time the total heat budget and the focusing of the energy at the south pole. I study here whether the non-uniform icy shell thickness can cause the north-south heating asymmetry by redistributing tidal heating either in the shell or in the core. Starting from the non-uniform tidal thin shell equations, I compute the volumetric rate, surface flux, and total power generated by tidal dissipation in shell and core. The micro approach is supplemented by a macro approach providing an independent determination of the core-shell partition of the total power. Unless the shell is incompressible, the assumption of a uniform Poisson's ratio implies non-zero bulk dissipation. If the shell is laterally uniform, the thin shell approach predicts shell dissipation with a few percent error while the error on core dissipation is negligible. Variations in shell thickness strongly increase the shell dissipation flux where the shell is thinner. For a hard shell with long-wavelength variations, the shell dissipation flux can be predicted by scaling with the inverse local thickness the flux for a laterally uniform shell. If Enceladus's shell is in conductive thermal equilibrium with isostatic thickness variations, the nominal shell dissipation flux at the south pole is about three times its value for a shell of uniform thickness, which remains negligible compared to the observed flux. The shell dissipation rate should be ten times higher than nominal in order to account for the spatial variations of the observed flux. Dissipation in an unconsolidated core can provide the missing power, but does not generate any significant heating asymmetry as long as the core is homogeneous. Non-steady state models, though not investigated here, face similar difficulties in explaining the asymmetries of tidal heating and shell thickness.

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Section: Dissipation In the Corementioning

confidence: 99%

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

Beuthe

2019

Icarus

103

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“…For the first constraint, the derived moment of inertia factor of Enceladus (Iess et al, 2014, McKinnon, 2015 indicates a low-density core, which can be explained if there is considerable water-filled porosity (Choblet et al, 2017. We consider flows of water between two connected reservoirs: a flow of water from the ocean to the core, and a reverse flow 8 from the core to the ocean.…”

Section: Model Framework and Constraintsmentioning

confidence: 99%

“…The current preferred mechanism to sustain tidal dissipation is through the resonance-locking mechanism (Nimmo et al, 2018), which suggests that the equilibrium tidal heating varies only slowly with satellite semimajor axis, and hydrothermal activity could even persist for billions of years (Choblet et al, 2017). Here, we adopt a very conservative minimal hydrothermal activity 10 timescale of 20 Myr calculated by Choblet et al (2017) for the case of lateral variations of the ice shell in which direct contact might occur between the porous core and the ice in its thickest equatorial regions, at the sub-Saturnian and anti-Saturnian points. Combining with the first constraint on the total duration (d) that a fluid spends in the core, the total duration of amino acid subjugation to hydrothermal temperatures would be in the range of 4-8 Myr.…”

Section: Referencesmentioning

confidence: 99%

Decomposition of Amino Acids in Water with Application to In-Situ Measurements of Enceladus, Europa and Other Hydrothermally Active Icy Ocean Worlds

Truong¹

2019

Preprint

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To test the potential of using amino acid abundances as a biosignature at icy ocean worlds, we investigate whether primordial amino acids (accreted or formed by early aqueous processes) could persist until the present time. By examining the decomposition kinetics of amino acids in aqueous solution based on existing laboratory rate data, we find that all fourteen proteinogenic amino acids considered in this study decompose to a very large extent (>99.9%) over relatively short lengths of time in hydrothermally active oceans. Therefore, as a rule of thumb, we suggest that if amino acids are detected at Enceladus, Europa, or other hydrothermally active ocean worlds above a concentration of 1 nM, they should have been formed recently and not be relicts of early processes. In particular, the detection of aspartic acid (Asp) and threonine (Thr) would strongly suggest active production within the ocean, as these amino acids cannot persist beyond 1 billion years even at the freezing point temperature of 273K. Identifying amino acids from the oceans of icy worlds can provide key insight into their history of organic chemistry.

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“…In addition, radiogenic heating is assumed to be the only heat source in the core, though tides may also be important (e.g., Choblet et al, 2017;Roberts, 2015). The rheology of pure water ice is assumed; inclusions of other molecules would change the viscosity of the ice shell (Durham et al, 2010) and may affect the thermal evolution of satellites.…”

Section: /2017je005404mentioning

confidence: 99%

One‐Dimensional Convective Thermal Evolution Calculation Using a Modified Mixing Length Theory: Application to Saturnian Icy Satellites

Kamata

2018

JGR Planets

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Solid‐state thermal convection plays a major role in the thermal evolution of solid planetary bodies. Solving the equation system for thermal evolution considering convection requires 2‐D or 3‐D modeling, resulting in large calculation costs. A 1‐D calculation scheme based on mixing length theory (MLT) requires a much lower calculation cost and is suitable for parameter studies. A major concern for the MLT scheme is its accuracy due to a lack of detailed comparisons with higher dimensional schemes. In this study, I quantify its accuracy via comparisons of thermal profiles obtained by 1‐D MLT and 3‐D numerical schemes. To improve the accuracy, I propose a new definition of the mixing length (l), which is a parameter controlling the efficiency of heat transportation due to convection, for a bottom‐heated convective layer. Adopting this new definition of l, I investigate the thermal evolution of Saturnian icy satellites, Dione and Enceladus, under a wide variety of parameter conditions. Calculation results indicate that each satellite requires several tens of GW of heat to possess a thick global subsurface ocean suggested from geophysical analyses. Dynamical tides may be able to account for such an amount of heat, though the reference viscosity of Dione's ice and the ammonia content of Dione's ocean need to be very high. Otherwise, a thick global ocean in Dione cannot be maintained, implying that its shell is not in a minimum stress state.

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Powering prolonged hydrothermal activity inside Enceladus

Cited by 185 publications

References 51 publications

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

Decomposition of Amino Acids in Water with Application to In-Situ Measurements of Enceladus, Europa and Other Hydrothermally Active Icy Ocean Worlds

One‐Dimensional Convective Thermal Evolution Calculation Using a Modified Mixing Length Theory: Application to Saturnian Icy Satellites

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