Origin and Evolution of Enceladus’s Tidal Dissipation

Abstract

Enceladus possesses a subsurface ocean beneath a conductive ice shell. Based on shell thickness models, the estimated total conductive heat loss from Enceladus is 25–40 GW; the measured heat output from the South Polar Terrain (SPT) is 4–19 GW. The present-day SPT heat flux is of order $100\text{ mW}\,\text{m}^{-2}$ 100 mW m − 2 , comparable to estimated paleo-heat fluxes for other regions of Enceladus. These regions have nominal ages of about 2 Ga, but the estimates are uncertain because the impactor flux in the Saturnian system may not resemble that elsewhere. Enceladus’s measured rate of orbital expansion implies a low dissipation factor $Q_{p}$ Q p for Saturn, with $Q_{p} \approx 3\times 10^{3}$ Q p ≈ 3 × 10 3 (neglecting the role of Dione). This value implies that Enceladus’s present-day equilibrium tidal heat production (roughly 50 GW, but with large uncertainties) is in approximate balance with its heat loss. If $Q_{p}$ Q p is constant, Enceladus cannot be older than 1.5 Gyr (because otherwise it would have migrated more than is permissible). However, Saturn’s dissipation may be better described by the “resonance-locking” theory, in which case Enceladus’s orbit may have only evolved outwards by about 35% over the age of the Solar System. In the constant-$Q_{p}$ Q p scenario, any ancient tidal heating events would have been too energetic to be consistent with the observations. Because resonance-locking makes capture into earlier mean-motion orbital resonances less likely, the inferred ancient heating episodes probably took place when the current orbital resonance was already established. In the resonance-locking scenario, tidal heating did not change significantly over time, allowing for a long-lived ocean and a relatively stable ice shell. If so, Enceladus is an attractive target for future exploration from a habitability standpoint.

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

“…(2021) concluded that hydrogenotrophic methanogenesis still appears to be the metabolism with the highest energy yield and biomass potential. If Enceladus is younger than once thought (Ćuk & Moutamid, 2023; Ćuk et al., 2016; though see also Nimmo et al., 2023) and/or life is in the early stages of evolution, metabolic inefficiencies may explain some of the high Gibbs free energies inferred. Finally, spatial constraints (e.g., restricted space for a habitat on the seafloor) could limit access to H 2 despite it being readily available in solution, particularly if the ocean plume is rotationally controlled and spatially isolated to a column during transit to the ice layer (e.g., Choblet et al., 2017; Schoenfeld et al., 2023), though wider mixing of seawater with HF and longer timescales of vertical transport have been proposed (e.g., Kang et al., 2022).…”

Effects of Transient Obliquity Tides Within Mimas' Warm, Icy Interior Preserved as a Frozen Fossil Figure

“…However, an overwhelming majority of measurable thermal emission from planetary bodies is due to blackbody radiation of absorbed sunlight and thus these measurements are better suited to calculating the body's albedo (e.g., Howett et al., 2010) rather than its background heat flux (cf. Howett et al., 2011a; Nimmo et al., 2023, for Enceladus, where the tiger stripes provide better measurement of internal heating). Radio‐tracked flybys could measure the spherical harmonic coefficients of gravity J 2 and C 2,2 , which we would expect to reveal a lower moment of inertia for a melted and differentiated ocean world compared to our inferred, ocean‐free Mimas.…”

Geologic Constraints on the Formation and Evolution of Saturn’s Mid-Sized Moons

“…These results require high dissipation in the present as well as in the past (see reviews by Nimmo et al. 2018 , 2023 ). Enhanced tidal dissipation within a porous (also called “fluffy”) core may be critical to explaining such high heat flows (Roberts 2015 ; Choblet et al.…”

“…Whether Enceladus is currently in a state of equilibrium tidal heating or oscillating around an equilibrium depends on how dissipation is partitioned between Enceladus and Dione, as well as the evolution of Saturn’s Q, as discussed in detail by Nimmo et al. ( 2023 ). In addition, Enceladus may have experienced past epochs of higher eccentricity during the complex dynamical evolution of all five mid-sized moons (e.g., Meyer and Wisdom 2008 ; Běhounková et al.…”