The Global Shape, Gravity Field, and Libration of Enceladus

Park, R. S.; Mastrodemos, N.; Jacobson, R. A.; Berne, A.; Vaughan, A. T.; Hemingway, D. J.; Leonard, E. J.; Castillo‐Rogez, J. C.; Cockell, C. S.; Keane, J. T.; Konopliv, A. S.; Nimmo, F.; Riedel, J. E.; Simons, M.; Vance, S.

doi:10.1029/2023je008054

Cited by 10 publications

(11 citation statements)

References 61 publications

(157 reference statements)

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“… 2016 ; Park et al. 2024 ). The Mimas results require that the interior is differentiated, but the data could be fit by either a subsurface ocean or a frozen outer layer over an elongated core (Tajeddine et al.…”

Section: Interior Structure Characterization Approaches and Observationsmentioning

confidence: 99%

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

Rhoden,

Ferguson,

Bottke

et al. 2024

Space Sci Rev

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Saturn’s mid-sized icy moons have complex relationships with Saturn’s interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn’s mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn’s mid-sized moons provide critical benchmarks for the development of formation theories.

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Section: Interior Structure Characterization Approaches and Observationsmentioning

confidence: 99%

“… 2018 and references therein; Park et al. 2024 ), Dione (Thomas 2010 ; Nimmo et al. 2011 ; Zannoni et al.…”

Section: Interior Structure Characterization Approaches and Observationsmentioning

confidence: 99%

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

Rhoden,

Ferguson,

Bottke

et al. 2024

Space Sci Rev

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“…More broadly, it is important to be aware that the understood parameter space of Enceladus' SW and HF is rapidly changing and updating through modeling (Fifer et al, 2022;Hsu et al, 2015;Kang et al, 2022;Lobo et al, 2021;Park et al, 2024), re-examining Cassini data (e.g., Peter et al, 2023;Postberg et al, 2023), and comparison to terrestrial analogs (e.g., Fukushi et al, 2020). The parameter space used here was broad and tried to capture conditions which fall within most current models to give an overview of Enceladus' possible habitability, biomass, and productivity.…”

Section: Increased Seawater/hf Mixing Boosts Likelihood Of Habitabili...mentioning

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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“…Yet while Enceladus experiences cryovolcanism and resurfacing, Mimas appears geologically inert. anomalously high libration (Nadezhdina et al, 2016;Park et al, 2024;Thomas et al, 2016), indicates the satellite is wobbling in response to periodic gravitational torques from Saturn on the satellite's tidal bulge more than one would expect for a fully solid, hydrostatic satellite. For Enceladus, this indicates its ice shell is decoupled from its interior by a subsurface ocean, in agreement with results from gravity data (Beuthe et al, 2016;Hemingway & Mittal, 2019;Iess et al, 2014).…”

Section: Motivation: the Mimas-enceladus Paradoxmentioning

confidence: 99%

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

Gyalay,

Nimmo,

Downey

2024

JGR Planets

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Mimas has a high eccentricity and an anomalously high physical libration like its neighbor, Enceladus, but does not appear to have a geologically active surface. We investigate Mimas' interior with a technique that infers spatial variations in tidal heating from its global shape. To account for its hydrostatic shape, we find Mimas' normalized moment of inertia is 0.375 ± 0.0025, indicating a relatively undifferentiated world. Its remaining topography is consistent with a ∼30 km thick conductive ice shell in Airy isostasy atop a weakly convecting ∼30 km thick layer that itself mantles a ∼140 km radius ice‐rock interior. The convective shell's density must be closer to the interior density to satisfy our moment of inertia and provide a denser compensating layer for Airy isostasy. This ice‐rock interior is elongated along the Mimas‐Saturn axis, which can match Mimas' observed physical libration without appealing to an ocean. The inferred ice shell thickness variations indicate a high obliquity (≈1.7°). We suggest that the obliquity damped rapidly, after which topography froze in when internal heat was conducted out of Mimas quicker than isostatic ice shell thickness variations could relax. We speculate on several possible explanations for this transient high obliquity, including excitation by ring‐forming material following the recent tidal disruption of an eccentric satellite. We cannot rule out a young Mimantean ocean, but our inferred moment of inertia favors a Mimas that was solid when it experienced a period of high obliquity, did not significantly melt during a recent resonance with Enceladus, and is solid today.

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The Global Shape, Gravity Field, and Libration of Enceladus

Cited by 10 publications

References 61 publications

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

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

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

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

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