Ice Shell Research Articles

Ice dynamic recrystallization within Europa's ice shell: Implications for solid-state convection

Solid-state convection has been proposed to occur within Europa's ice shell based both on the interpretation of observed geological activity during Galileo spacecraft exploration and theoretical investigations. Laboratory experiments have investigated the effect of grain size insensitive creep and grain size sensitive creep on the ductile behaviour of polycrystalline ice. The ice grain size and the ice impurities content and, consequently, the viscosity of the ice within Europa's ice shell are poorly constrained, limiting the possibility to understand if solid-state convection can occur under Europa's ice shell conditions. To investigate how diurnal tidal flexing and the internal dynamics of Europa's ice shell influence the ice grain crystals' evolution, we adopted a thermal-mechanical numerical model that uses finite differences and marker-in-cell techniques, implementing the dynamic recrystallization of the ice and the ice grain evolution in a self-consistent way with the numerical model. We found that solid-state convection within Europa's ice shell can occur if it is diurnally tidally deformed, as the tidal stresses within the ice shell operate to reduce the ice grain sizes and the ice viscosity. We discuss future radio science experiments, in combination with radar sounder investigations, that will be capable of characterizing the possible presence of solid-state convection within Europa's ice shell.

Microwave Observations of Ganymede's Sub‐Surface Ice: I. Ice Temperature and Structure

AbstractOn 7 June 2021, Juno flew within 1,000 km of Ganymede's surface, partially mapping its ice shell at six frequencies ranging from 0.6 to 22 GHz. The radiance at these frequencies originates from successively deeper layers of the sub‐surface and may reach depths of 24 km at 0.6 GHz. The MWR observations cover a latitude range from 20°S to 60°N and a longitude range from 120°W to 60°E. We present brightness temperature and derived reflectivity maps of Ganymede with a spatial resolution of up to ∼140 km. The microwave brightness temperature at all MWR wavelengths is anti‐correlated with the visible brightness of the terrain. Normalizing the MWR brightness temperatures using a thermal model for the ice shell reveals that the brightest regions are significantly more reflective in the microwave than the dark regions and that all terrain types are more reflective than is expected from a solid ice surface. We suggest that multiple reflections of the colder sky background at sub‐surface interfaces (e.g., fractures) explain the depressed brightness temperatures observed in brighter terrain types. A thin silicate or salt contaminant surface layer, which is significantly more reflective than ice in the microwave, could explain the microwave reflectivity in the dark regions with little to no contribution from sub‐surface fractures. The observed 0.6–1.2 GHz brightness temperature difference suggests an upper bound on the ice shell conducting layer depth of 150 km in the observation area.

Inferring the Mean Thickness of the Outer Ice Shell of Enceladus From Diurnal Crustal Deformation

AbstractThe thickness of the outer ice shell plays an important role in several geodynamical processes at ocean worlds. Here, we show that observations of tidally driven diurnal surface displacements can constrain the mean ice shell thickness, . Such estimates are sensitive to any significant structural features that break spherical symmetry such as faults and lateral variation in ice shell thickness and structure. We develop a finite‐element model of Enceladus to calculate diurnal tidal displacements for a range of values in the presence of such structural heterogeneities. Consistent with results from prior studies, we find that the presence of variations in ice shell thickness can significantly amplify deformation in thinned regions. If major faults are also activated by tidal forcing—such as Tiger Stripes on Enceladus—their characteristic surface displacement patterns could easily be measured using modern geodetic methods. Within the family of Enceladus models explored, estimates of that assume spherical symmetry a priori can deviate from the true value by as much as ∼41% when structural heterogeneities are present. Additionally, we show that crustal heterogeneities near the South Pole produce differences of up to 35% between Love numbers evaluated at different spherical harmonic orders. A ∼41% range in estimates of from Love numbers is smaller than that found with approaches relying on static gravity and topography (∼250%) or analyzing diurnal libration amplitudes (∼85%) to infer at Enceladus. As such, we find that analysis of diurnal tidal deformation is a relatively robust approach to inferring mean crustal thickness.

Mimas: Frozen Fragment, Ring Relic, or Emerging Ocean World?

Mimas, the smallest and innermost of Saturn's mid-sized moons, has a heavily cratered surface devoid of the intricate fracture systems of its neighbor, Enceladus. However, Cassini measurements identified a signature of an ocean under Mimas’ ice shell, although a frozen ice shell over a rocky interior could not be ruled out. The Mimas ocean hypothesis has stimulated inquiry into Mimas’ geologic history and orbital evolution. Here, we summarize the results of these investigations, which (perhaps surprisingly) are consistent with an ocean-bearing Mimas as long as it is geologically young. In that case, a ring origin for Mimas is favored over primordial accretion. An independently developed model for the formation of a gap in Saturn's rings provides a potential mechanism for generating a late-stage ocean within Mimas and may have assisted in the development of Enceladus’ ocean and associated geologic activity. Rather than a battered relic, Mimas may be the youngest ocean moon in the Saturn system, destined to join Enceladus as an active world in the future. The presence of oceans within Saturn's mid-sized moons also has implications for the habitability of Uranus’ moons; the Uranus system was chosen as the highest priority target for the next NASA Flagship-class mission. ▪ Models of Mimas’ tides and rotation state support a present-day internal ocean. ▪ Mimas’ craters, impact basin, and lack of widespread tectonism are compatible with a stable/warming ocean. ▪ The formation of the Cassini Division within Saturn's rings provides a potential pathway to a present-day ocean within Mimas. ▪ If Mimas has an ocean today, it is geologically young.

Assessment of thermal environment and thermal comfort in a typical composite ice shell building

Composite ice shell buildings are a special type of architecture found in severe cold regions and have played an important role in ice and snow tourism in recent years owing to their ability to offer tourists a romantic and novel experience. We conducted a field study to examine the thermal environment of a typical composite ice shell building in Harbin, China. In the unheated hall and separate small room, the indoor air had an average temperature and relative humidity of −20.64 °C and 76.94%, and −17.71 °C and 79.57%, respectively. Surface heat flux measurement of the north-facing building envelope revealed that the thermal properties of composite and plain ice differed significantly, suggesting that composite ice may have a lower thermal conductivity. The natural ventilation rate of 48,041 m3/h corresponded to an air change rate of 6.86 h−1, making the thermal environment highly susceptible to outdoor meteorological conditions. In addition, we conducted a questionnaire survey to study the thermal sensation and thermal comfort of the tourists. The results showed that the mean thermal sensation for the foot and head were approximately “cold " and “slightly cool”, respectively. The overall and mean thermal sensation for the hand were both between “cool” and “cold”, and the mean thermal comfort was approximately “slightly uncomfortable”. An adaptive thermal sensation prediction model suitable for composite ice shell buildings was developed through the study. These findings can assist in the design and construction of thermal environments that are better suited for short-term stays and specialty tourism.

Non‐Synchronous Rotation on Europa Driven by Ocean Currents

AbstractIt has been suggested that the ice shell of Jupiter's moon Europa may drift non‐synchronously due to tidal torques. Here we argue that torques applied by the underlying ocean are also important and can result in non‐synchronous rotation. The resulting spin rate can be slightly slower than the synchronous angular rate that would have kept the same point of the ice shell facing Jupiter. We develop an ice shell rotation model, driven by ocean stress calculated using a high‐resolution state‐of‐the‐art ocean general circulation model, and take into account the viscoelastic deformation of the ice shell. We use the ice shell model results together with observed limits on the ice shell drift speed to constrain ice shell parameters such as effective viscosity, which is currently uncertain by at least four orders of magnitude. Our results suggest, at best, sluggish ice shell convection. Depending on the relaxation time scale of the ice shell and on the ocean currents, the ice shell may exhibit negligible drift, constant drift, or oscillatory drift superimposed on random fluctuations. The expected rotation rate exceeds ∼30 m/yr; future spacecraft observations can be used to test these predictions and yield insight into the properties of the ice shell and underlying ocean.

Numerical simulations of heat exchange and vapor flow in ice fractures on Enceladus

Cassini spacecraft observed plume of water vapor and ice particles above the south polar terrain (SPT) of Saturn's moon Enceladus in 2005. The area of the SPT is marked by four prominent linear depressions in the ice dubbed “tigers stripes” (TS) that appear to be the source of the plume. The area around TS is also the source of anomalously high heat flux. The current consensus based on analysis of Cassini observations and modeling suggests that TS are fractures in the ice that penetrate to warm sub-ice ocean. Escaping vapor laden with ice particles forms the visible plume and latent heat of condensing vapor serves as the source of the surface thermal anomaly. In the present work previously developed numerical model of vapor flow and heat exchange inside TS fractures is refined and used to explain Cassini observations. Model improvements include consideration of a possible snow layer on the surface and inclusion of surface ice sublimation into energy balance model. Results of the simulations with improved model suggest that Cassini observations of vapor mass flow and endogenic heat flux can be explained if the width of the ice fractures is in the range 0.05–0.1 m, similar to previously published results. Wider fractures are possible if only a small portion of TS total length produces vapor plume, but the whole ~500 km length of TS fractures still contributes to the surface thermal anomaly. Alternatively, wider fractures are also possible in fractures with high effective friction due to wall roughness or channel tortuosity. Simulated heat output of fractures with different widths and depths in the range of 1.5–3 km is ~2.5–3.0 GW and is only weakly dependent on fracture width. A 2-m-thick surface snow layer covering the fracture can significantly lower outgoing heat flux to 0.9–1.9 GW and reduce surface temperatures by 20–40 K. The total heat flow from different fractures (including conduction from the water-filled portion) is remarkably similar - it is ~3.3 GW and 2.3 GW for fractures without and with a surface snow layer, respectively. The simulated heat flow is lower than the observed heat flow, suggesting that each TS depression may host 2 to 6 narrow ice fractures. The long-standing puzzle of the large difference between the observed heat flow from near TS (~4.2 GW) and from the whole SPT (~15.8 GW) is explained by conduction of ~11 GW of heat from the ocean through the ~5 km-thick ice shell underlying the SPT. Predicted high surface temperatures near fracture exit result in high rates of ice sublimation, which may produce a secondary source of water vapor feeding the observed plume. Fracture outlet likely evolves into a cone-like cross-section widening towards the surface on timescales from 1 to 100 years.

Revealing the Interior Structure of Icy Moons with a Bayesian Approach to Magnetic Induction Measurements

Some icy moons and small bodies in the solar system are believed to host subsurface liquid water oceans. The interaction of these saline, electrically conductive oceans with time-varying external magnetic fields generates induced magnetic fields. Magnetometry observations of these induced fields in turn enable the detection and characterization of these oceans. We present a framework for characterizing the interiors of icy moons using multifrequency induction and Bayesian inference applied to magnetometry measurements anticipated from the upcoming Europa Clipper mission. Using simulated data from the Europa Clipper Magnetometer, our approach can accurately retrieve a wide range of plausible internal structures for Europa. In particular, the ocean conductivity is recovered to within ±50% for all internal structure scenarios considered, and the ocean thickness can be retrieved to within ±25 km for five out of seven scenarios. Characterization of the ice shell thickness to ±50% is possible for six of seven scenarios. Our recovery of the ice shell thickness is highly contingent on accurate modeling of magnetic fields arising from the interaction of Europa with the ambient magnetospheric plasma, while the ocean thickness is more modestly affected and the ocean conductivity retrieval is largely unchanged. Furthermore, we find that the addition of a priori constraints (e.g., static gravity measurements) can yield improved ocean characterization compared to magnetometry alone, suggesting that multi-instrument techniques can play a key role in revealing the interiors of Europa and other ocean worlds.

The formation and evolution of Pluto's Sputnik basin prior to nitrogen ice fill

We simulate Sputnik Planitia (SP), Pluto's largest impact basin, from formation through cooling and relaxation prior to loading of the basin with N2 ice. To assess potential conditions for Pluto's interior that permit the formation of an SP-like basin in terms of depth and diameter, we consider impacts into targets that possess a subsurface ocean with variable ice shell thicknesses and thermal structures. We use a shock physics code to model excavation and transient crater collapse. Then, using the final temperature and density structure from the shock physics code as initial conditions, we simulate the subsequent cooling and viscoelastic relaxation of basin topography using a finite element model (FEM). We show that a thin ice shell (on the order of 100 km) overlying a thick ocean (on the order of 228 km) produces an SP-like basin in terms of diameter and pre-N2 depth. A basin formed in a much thicker ice shell (200 km) overlying a thin ocean (128 km) can reproduce SP if the ice shell is relatively warm and possesses a strong rheology. Our results suggest that SP was not a mascon basin prior to subsequent loading by N2 ice, but rather close to isostatically compensated. We find that SP could have developed into a mascon basin following N2 loading if the ice shell is thin, conductive and possesses a lithosphere capable of supporting the N2 load.

Geometry of Freezing Impacts Ice Composition: Implications for Icy Satellites

AbstractNon‐ice impurities within the ice shells of ocean worlds (e.g., Europa, Enceladus, Titan, Ganymede) are believed to play a fundamental role in their geophysics and habitability and may become a surface expression of subsurface ocean properties. Heterogeneous entrainment and distribution of impurities within planetary ice shells have been proposed as mechanisms that can drive ice shell overturns, generate diverse geological features, and facilitate ocean‐surface material transport critical for maintaining a habitable subsurface ocean. However, current models of ice shell composition suggest that impurity rejection at the ice‐ocean interface of thick contemporary ice shells will be exceptionally efficient, resulting in relatively pure, homogeneous ice. As such, additional mechanisms capable of facilitating enhanced and heterogeneous impurity entrainment are needed to reconcile the observed physicochemical diversity of planetary ice shells. Here we investigate the potential for hydrologic features within planetary ice shells (sills and basal fractures), and the unique freezing geometries they promote, to provide such a mechanism. By simulating the two‐dimensional thermal and physicochemical evolution of these hydrological features as they solidify, we demonstrate that bottom‐up solidification at sill floors and horizontal solidification at fracture walls generate distinct ice compositions and provide mechanisms for both enhanced and heterogeneous impurity entrainment. We compare our results with magmatic and metallurgic analogs that exhibit similar micro‐ and macroscale chemical zonation patterns during solidification. Our results suggest variations in ice‐ocean/brine interface geometry could play a fundamental role in introducing compositional heterogeneities into planetary ice shells and cryoconcentrating impurities in (re)frozen hydrologic features.

Turbulent Drag at the Ice‐Ocean Interface of Europa in Simulations of Rotating Convection: Implications for Nonsynchronous Rotation of the Ice Shell

AbstractEuropa's geologically scarred surface shows significant evidence that the ice shell may have rotated nonsynchronously in the past. The long‐term spin state of the ice shell is controlled by the time‐mean torques acting upon it. A torque that has not been previously considered is exerted due to drag from oceanic currents beneath the ice. We estimate this torque for the first time by performing global, nonhydrostatic, three‐dimensional simulations of Europa's ocean, including nonlinear turbulent boundary layer drag at the seafloor and ice‐ocean interface. Our simulations show that ocean dynamics, which manifest in alternating east‐west jets, result in a net torque on the ice shell. The torque can act to either spin up or spin down the ice shell depending on the strength of convection, suggesting that a torque reversal can occur as Europa's interior thermally evolves. Scaling analysis indicates that an average jet speed of at least ∼1 cm s−1 is required for the ice‐ocean torque to be comparable to the tidal torque acting to spin up the ice shell. Our results suggest that ocean currents may contribute to any nonsynchronous rotation of the ice shell. Consequently, Europa's present‐day spin state may hold information about the dynamics of its subsurface ocean.

Spectroscopic Detection of Biosignatures in Natural Ice Samples as a Proxy for Icy Moons.

Some of the icy moons of the solar system with a subsurface ocean, such as Europa and Enceladus, are the targets of future space missions that search for potential extraterrestrial life forms. While the ice shells that envelop these moons have been studied by several spacecrafts, the oceans beneath them remain unreachable. To better constrain the habitability conditions of these moons, we must understand the interactions between their frozen crusts, liquid layers, and silicate mantles. To that end, astrobiologists rely on planetary field analogues, for which the polar regions of Earth have proven to be great candidates. This review shows how spectroscopy is a powerful tool in space missions to detect potential biosignatures, in particular on the aforementioned moons, and how the polar regions of the Earth are being used as planetary field analogues for these extra-terrestrial environments.

Constraints on the composition and thermal structure of Ariel's icy crust as inferred from its largest observed impact crater

The large graben-like troughs and smooth plains visible on the surface of Ariel are indicative of a period of high heat flow in the Uranian moon's past. High heat flows on icy moons like Ariel can also enable viscous flow that removes impact crater topography, a process called viscous relaxation. Here we use numerical modeling to investigate the conditions necessary to viscously relax Ariel's largest impact crater, Yangoor, which is 80 km in diameter and unusually shallow. If we assume that Ariel's crust consists of non-porous water ice, heat fluxes ≥60 mW m−2 are required to reduce an initially deep Yangoor-like crater to its current observed depth. Lower fluxes are required if a high-porosity (30%), low-conductivity surface layer several kilometers thick is assumed to exist, but in any case, fluxes in excess of 30 mW m−2 are necessary to substantially reduce Yangoor's topography. The inclusion of ammonia dihydrate has a negligible effect on our results despite decreasing the viscosity of Ariel's deep ice. Our results are consistent with previous inferences of high heat fluxes on Ariel, but exceed both expected radiogenic heat fluxes and known equilibrium tidal heat fluxes by an order of magnitude. If Yangoor's shallow depth is the result of tidal heating, then short-lived non-equilibrium tidal dissipation or some other source of energy is required. Notably, although our results do not require the presence of an ocean within Ariel, the thermal conditions necessary to viscously relax Yangoor also imply a relatively thin ice shell (∼10-km thick) if conductive heat transport is assumed.

Radar Attenuation in Enceladus' Ice Shell: Obstacles and Opportunities for Constraining Shell Thickness, Chemistry, and Thermal Structure

AbstractEnceladus is a dynamic icy moon of Saturn and a leading target for future planetary missions focused on the search for life beyond Earth. For such missions, instruments that can provide geophysical and geochemical context for ice shell and ocean processes are critical to evaluate whether conditions are suitable for life and biosignature detection. Radar sounding is a powerful geophysical technique to probe the thermophysical and chemical properties of icy moons, like Enceladus, and to investigate the subsurface context for the exchange of material and energy between their subsurface oceans, ice shells, and plumes. To inform the scientific potential and instrument performance demands of such a radar‐sounding investigation of Enceladus' ice shell, we adapt and extend previous radar attenuation analysis done for Europa to the configuration and conditions of Enceladus. We also discuss how attenuation (both as an obstacle for the detection of ice shell reflectors and as a signal itself) can help constrain the thermal, physical, and chemical configuration of Enceladus' ice shell and reveal the processes governing the moon's ocean/shell/plume system.

Estimates for Tethys' Moment of Inertia, Heat Flux Distribution, and Interior Structure From Its Long‐Wavelength Topography

AbstractWe examine if Saturn's moon Tethys may be an ocean world by assuming that spatial variations in tidal heating are responsible for thickness or temperature variations in an isostatic ice shell, which manifests as surface topography. Because patterns of tidal heating depend on average ice shell thickness and whether the shell overlies a rigid or liquid layer, we can use Tethys' long‐wavelength topography to infer its interior structure. We test a wide range of assumed parameters to hone in on the characteristics of Tethys that produce self‐consistent and physically plausible interior models. To verify our technique, we apply it to Enceladus and recover the signature of a sub‐surface global ocean with an appropriately thick ice shell and moment of inertia. Our best‐fit Tethys models require Pratt isostasy and obliquity tides, with a normalized moment of inertia 0.340–0.345 and an average surface heat flux 1–2 mW m−2. The best‐fit basal heat flux distribution indicates that Tethys does not have an ocean. The total power inferred (4–8 GW) to produce Tethys' shape from tidal heating indicates either a highly dissipative interior or an obliquity higher than previously estimated. The topography may also be a relic of a warmer past when the obliquity was higher.

Estimating the 3D structure of the Enceladus ice shell from Flexural and Crary waves using seismic simulations

A seismic investigation on Saturn's moon Enceladus could determine the thickness of the ice shell, along with variations from the mean thickness, by recovering phase and group velocities, and through the frequency content of surface waves. Here, we model the Enceladus ice shell with uniform thicknesses of 5 km, 20 km, and 40 km, as well as with ice topography ranging from 5-40 km. We investigate several approaches for recovering the mean ice shell thickness. We show that surface wave dispersions could be used to determine the mean ice shell thickness. Flexural waves in the ice only occur if the shell is thinner than a critical value < 20 km. Rayleigh waves dominate only in thicker ice shells. The frequency content of Crary waves depends on the ice shell thickness.

Surviving in Ocean Worlds: Experimental Characterization of Fiber Optic Tethers across Europa-like Ice Faults and Unraveling the Sliding Behavior of Ice

As an initial step toward in situ exploration of the interiors of Ocean Worlds to search for life using cryobot architectures, we test how various communication tethers behave under potential Europa-like stress conditions. By freezing two types of pretensioned insulated fiber optic cables inside ice blocks, we simulate tethers being refrozen in a probe’s wake as it traverses through an Ocean World’s ice shell. Using a cryogenic biaxial apparatus, we simulate shear motion on preexisting faults at various velocities and temperatures. These shear tests are used to evaluate the mechanical behavior of ice, characterize the behavior of communication tethers, and explore their limitations for deployment by a melt probe. We determine (a) the maximum shear stress tethers can sustain from an ice fault, prior to failure (viable/unviable regimes for deployment), and (b) optical tether performance for communications. We find that these tethers are fairly robust across a range of temperature and velocity conditions expected on Europa (T = 95–260 K, velocity = 5 × 10−7 m s−1 to 3 × 10−4 m s−1). However, damage to the outer jackets of the tethers and stretching of inner fibers at the coldest temperatures tested both indicate a need for further tether prototype development. Overall, these studies constrain the behavior of optical tethers for use at Ocean Worlds, improve the ability to probe thermomechanical properties of dynamic ice shells likely to be encountered by landed missions, and guide future technology development for accessing the interiors of (potentially habitable ± inhabited) Ocean Worlds.

Tracking the Evolution of an Ocean Within Mimas Using the Herschel Impact Basin

AbstractMimas' small size, lack of geologic activity, and high eccentricity suggest a frozen, inactive history. However, Cassini libration measurements are best explained by a present‐day liquid ocean under an ice shell 24–31 km thick, a configuration that tidal heating can support. These unexpected findings have motivated further study of Mimas' surface geology to constrain its interior evolution. Here, we model the formation of Mimas' large impact basin, Herschel, using iSALE‐2D. Our goal is to determine whether the current estimated ice shell thickness is sufficient to withstand the impact and produce the observed basin morphology. We find that, if Mimas had an ocean at the time of the Herschel‐forming impact, the ice shell had to have been 10s of km thicker than today. These findings are consistent with inferences from minimal tectonic activity that Mimas must be a young ocean world, if it indeed has an ocean today.

Surface‐To‐Ocean Exchange by the Sinking of Impact Generated Melt Chambers on Europa

AbstractImpacts into icy bodies often generate near‐surface melt chambers and thermal perturbations that soften the ice. We explore the post‐impact evolution of non‐penetrating impacts into Europa's ice shell. Simulations of viscous ice deformation show that dense impact melts founder before refreezing. If the transient cavity depth exceeds half the ice shell thickness, over 40% of the impact melt drains into the underlying ocean. Drainage of impact melts from the near‐surface to the ocean occurs on timescales of 103–104 years. The drainage of melts to the ocean occurs for all plausible ice shell thicknesses and ice viscosities, suggesting that melt foundering is a natural consequence of impacts on icy worlds. Post‐impact viscous deformation is an important process on icy worlds that affects cryovolcanism, likely modifies crater morphology, creates porous columns through the ice for surface‐to‐ocean exchange, and may supply the oxidants required for habitability to subsurface oceans.

The challenges of driving Charon's cryovolcanism from a freezing ocean

A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon's surface with its ocean and facilitating ocean-sourced cryovolcanism. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was <10 km thick when the flows occurred (as opposed to >100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon's canyons. Plain language summaryWhen ocean-bearing moons begin to cool down, their oceans can freeze. As new ice accretes to the bottom of the existing ice shell, the added volume of the ice can stress the shell. Pluto's largest moon, Charon, has canyons and cryovolcanic flows that may have formed in response to a freezing ocean. Here, we model the formation of fractures within Charon's ice shell as the ocean underneath it freezes to explore the evolution of Charon's interior and surface. We find that an ocean source for cryovolcanic flows is unlikely because the ice shell would have had to be much thinner than current thermal evolution models imply. However, freezing the ocean may have produced the stresses that formed canyons later in Charon's history.