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Water is a key ingredient to the potential for life, and several icy moons, such as Europa and Enceladus, are thought to harbor a dynamic subsurface ocean of liquid water. An ice probe or “cryobot” is a proposed technology to penetrate the surrounding ice shell and deliver a life-detection payload to these oceans. As the ice probe descends, it is surrounded by a thin melt film of liquid water. This paper focuses on the fluidics of the melt film for the case of a general ice probe shape, leading to a new approximate equation for the force balance constraints (validated to 0 . 4 % error by numerical model). In addition to the introduction of water by melting, the analysis allows for consideration of additional volumetric flow via a pumped jet of recirculated water. The general force balance equation is explored in more detail for the example of a cylindrical probe geometry with flat circular end caps. The thickness of the melt film below and to the side of the cylindrical probe is constrained by the force balance such that the probe typically operates in one of two asymptotic regimes, moving or stalled.

The recent expansion of studies at hydrothermal submarine vents from investigation of abiotic methane formation to include abiotic production of organics such acetate and formate, and rising interest in processes of abiotic organic synthesis on the ocean-world moons of Saturn and Jupiter, have raised interest in potential Earth analogs for investigation of prebiotic/abiotic processes to an unprecedented level. The deep continental subsurface provides an attractive target to identify analog environments where the influence of abiotic carbon cycling may be investigated, particularly in hydrogeological isolated fracture fluids where the products of chemical water-rock reactions have been less overprinted by the biogeochemical signatures of the planet’s surficial water and carbon cycles. Here we report, for the first time, a comprehensive set of concentration measurements and isotopic signatures for acetate and formate, as well as the dissolved inorganic and organic carbon pools, for saline fracture waters naturally flowing 2.4 km below surface in 2.7 billion year-old rocks on the Canadian Shield. These geologically ancient fluids at the Kidd Creek Observatory were the focus of previous investigations of fracture fluid geochemistry, microbiology and noble gas-derived residence times. Here we show the fracture waters of Kidd Creek contain high concentrations of both acetate and formate with concentrations from 1200 to 1900 µmol/L, and 480 to 1000 µmol/L, respectively. Acetate and formate alone account for more than 50–90% of the total DOC – providing a very simple “organic soup”. The unusually elevated concentrations and profoundly 13C-enriched nature of the acetate and formate suggest an important role for abiotic organic synthesis in the deep carbon cycle at this hydrogeologically isolated site. A variety of potential abiotic production reactions are discussed, including a radiolytically driven H, S and C deep cycle that could provide a mechanism for sustaining deep subsurface habitability. Scientific discoveries are beginning to reveal that organic-producing reactions that would have prevailed on Earth before the rise of life, and that may persist today on planets and moons such as Enceladus, Europa and Titan, can be accessed in some specialized geologic settings on Earth that provide valuable natural analog environments for the investigation of abiotic organic chemistry outside the laboratory.

Mission analysis using aerogravity-assist (AGA) maneuver at Titan is presented in this paper. Considering Saturn arrival, Titan encounter, and different post-AGA orbits, the analysis uses vector diagrams to illustrate the relations among all conditions as well as design constraints. The analysis focuses on Enceladus mission concept and also present a summary result for Cassini-type Saturn system missions. Turn angles using AGA maneuver at Titan are quantified via numerical simulation, showing that 25-deg AGA turn angles are achievable with a Titan inbound excess velocity ( V ∞ , IN ) of 10–20 km/s. Results show that for an Enceladus mission, V ∞ , IN of 10 km/s is the minimum velocity required for AGA maneuver to achieve an outbound transfer orbit directly to Enceladus; the corresponding peak g-load is less than 6 g, peak heat rate less than 200 W/cm2, and total heat load of around 25 kJ/cm2. For a Cassini-type Saturn mission, a minimum V ∞ , IN of 5 km/s can be used for Titan AGA. The results showed are for V ∞ , IN of 6 km/s; and the corresponding peak g-load, peak heat rate, and total heat load are 1.5 g, 20 W/cm2, and 5 kJ/cm2 respectively.

The ice plumes emanating from the geyser vents at Enceladus’ south pole offer an enticing route for direct access to acquire pristine astrobiological samples. Unlike traditional landing scenarios where the location of the landing target is known a priori, we propose that one or more penetrators explore the plume environment via lateral maneuvers during the descent to Enceladus’ surface to infer the location of promising landing sites by autonomously measuring the plume ice/vapour concentration at multiple locations within the plume. This offers the prospect of targeting the vent sources of the plumes for direct access to subsurface material prior to its ejection. We examine four types of “viffing” (vector-in-forward flight) descent profiles and the impulsive velocity increment ( Δ V ) required for the lateral maneuvers: (i) a ballistic descent trajectory with a minimal Δ V cost represents the reference trajectory that implements no lateral search maneuvers; (ii) a planar descent with a decaying lateral oscillation which has an intermediate Δ V cost and a small search area; (iii) a nested series of decreasing sized box trajectories offering intermediate Δ V costs of varying degrees depending on the specific parameters; and, (iv) a quasi-spiral trajectory of decreasing radius which imposes the highest Δ V cost but with the largest lateral search area. Computations and simulations determined that the proposed viffing maneuvers are feasible and permit accurate targeting of Enceladian geyser vents at modest cost in Δ V . We also briefly assessed the problem of mapping the Enceladian plume during the descent, which will ultimately be necessary for viffing.

Search for habitability in Outer Planets moons, requires presence of proper chemistry, water, and energy. Dissipation from tidal forces is major energy source in evolution of Icy moon systems, generically exhibiting a multi-layer structure, with outer solid (ice) and intermediate liquid layers, and solid (rocky) core. Saturn moon Enceladus has been found to eject plumes of water vapour and ice, and very recently, hydrogen molecules, a tentative sign of chemistry supporting microbial life. Purpose of the present work is a preliminary design of a minor electrodynamic tether mission to visit Enceladus. Free tether capture and maneuvering would take the S/C to a Saturn orbit with periapsis very close to the planet and apoapsis at Enceladus orbit, the S/C proved to be then in a 1:2 resonance with the moon, allowing parallel, conveniently slow flybys. Such mission could also involve visiting Dione, the 4 th largest Saturn moon, Enceladus and Dione being in a natural 1:2 Laplace resonance; it was recently brought to light that Dione exhibits multiple, parallel color lines, hundred kilometers long.

Its extraordinary level of geologic activity, its potential for habitability, and the prospects of returning samples from its plume of erupting water ice make Saturn's small (∼500 km diameter) moon Enceladus a high priority target for future exploration and a key to our developing understanding of icy ocean worlds. The structure of its outer ice shell is particularly important as it relates to the global heat budget, the global-scale response to tidal forces, and the nature of the ongoing eruptions. It is also diagnostic of how and where heat is dissipated internally. Here, using the most recent shape model and a new approach to modeling isostasy, we obtain a shell structure that simultaneously accommodates the shape, gravity, and libration observations and suggests that tidal dissipation near the base of the ice shell is likely an important mode of internal heating. The implied conductive heat loss is greater than the heat loss associated with the eruptions but is nevertheless compatible with the condition of steady state.

We model the stability fields of minerals in contact with an interior water-ammonia ocean on Titan. Using constraints on the ammonia abundance from Titan's orbital state, and updated kinetic data, we show that earlier pre-Cassini work by Engel et al. (1994) was qualitatively but not quantitatively correct. We calculate a salinity of the ocean of 1%, consistent with the ocean density derived from Cassini data on Titan's tidal response, and within the range found for Enceladus' ocean. We also consider the sources and sinks of the observed 40Ar in the atmosphere, and conclude that what is observed is only a small fraction of what might be dissolved in the ocean or trapped in clathrate in the crust. Thus, the efficiency of degassing from the core must be much higher than in previous estimates (e.g., McKinnon, 2010) to account for what is observed in the atmosphere. One self-consistent model is that of a partially hydrated core from which much of the potassium has been leached into the ocean, as suggested by the thermal model of Castillo-Rogez and Lunine (2010). Our picture of present-day Titan is that of a slightly salty ocean—about 1% salinity— atop an actively heated and hydrated rock core.

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. 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., 56 pages, 16 figures, 9 tables. Submitted to Icarus

Saturn's moon Enceladus is known to have a water vapor plume erupting from fissures across its south polar region. The plume was detectable in an observation of Enceladus transiting Saturn by Cassini's Ultraviolet Imaging Spectrograph (UVIS), but only at 1216 A (Lyman alpha). Jupiter's moon Europa also may have multiple water vapor plumes, detected via similar ultraviolet observations of Europa transiting Jupiter (after being discovered via emission features) by Hubble Space Telescope. Comparison of the UVIS Enceladus transit observation to published Europa transit results reveals that Europa's plumes have very different properties than Enceladus' plume using the same observational technique. For example, the mass of water expelled is two orders of magnitude less at Enceladus compared to Europa.

Current astrobiological exploration of the outer solar system is largely driven by understanding planetary ice-ocean systems such as those of Europa and Enceladus. The ice-shell thickness that affects the dynamics of the ice-ocean system is determined by the balance between heat loss via convection and conduction, and the heat production via tidal dissipation and radiogenic decay. In this study, we investigate how ice-shells of such ocean moons grow, and how convection inside them evolves as the system cools. We perform geodynamical calculations to investigate the ice-shell thickness and growth rate as the system cools from a warm initial condition. We study both diffuse and localized end member modes of tidal dissipation within the ice-shell. We find that as the ice-shell cools and thickens, there is a reconfiguration of the convection patterns, increasing the size of convection cells that leads to a relatively significant increase in the ice-shell growth rate.

Analysis of Enceladus’ gravity, topography and libration data suggests that the thickness of the ice crust significantly varies, which may have important consequences for the heat transfer across and the tidal dissipation within the ice shell. Understanding these processes is a critical prerequisite for analyzing Enceladus’ thermal evolution and assessing the long-term stability of its subsurface ocean. Here, we investigate the impact of ice shell thickness variations on the tidal deformation of the moon and the associated heat production using a finite-element model that includes faults (“tiger stripes”) in the south polar region (SPR). Since the tidal deformation and the thermal structure of the ice shell are coupled through temperature and deviatoric stress, we simultaneously solve the equations governing the anelastic deformation of ice and also equations which model conductive heat transfer in the ice shell. We find that tidal heating is concentrated in a narrow low viscosity zone near the base of the ice shell and along faults. Outside the SPR, the thickness of this zone is about 1/10 of the local ice thickness and the associated volumetric heating is ≲ 1 0 − 6 W/m 3 , corresponding to less than 1 GW of dissipated power. In the SPR, the tidal effects are enhanced by the combined action of faults and ice shell thinning. Although the volumetric heating in this relatively small region may be larger than 10 −4 W/m 3 , the total heat production in this region does not exceed 1.1 GW. Our computations show that tidal heating in the ice shell can explain only a small fraction of Enceladus’ heat production derived from astrometric observations, implying that Enceladus’ heat engine is powered by dissipation in the core or in the ocean.

In this study we performed a new analysis of ground-based observations that were taken on 17 April 2011 using the 10-metre Keck telescope on Mauna Kea, Hawaii. Emissions from H 3 + , a major ion in Saturn’s ionosphere, were previously analyzed from these observations, indicating that peaks in emission at specific latitudes were consistent with an influx of charged water products from the rings known as ‘ring rain’. Subsequent modeling showed that these peaks in emission are best explained by an increase in H 3 + density, rather than in column-averaged H 3 + temperatures, as a local reduction in electron density (due to charge exchange with water) lengthens the lifetime of H 3 + . However, what has been missing until now is a direct derivation of the H 3 + parameters temperature, density and radiative cooling rates, which are required to confirm and expand on existing models and theory. Here we present measurements of these H 3 + parameters for the first time in the non-auroral regions of Saturn, using two H 3 + lines, Q(1,0 − ) and R(2,2). We confirm that H 3 + density is enhanced near the expected ‘ring rain’ planetocentric latitudes near 45°N and 39°S. A low H 3 + density near 31°S, an expected prodigious source of water, may indicate that the rings are ‘overflowing’ material into the planet such that H 3 + destruction by charge-exchange with incoming neutrals outweighs its lengthened lifetime due to the aforementioned reduction in electron density. Derived H 3 + temperatures were low while the density was high at 39°S, potentially indicating that the ionosphere is most affected by ring rain in the deep ionosphere. Saturn’s moon Enceladus, a known water source, is connected with a dense region of H 3 + centered on 62°S, perhaps indicating that charged water from Enceladus is draining into Saturn’s southern mid-latitudes. We estimated the water product influx using previous modeling results, finding that 432 - 2870 kg s − 1 of water delivered to Saturn’s mid-latitudes is sufficient to explain the observed H 3 + densities. Assuming that our Saturn northern Spring measurement represents all seasons, and that the rings are able to reorganize over time, the ring rain mechanism alone will drain Saturn’s rings to the planet in 292 − 124 + 818 million years.

Some of the moons of the outer solar system harbour subsurface liquid oceans. Tidal dissipation plays an important role in preventing these oceans from freezing. In the past, most studies considered only tidal dissipation in the solid layers of these bodies (rock and ice). Recently, new studies considering tidal dissipation in the oceans of these moons have appeared. All of them make use of the shallow water approximation. However, the use of this approximation might not be adequate. Here we consider the linear non-hydrostatic three dimensional response of these oceans to tidal forcing with the full Coriolis force. To do so we consider an ocean of homogeneous density contained within a perfectly spherical shell and neglect the effect of the ice shell. We force the ocean with a time changing tidal potential and observe patterns of periodic inertial waves that take energy from the global tidal forcing and focus it along thin shear layers that propagate in the fluid. We focus on Europa and Enceladus, showing that inertial waves result in fluid flows of significant amplitude (a few cm/s). Nevertheless, we find that under the previously mentioned assumptions tidal dissipation due to inertial waves is several orders of magnitude smaller than Europa's radiogenic heating and Enceladus’ observed heat flux. Finally, we propose additional dissipation mechanisms that might play a relevant role in Europa and Enceladus and could be further investigated.

Numerous studies of the south polar region of Saturn's moon Enceladus have focused on the four main fractures (the tiger stripes), their associated water plume, and the effects of diurnal tidal stresses on these fractures. However, due to their small magnitude (10–100 kPa), diurnal stresses alone are unable to completely explain the initial formation of the tiger stripes and the many additional fractures in the region. We suggest nonsynchronous rotation (NSR) stresses, induced by a freely rotating ice shell over a global ocean, are large enough to overcome the tensile strength of the ice shell and initiate fracturing on Enceladus. Using the tidal stress calculation program SatStressGUI, we demonstrate the dependence of the magnitude of the NSR stress on the thickness of the ice shell, particularly the brittle outer layer of the ice shell. We suggest the brittle layer in the south polar region is 2–4 km thick whereas in the more northern regions, it is 6–8 km. The difference is most likely due to the elevated energy flux at the south pole, compared to the regions to the north, resulting in a warmer and thinner ice shell in the south polar region. The thinner ice around the south pole permits NSR tensile stresses of ∼5 MPa, enough to fracture the ice. However, the thicker ice to the north reduces the NSR stress to less than 2 MPa making fracturing less likely. The difference in ice shell thickness, and resultant NSR stress, can help explain the activity in the south, but relative quiescence in the north. Our modeled NSR stress magnitudes and orientations suggests a NSR period on the order of 1 Myr and advocate that the tiger stripes have rotated ∼45° clockwise about the south pole since their initial formation in response to NSR of the ice shell. If the ice shell is rotating at a constant rate, the tiger stripes would thus be up to ∼100,000 years old. An apparent decreasing angular separation between consecutive fracture sets that formed in the south polar region suggests an ice shell that has weakened in strength over the course of its recent geologic history; this could be a result of increased heating, thinning of the ice shell, or a combination of the two. As the ice shell thinned, the NSR stresses were enhanced, making fracturing more likely after a smaller amount of NSR compared to the previous fracturing episode.

The field parameters – number density, velocity components and temperature – for the Enceladus geysers, and possibly similar jets from other bodies, such as Europa, Ceres or comets, are expensive to obtain using physically correct simulation methods, such as the Direct Simulation Monte Carlo (DSMC) approach. It would be very useful to be able to correctly reproduce all the different states that the flow is undergoing while expanding into vacuum, from a high density and collisional state near the surface to free-molecular and collisionless at high altitude without resorting to expensive DSMC simulations in every case. In this work, we consider a two-phase water vapor/grain mixture exiting a circular vent, assuming a uniform radius of the water ice grains, and study how the field parameters can be fitted at an altitude of 10 km, where the flow is collisionless. To do so, we define simple functional forms for each of these fitted parameters, and we study how their coefficients vary as a function of the vent exit parameters, i.e. the vent radius, the water mixture mass flow, the water vapor/water ice mass ratio, the water ice grain radius, the water vapor and water ice exit speed, the vent exit angle and flow temperature. We define polynomial approximations to model these variations. We show that all the vent parameters have nearly-independent influences on the radial profiles at 10 km, except for the water vapor and water ice exit speed, for which we considered cross-correlations. We finally show that the geyser field parameters can be reconstructed using our parametric study for variations of the vent parameters within the range considered here, and in a time frame of a few milliseconds. The results of the parametrizations presented in this study can now be used to propagate the geyser field parameters using computationally inexpensive free molecular/ballistic codes up to higher altitudes. The DSMC results that have been run for this paper are available at an online repository.

Subsurface ocean tides act as a mechanism to dissipate tidal energy in icy satellite interiors. We numerically model the effect of an ice shell on ocean tides using non-linear bottom drag for the first time. We demonstrate that subsurface oceans experience tidal pressurization due to the confining nature of the ice shell, and find that Enceladus’ eccentricity forcing can generate up to 2.2 kPa of pressure excess at the ocean surface. Existing free-surface oceanic energy dissipation scaling laws are extended to subsurface oceans, and are benchmarked against our numerical results to within 10 %. We show that for the large bodies Ganymede, Europa and Titan, an ice shell increases eccentricity tidal heating due to self-gravity, whereas the shell’s suppressive mechanical forcing reduces eccentricity tide dissipation on Enceladus and Dione by several orders of magnitude due to their high effective rigidities. In contrast, the ice shell enhances obliquity-forced dissipation in all satellites investigated, except Triton, because the largely divergence-free ocean response is unaffected by the shell’s rigidity but is still enhanced by self-gravity. We conclude that the fundamental difference in ocean response to obliquity and eccentricity forcing, combined with self-gravity, results in increased obliquity heating and suppressed eccentricity heating in small satellites. For large satellites with low effective rigidities, the type of ocean response is less important because the shell’s mechanical forcing has little impact on the flow, whereas self-gravity will enhance the response, and consequently dissipation, regardless of the forcing. Overall, obliquity tides are likely to dominate the tidal heating budget of icy satellite oceans, remaining the most prominent source of fluid dissipation in subsurface barotropic ocean tides.

Since the discovery of an ice particle plume erupting from the south polar terrain on Saturn’s moon Enceladus, the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus’ surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instruments. Here, an updated and detailed set of deposition maps is provided based on a deep-source plume model (Schmidt et al., 2008), for the eight ice-particle jets identified in Spitale and Porco (2007), the updated set of jets proposed in Porco et al. (2014), and a contrasting curtain-style plume proposed in Spitale et al. (2015). Methods for computing the surface deposition are detailed, and the structure of surface deposition patterns is shown to be consistent across changes in the production rate and size distribution of the plume. Maps are also provided of the surface deposition structure originating in each of the four Tiger Stripes. Finally, the differing approaches used in Porco et al. (2014) and Spitale et al. (2015) have given rise to a jets vs. curtains controversy regarding the emission structure of the Enceladus plume. Here we simulate each, leading to new insight that, over time, most emissions must be directed relatively orthogonal to the surface because jets “tilted” significantly away from orthogonal lead to surface deposition patterns inconsistent with surface images. Data for maps are available in HDF5 format for a variety of particle sizes at http://impact.colorado.edu/southworth_data .

We present a new model of Enceladus’ internal structure based on the recent shape model by Tajeddine et al. (2017) and the gravity model by Iess et al. (2014). Assuming that the rocky core is homogeneous and in hydrostatic equilibrium, we derive a set of structural models that accurately reproduce the main characteristics of Enceladus’ gravity field. In order to restrict the range of acceptable models we analyze the degree of compensation (ratio of the bottom to the surface load) at spherical harmonic degrees well constrained by the gravity data. In agreement with previous studies, we find that Enceladus’ ice shell is close to equilibrium, with the degree of compensation approaching one for models with a hydrostatic core having a radius between 190 km and 195 km. By computing the flow of ice driven by variations in hydrostatic pressure on the ice water/interface, we demonstrate that the ice shell is in steady state, as suggested by the gravity and shape data, only if the viscosity of ice at the melting temperature is equal to or higher than 3 × 1014 Pa s, corresponding to diffusion creep with a grain size of 1 mm or larger. The topographic anomalies are maintained by phase changes at the ice/water interface with a melting/freezing rate of a few mm/yr or smaller. This process is controlled by the heat flux at the top of the ocean, characterized by a strong degree-2 zonal component with amplitudes exceeding 60 mW/m2 in both south and north polar regions.

The ‘Ocean Worlds’ of our Solar System are covered with ice, hence the water is not directly accessible. Using melting probe technology is one of the promising technological approaches to reach those scientifically interesting water reservoirs. Melting probes basically consist of a heated melting head on top of an elongated body that contains the scientific payload. The traditional engineering approach to design such melting probes starts from a global energy balance around the melting head and quantifies the power necessary to sustain a specific melting velocity while preventing the probe from refreezing and stalling in the channel. Though this approach is sufficient to design simple melting probes for terrestrial applications, it is too simplistic to study the probe’s performance for environmental conditions found on some of the Ocean Worlds, e.g. a lower value of the gravitational acceleration. This will be important, however, when designing exploration technologies for extraterrestrial purposes. We tackle the problem by explicitly modeling the physical processes in the thin melt film between the probe and the underlying ice. Our model allows to study melting regimes on bodies of different gravitational acceleration, and we explicitly compare melting regimes on Europa, Enceladus and Mars. In addition to that, our model allows us to quantify the heat losses due to convective transport around the melting probe. We discuss to which extent these heat losses can be utilized to avoid the necessity of a side wall heating system to prevent the probe from stalling, and introduce the notion of the ‘Critical Refreezing Length’. Our results allow to draw important conclusions towards the design of melting probe technology for future missions to icy bodies in our Solar System.

We present the first map of CO2 at the surface of Enceladus using data obtained by the Cassini Visible-Infrared Mapping Spectrometer (VIMS). In order to measure the weak and narrow CO2 absorption band depths around 4.26 µm, we improved: (1) the calibration of VIMS spectra; (2) the calculation of geographic coordinates; and (3) the projection techniques. We averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the CO2 abundances. CO2 is reliably detected mostly in the South Polar Region. This region includes active faults (Tiger Stripes), the highest observed surface temperatures, and locations of active plume eruptions. The occurrence here of CO2 is consistent with an endogenic origin controlled by tectonics. Both pure CO2 ice and complexed CO2 are detected from the positions of absorption bands between 4.27 and 4.24 µm. The highest concentrations of CO2 are between the main active faults of the South Polar Region, where the surface temperature is low. Pure CO2 ice deposits at the surface of Enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean. These pockets eventually release cold CO2 gas (∼70 to ∼119 K) at low-velocity (seeping) between the Tiger Stripes [Matson et al., 2018, Icarus, 302, 18–26]. CO2 clathrate hydrates may form in the ocean and may be subsequently released when a CO2 gas pocket blows out and erupts. Other mechanisms may contribute to reinforcing the anti-correlation of the CO2 distribution (of any type) with respect to the location of the Tiger Stripes, such as successive sublimation of CO2 and condensation on colder areas, and partial frost cover by H2O releases from plume eruptions.

Several large ridges, termed dorsa, stand over 800 m above their surroundings in a region centered on the trailing hemisphere of Saturn's icy moon Enceladus. In map view, these dorsa are linear to curvilinear, 20 km to more than 50 km long, and display near-orthogonal trends. They cross-cut (are younger than) most other geological features in the region. High-resolution limb profiles show the dorsa to be asymmetric in cross-sectioned profile and 5–6 km in width, and high-resolution images show striations along their crests. The structure and morphology of the dorsa suggest they are thrust blocks, possibly analogous to lobate scarps or wrinkle-ridges found on the terrestrial planets. The low slopes of their backlimbs and steeper forelimbs, suggest the dorsa were formed as wrinkle ridges or lobate scarps overlying thrust faults that penetrate 1–4 km deep to a detachment, most likely at the brittle-ductile transition (BDT). Their near-orthogonal trends are consistent with biaxial horizontal shortening. These relationships suggest that the central trailing hemisphere was recently subjected to a relatively high heat flow at the time that deformation occurred.

Chlorinated compounds have been recently identified or suggested to exist in a number of planetary bodies such as Ceres, Europa, Ganymede, Enceladus and Mars, on the basis of remote sensing and in-situ measurements as well as Earth-based telescopic observations. Sodium and magnesium-bearing chlorinated compounds with different levels of hydration have been suggested to reach the surface of these bodies through ascending flows from the interior; thus, such materials could be geochemically related to the putative presence of subsurface liquid reservoirs and could bring precious information about those layers' salty composition. Laboratory spectroscopic data of chlorinated salts carried out at temperatures representative of real planetary bodies, fundamental to correctly interpret the observational data, are currently missing or incomplete. Here we present laboratory reflectance spectra of two hydrated magnesium chlorides, namely MgCl2•2H2O and MgCl2•6H2O, measured at three different grain sizes and at twelve temperature values in the range 80÷295 K. All spectra have been measured in the visible and infrared spectral range 0.5÷4.7 μm. We examined the absorption features and related spectral parameters as a function of both temperature and grain size. These reflectance spectra, as far as the infrared range beyond 2.5 μm is concerned, are the first measured and published at cryogenic temperatures, therefore these data may prove very useful for the interpretation and modeling of remote sensing spectral data from planetary missions. In particular, this new dataset will be helpful in applying spectral unmixing models to past and future observations of some icy satellites, Mars, Ceres, and possibly other Solar System bodies.

The process by which the subsurface ocean on Enceladus is heated remains a puzzle. Tidal interaction with Saturn and Dione is the leading candidate but whether the dominant heating occurs in the solid core, ice crust or in the ocean itself is an outstanding question. Here we consider the driving effect of the longitudinal libration of the ice crust on the subsurface ocean and argue that the flow response should be turbulent even in the most benign situation of a smooth spherical ice shell when the motion of the boundary is only transmitted viscously. A rigorous upper bound on the turbulent viscous dissipation rate is then derived and used to argue that libration should be potent enough to explain the observed heat flux emanating out of Enceladus when the effects of tidal distortion and roughness of the ice crust are included.

The Cassini-Huygens mission turned the moons of Saturn into tangible worlds. Although the discoveries from the spacecraft have been compiled in various review articles (e. g., Dougherty et al., 2009), there is no single publication that summarizes the remaining outstanding questions. Drawing on a workshop sponsored by the Cassini Project, we summarize the unanswered questions for the main icy moons of Saturn – Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe - for the disciplines of surface composition, geology, thermal properties, Enceladus's plume activity, interiors, and the interactions between Saturn's magnetosphere and the moons.

Enceladus is the main source of neutral and charged particles in the Saturnian magnetosphere. The particles originate at more than 100 active geysers forming a plume above the south pole of the moon and are continuously released into Saturn’s magnetosphere. Therefore the understanding of the interaction of those particles and the local magnetospheric environment of the moon is very important. One technique to study that interaction is to study the typical motion of charged particles in the perturbed plasma flow and the associated magnetic field lines in the vicinity of the moon especially during close flybys. The Cassini spacecraft flew by Enceladus 23 times between 2005 and 2015 at distances between 25 and 5000 km. During some of the flybys Cassini went directly through the south polar plume. Other flybys happened north of the moon or on high-latitude trajectories with respect to the moon. In this paper we present the energetic electron measurements during those flybys obtained by the Low Energy Magnetosphere Measurement System LEMMS, part of the Magnetosphere Imaging Instrument MIMI onboard Cassini (Krimigis et al., 2004). As already shown in Krupp et al. (2012) for the first 14 flybys MIMI/LEMMS typically observes dropouts in the particle intensities in the region of disturbed field lines and in the presence of the moon itself or dense material blocking the bounce and drift motions of the particles. We present in this paper a continuation of the Krupp et al. (2012) results and add a full classification for all 23 flybys using the full data set of energetic electron measurements of MIMI/LEMMS. We distinguish the observed absorption and dust signatures into four different categories: (1) full absorption signatures when all the particles within a fluxtube connecting the spacecraft with the moon are lost onto the moon during one of the particle motions; (2) partial dropouts (ramp-like feature) when not all the particles inside the fluxtube are lost; (3) short dropouts in the fluxes when particles are suddenly lost for a short period in time; and interpret those features as full or partial losses onto the moon or its environment as a result of different plasma and dust regimes in the vicinity of Enceladus. We compare the results with those of Meier et al. (2014) and Engelhardt et al. (2015); (4) In addition we also show dust-related “false electron” measurements for those flybys when Cassini directly went through the dense regions of the south polar plume. Those “dust-peaks” can be interpreted as the result of impacting dust particles inside the LEMMS aperture or nearby creating a plasma cloud.

On day 138 of 2010, the plume of dust and gas emerging from Enceladus' South Polar Terrain passed between the Sun and the Cassini spacecraft. This solar occultation enabled Cassini's Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to obtain simultaneous measurements of the plume's gas and dust components along the same lines of sight. The UVIS measurements of the plume's gas content are described in Hansen et al. (2011, GRL 38:11202) , while this paper describes the VIMS data and the information they provide about the plume's particle content. Together, the VIMS and UVIS measurements reveal that the plume material above Baghdad and Damascus sulci has a dust-to-gas mass ratio that is roughly an order of magnitude higher than the material above Alexandria and Cairo sulci. Similar trends in the plume's dust-to-gas ratio are also found in data obtained when Cassini flew through the plume in 2009, during which time the Ion and Neutral Mass Spectrometer (INMS), Radio and Plasma Wave Science instrument (RPWS) and Cosmic Dust Analyzer (CDA) instruments made in-situ measurements of the plume's gas and dust densities (Dong et al. 2015 JGR 120:915-937). These and other previously-published systematic differences in the material erupting from different fissures likely reflect variations in subsurface conditions across Encealdus' South Polar Terrain., 24 pages, 14 figures, submitted for publication in Icarus

The geologic activity at Enceladus's south pole remains unexplained, though tidal deformations are probably the ultimate cause. Recent gravity and libration data indicate that Enceladus's icy crust floats on a global ocean, is rather thin, and has a strongly non-uniform thickness. Tidal effects are enhanced by crustal thinning at the south pole, so that realistic models of tidal tectonics and dissipation should take into account the lateral variations of shell structure. I construct here the theory of non-uniform viscoelastic thin shells, allowing for depth-dependent rheology and large lateral variations of shell thickness and rheology. Coupling to tides yields two 2D linear partial differential equations of the 4th order on the sphere which take into account self-gravity, density stratification below the shell, and core viscoelasticity. If the shell is laterally uniform, the solution agrees with analytical formulas for tidal Love numbers; errors on displacements and stresses are less than 5% and 15%, respectively, if the thickness is less than 10% of the radius. If the shell is non-uniform, the tidal thin shell equations are solved as a system of coupled linear equations in a spherical harmonic basis. Compared to finite element models, thin shell predictions are similar for the deformations due to Enceladus's pressurized ocean, but differ for the tides of Ganymede. If Enceladus's shell is conductive with isostatic thickness variations, surface stresses are approximately inversely proportional to the local shell thickness. The radial tide is only moderately enhanced at the south pole. The combination of crustal thinning and convection below the poles can amplify south polar stresses by a factor of 10, but it cannot explain the apparent time lag between the maximum plume brightness and the opening of tiger stripes. In a second paper, I will study tidal dissipation in a non-uniform crust., 71 pages, 12 figures, 5 tables

Solid CO2 surface deposits were reported in Enceladus’ South Polar Region by Brown et al. (2006). They noted that such volatile deposits are temporary and posited ongoing replenishment. We present a model for this replenishment by expanding on the Matson et al. (2012) model of subsurface heat and chemical transport in Enceladus. Our model explains the distributions of both CO2 frost and complexed CO2 clathrate hydrate as seen in the Cassini Visual and Infrared Mapping Spectrometer (VIMS) data. We trace the journey of CO2 from a subsurface ocean. The ocean-water circulation model of Matson et al. (2012) brings water up to near the surface where gas exsolves to form bubbles. Some of the CO2 bubbles are trapped and form pockets of gas in recesses at the bottom of the uppermost ice layer. When fissures break open these pockets, the CO2 gas is vented. Gas pocket venting is episodic compared to the more or less continuous eruptive plumes, emanating from the “tiger stripes”, that are supported by plume chambers. Two styles of gas pocket venting are considered: (1) seeps, and (2) blowouts. The presence of CO2 frost patches suggests that the pocket gas slowly seeped through fractured, cold ice and when some of the gas reached the surface it was cold enough to condense (i.e., T ∼70 to ∼119 K). If the fissure opening is large, a blowout occurs. The rapid escape of gas and drop in pocket pressure causes water in the pocket to boil and create many small aerosol droplets of seawater. These may be carried along by the erupting gas. Electrically charged droplets can couple to the magnetosphere, and be dragged away from Enceladus. Most of the CO2 blowout gas escapes from Enceladus and the remainder is distributed globally. However, CO2 trapped in a clathrate structure does not escape. It is much heavier and slower moving than the CO2 gas. Its motion is ballistic and has an average range of about 17 km. Thus, it contributes to deposits in the vicinity of the vent. Local heat flow indicates that gas pockets can be located as deep as several tens of meters below the surface. Gas pockets can be reused, and we explore their life cycle.

Missions based on low-energy orbits in the vicinity of planetary moons, such as Titan or Enceladus, involve significant end-to-end trajectory design challenges due to the gravitational effects of the distant larger primary. To address these challenges, the current investigation focuses on the visualization and use of multidimensional Poincare maps to perform preliminary design of orbits with significant out-of-plane components, including orbits that provide polar coverage. Poincare maps facilitate the identification of families of solutions to a given orbit problem and provide the ability to easily respond to changing inputs and requirements. A visual-based design process highlights a variety of trajectory options near Saturn's ocean worlds, including both moon-centered orbits and libration point orbits.

In this paper parallel flow velocity shear Kelvin-Helmholtz instability has been studied in two different extended regions of the inner magnetosphere of Saturn. The method of the characteristic solution and kinetic approach has been used in the mathematical calculation of dispersion relation and growth rate of K-H waves. Effect of magnetic field (B), inhomogeneity (P/a), velocity shear scale length ( A i ), temperature anisotropy ( T ⊥ / T | | ), electric field (E), ratio of electron to ion temperature ( T e / T i ), density gradient ( e n ρ i ) and angle of propagation ( θ ) on the dimensionless growth rate of K-H waves in the inner magnetosphere of Saturn has been observed with respect to k ⊥ ρ i . Calculations of this theoretical analysis have been done taking the data from the Cassini in the inner magnetosphere of Saturn in the two extended regions of Rs ∼4.60–4.01 and Rs ∼4.82–5.0. In our study velocity shear, temperature anisotropy and magnitude of the electric field are observed to be the major sources of free energy for the K-H instability in both the regions considered. The inhomogeneity of electric field, electron-ion temperature ratio, and density gradient have been observed playing stabilizing effect on K-H instability. This study also indicates the effect of the vicinity of icy moon Enceladus on the growth of K-H instability.

The Laplace Tidal Equations (LTEs) play a basic role for investigating the dissipation of long-term orbital energy as heat in surface oceans of icy satellites. Here we address solving LTEs by means of finite elements using PDE2D, a general-purpose finite element program for solving multidimensional PDEs. We use PDE2D to solve the equations for eccentricity and obliquity gravitational potentials for a shallow water approximation, and calculate ocean dissipation in three icy satellites, Enceladus, Europa and Titan. Compared with previous modeling procedures, PDE2D solves for unknowns that are approximated by piecewise cubic polynomials, thus attaining a higher O(dφ4) + O(dθ4) numerical precision. Our modeling results show, within few percent errors, a good agreement with earlier semi-analytical models and numerical solutions. We provide an open access to our PDE2D codes which are intended to be simple and easy to adjust for a wide range of tidal heating related problems. Our modeling results show that PDE2D can be a useful tool for researchers studying tidal heating related problems.

In stark contrast with its neighbor moon Enceladus, Mimas is surprisingly geologically quiet, despite an eccentric orbit and distance to Saturn prone to levels of tidal dissipation 30 times higher. While Mimas’ lack of geological activity could be due to a stiff, frigid interior, libration data acquired using the Cassini spacecraft suggest that its interior is not homogeneous. Here, we present one-dimensional models of the thermal, structural, and orbital evolution of Mimas under two accretion scenarios: primordial, undifferentiated formation in the Saturnian sub-nebula, and late, layered formation from a debris ring created by the disruption of one or more previous moons. We find it difficult to reproduce a differentiated, eccentric Mimas under a primordial accretion scenario: either Mimas never differentiates, or the internal warming that leads to differentiation increases tidal dissipation, yielding runaway heating that produces a persistent ocean, thereby circularizing Mimas’ orbit. Only if Mimas accretes very early (so that the decay of short-lived radionuclides initiates differentiation) but its rheology is not highly dissipative (in order to stop runaway tidal heating even if the eccentricity is not negligible) can the simulations match the observational constraints. Alternatively, a late, layered accretion scenario yields a present-day Mimas that matches observational constraints, independently of the magnitude of tidal dissipation. Consistent with previous findings, these models do not produce an ocean on Enceladus unless its orbital eccentricity is higher than today’s value.

Many obsects in the solar system are suspected to have experience reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic of exogenic processes, can change objects' moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such phenomenon is called True Polar Wander (TPW). Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by ~55{\deg} about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a topographic basin chain tracing a non-equatorial circumglobal belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of an earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images.

Enceladus is the first outer solar system body on which pit chains have been positively identified. We map the global distribution of pit chains and show that pit chains are among the youngest tectonic features on Enceladus's surface, concentrated in the cratered plains centered on Enceladus's Saturnian and anti-Saturnian hemispheres. Pit chains on Enceladus are interpreted as the surface expressions of subsurface dilational fractures underlying a cover of unconsolidated material, which we infer to be a geologically young cover of loose regolith that mantles the surface of Enceladus. A widespread layer of regolith may act to insulate the surface, which has implications for the thermal state of Enceladus's ice shell. The widespread distribution of pit chains across the cratered plains indicates that this ancient surface has recently been tectonically active.

The geophysical evolution of many icy moons and dwarf planets seems to have provided opportunities for interaction between liquid water and rock (silicate and organic solids). Here, we explore two ways by which water-rock interaction can feed back on geophysical evolution: the production or consumption of antifreeze compounds, which affect the persistence and abundance of cold liquid; and the potential leaching into the fluid of lithophile radionuclides, affecting the distribution of a long-term heat source. We compile, validate, and use a numerical model, implemented with the PHREEQC code, of the interaction of chondritic rock with pure water and with C, N, S-bearing cometary fluid, thought to be the materials initially accreted by icy worlds, and describe the resulting equilibrium fluid and rock assemblages at temperatures, pressures, and water-to-rock ratios of 0–200 ° C, 1–1000 bar, and 0.1–10 by mass, respectively. Our findings suggest that water-rock interaction can strongly alter the nature and amount of antifreezes, resulting in solutions rich in reduced nitrogen and carbon, and sometimes dissolved H2, with additional sodium, calcium, chlorine, and/or oxidized carbon. Such fluids can remain partially liquid down to 176 K if NH3 is present. The prominence of Cl in solution seems to hinge on its primordial supply in ices, which is unconstrained by the meteoritical record. Equilibrium assemblages, rich in serpentine and saponite clays, retain thorium and uranium radionuclides unless U-Cl or U-HCO3 complexing, which was not modeled, significantly enhances U solubility. However, the radionuclide 40 K can be leached at high water:rock ratio and/or low temperature at which K is exchanged with ammonium in minerals. We recommend the inclusion of these effects in future models of the geophysical evolution of ocean-bearing icy worlds. Our simulation products match observations of chloride salts on Europa and Enceladus; CI chondrites mineralogies; the observation of serpentines, NH4-phyllosilicates, and carbonates on Ceres’ surface; and of Na and NH4-carbonate and chloride in Ceres’ bright spots. They also match results from previous modeling studies with similar assumptions, and systematically expand these results to heretofore unexplored physico-chemical conditions. This work involved the compilation and careful validation of a comprehensive PHREEQC database, which combines the advantages of the default databases phreeqc.dat (carefully vetted data, molar volumes) and llnl.dat (large diversity of species), and should be of broad use to anyone seeking to model aqueous geochemistry at pressures that differ from 1 bar with PHREEQC.

There is abundant petrologic evidence for the oxidation of Fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. The H2 lost from the chondrite parent bodies as a result of Fe oxidation would have been isotopically very light, enriching any residual water in D. The extents of the D enrichments will have depended on the fractions of water consumed and the temperatures during Fe oxidation. Here we have estimated the likely ranges of water consumed by Fe oxidation in the CI, CM, CR and LL parent bodies, as well as the likely range of changes in water H isotopic compositions this would have produced. We first used Fe XANES to determine the Fe valences of bulk meteorite powders in Orgueil (CI1), a number of CMs and CRs that experienced varying degrees of alteration, and Semarkona (LL3.00). The total ranges of bulk Fe valences we obtained were: Orgueil 2.77, CMs 2.40–2.63, CRs 1.46–2.54, and Semarkona 2.10. Combining previous estimates of the present water/OH contents of our samples with the present bulk Fe valences and an estimated range of initial bulk Fe valences, we estimate the likely ranges of fractional water losses to have been: Orgueil 15–26%, Semarkona 73–83%, CMs 23–48%, and CRs 39–62%. The associated maximum and minimum changes in the H isotopic compositions of the remaining water were estimated assuming the equilibrium H2-H2O isotopic fractionation factor, Rayleigh fractionation of the H2, and oxidation temperatures of 0–200 °C. Using previous estimates of the water H isotopic compositions in the chondrites, the ranges of estimated δD values for the initial chondritic waters are: Orgueil −672‰ to −422‰, CMs −676‰ to −493‰, CRs −527‰ to −56‰, and Semarkona −527‰ to 154‰. The CI, CM, CR and ordinary chondrites all accreted water with similar H isotopic compositions that were distinct from the compositions of comets or Saturn’s moon Enceladus. Thus, the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the Asteroid Belt from comet forming regions by orbital migration of the giant planets. If the carbonaceous chondrites did form in the outer Solar System, as some models predict, it was probably not beyond 7 AU. However, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner Solar System.

Particle properties of individual fissure eruptions within Enceladus’ plume have been analyzed using high spatial resolution Visible and Infrared Mapping Spectrometer (VIMS) observations from the Cassini mission. To first order, the spectra of the materials emerging from Cairo, Baghdad and Damascus sulci are very similar, with a strong absorption band around 3 µm due to water-ice. The band minimum position indicates that the ice grains emerging from all the fissures are predominantly crystalline, which implies that the water-ice particles’ formation temperatures are likely above 130 K. However, there is also evidence for subtle variations in the material emerging from the different source fissures. Variations in the spectral slope between 1–2.5 µm are observed and probably reflect differences in the size distributions of particles between 0.5 and 5 µm in radius. We also note variations in the shape of the 3 µm water-ice absorption band, which are consistent with differences in the relative abundance of > 5 µm particles. These differences in the particle size distribution likely reflect variations in the particle formation conditions and/or their transport within the fissures. These observations therefore provide strong motivation for detailed modeling to help place important constraints on the diversity of the sub-surface environmental conditions at the geologically active south-pole of Enceladus.

We propose a dynamic model relating the non-hydrostatic long-wavelength topography on Enceladus to loading at the base of the ice shell. In dynamic equilibrium, the surface topography induced by a bottom load is very close to that inferred for Airy isostasy. However, due to the small size of the moon the relaxation to equilibrium is significantly longer than the maximum Maxwell relaxation time. During the relaxation, the upper load/bottom load ratio can be either smaller than 1 or larger than 1, depending on the initial state and the loading history.

Relating relaxation of impact crater topography to past heat flow through the crusts of icy satellites is a technique that has been applied to satellites around Jupiter and Saturn. We use global digital elevation models of the surfaces of Dione and Tethys generated from Cassini data to obtain crater depth/diameter ( d/D ) data. Relaxation is found to affect craters down to smaller diameters on these satellites compared to Rhea. We perform relaxation simulations in order to assess the heat flow necessary to relax craters on Dione and Tethys to their present morphologies. Heat flows exceeding 60 mW m −2 are required to relax several craters on both satellites, and relaxation appears to be subject to geographical controls. On Dione, we define a ‘relaxation dichotomy’ that separates the more relaxed craters in sparsely cratered plains from the less relaxed craters in heavily cratered terrain. The configuration of this dichotomy resembles that of the structural-geological dichotomy on Enceladus, implying that a similar resonance-induced tidal heating mechanism concentrated in the southern hemisphere may have affected both satellites. Defining geographical distribution of relaxation on Tethys is hindered by the presence of the young Odysseus impact and its associated ejecta.

Recent geodetic measurements for Enceladus suggest a global subsurface ocean that is thicker beneath the south pole. In order to maintain such an ocean, viscous relaxation of topography at the base of the ice shell and melting of ice need to be balanced. In this study, we investigate the interior thermal state that can lead to the relaxation timescale being comparable to the melting timescale. Our results indicate that a basal heat flux about ten times higher than that due to radiogenic heating, or an ice shell tidal heating rate about ten times higher than the conventional estimate of 1.1 GW is necessary if the ice shell is in thermal equilibrium. These requirements are concordant with recent astrometric studies.

Based on magnetic field data from Cassini's high-inclination orbits of Saturn (radius R S = 60 , 268 km ), we analyze the latitudinal distribution of ion cyclotron waves in the giant planet's magnetosphere. Our survey takes into account magnetic field data from all high-inclination orbits between 2004 and 2015. We analyze the dependency of the occurrence rate and amplitude of the ion cyclotron waves on radial distance ρ to Saturn's rotation axis, vertical distance z to Saturn's equatorial plane, and magnetic latitude λ. The occurrence rate of ion cyclotron waves is approximately 100% in Saturn's equatorial plane between the orbits of Enceladus and Dione and decreases to 50% at altitudes of | z | ≈ 0.6 R S . Ion cyclotron waves were detected up to | z | = 2.0 R S . The occurrence rate displays strong, non-monotonic variations with respect to ρ, z, and λ. The vertical amplitude profile of the waves exhibits an M-like pattern with two distinct peaks near z = ± 0.3 R S and the central minimum at z=0. Compared to earlier observations, we find this M-like structure to be inflated in±z direction by a factor of three. The available magnetic field data provides only weak evidence for a local impact of Enceladus and Dione on the ion cyclotron wave field. Using the observed Doppler shift of the ion cyclotron wave frequency during Cassini's high-inclination orbits, we demonstrate the existence of a narrow band of bidirectional wave propagation. This band is centered around Saturn's equatorial plane and possesses a half-width of | z | = 0.15 R S , which agrees well with the vertical scale height of Saturn's neutral cloud. To the north of this band, all ion cyclotron waves propagate towards the north ( z > 0 ); and to the south, all waves propagate towards the south ( z 0 ). In companion with our previous study (Meeks et al., 2016), this survey provides the complete three-dimensional picture of the ion cyclotron wave field between the orbits of Enceladus and Rhea during the Cassini era.

The origin of the unique south polar terrain on Enceladus is not well understood. Gravity measurements made by Cassini suggest that the ice shell is thinner at the south pole than in the north. Tides are recognized to be the ultimate energy source for the observed thermal anomaly, and are believed to regulate the activity of jets, and movement along the tiger-stripes. However, tides alone are insufficient to explain the observed pattern of heat flow and activity, because the tidal potential is symmetric about the equator and no corresponding thermal anomaly or activity is observed in the north. Tidal heating is highly sensitive to the thickness and the mechanical properties of the ice shell, and the presence of significant lateral variations in the mechanical properties of the ice shell, and in general is stronger when the ice is thinner and weaker. Although the tidal heating might be able to sustain these variations once in place, a non-tidal, physical mechanism is therefore needed to break the tidal symmetry. Here we explore the possibility that a large impact into what is now the SPT could break through the ice shell of Enceladus, and break the hemispheric symmetry as well. We have performed hydrocode simulations of the impacts of icy projectiles into the ice shell of Enceladus, and have modeled the thermal evolution of the ice shell post-impact. We find that an impact capable of forming a crater the size of the present-day south polar terrain could have completely penetrated a 20-km thick ice shell, and would have excavated a cavity extending ~30 km deep in a thicker ice shell. Although the impact does not completely punch through a thicker ice shell, the floor of the crater is extensively fractured, exposing the ocean to space. We find that the impact shock heating will be retained for a few Myr, and that it locally softens the ice to permit more tidal dissipation. The ice shell beneath the impact site thickens over time, but a 1–2 km reduction in topography is retained at the surface.

Science goals for icy ocean worlds missions include characterizing the chemical composition of the surface and interior using remote sensing and in situ techniques. The next class of flight mass spectrometers for these missions will obtain compositional identifications and isotope ratio measurements of volatiles evolved from the ice surface (exosphere) and plumes. These mass spectra will be combined with data from other flight instruments to infer the composition of the interior from volatiles. To ensure accurate interpretation of these measurements, it is critical to verify whether the fundamental assumption that volatiles observed in icy ocean world exospheres or plumes will be a direct reflection of the sub-ice ocean. The present study evaluates whether isotopologues from an initial CO2 gas fractionate by interacting with seawater (brine) of varying salt composition and concentration. δ13CCO2 are affected by the pH of the brine and subsequent speciation of {CO2}, where {CO2} represents the combination of CO2, H2CO3, HCO3-, and CO32-. {CO2} for low pH brines hypothesized for Europa will preferentially speciate as CO2. Analyzed δ13CCO2 for low pH brines are within error of the original δ13CCO2, demonstrating that volatile CO2 in a low pH system will be a direct reflection of the original CO2. However, Europa’s radiative environment and rapid depressurization due to plume ejection may impose fractionation effects. In contrast, high pH systems relevant to Enceladus or a more alkaline Europa are expected to form all carbonate species, while favoring speciation as HCO3-. High pH brines in these experiments include both an original CO2 gas and an isotopically distinct HCO3- (from NaHCO3 salt). These alkaline experiments demonstrate that δ13CCO2 values are highly variable, and depend on the concentration of the dominant carbonate species, (Na)HCO3-. Mass balance estimates indicate that measured δ13CCO2 is a thermodynamically predictable mixture of both carbon sources, suggesting that measurements of δ13C at Enceladus would directly reflect of the sources of CO2 and carbonate buffering in the ocean. δ18O measurements for CO2 interacting with KCl and MgCl2 follow established models for δ18O-ionic strength. CO2 interacting with MgSO4, Na2SO4, and NaCl demonstrate an offset from established δ18O-ionic strength models depending on the concentration of initial CO2. These results suggest that current predictive models for δ18O in brines need to be resolved for changing concentrations of CO2.

Icy satellites that contain subsurface oceans require sufficient thermal energy to prevent the liquid portion of their interiors from freezing. We develop a numerical finite difference model to solve the Laplace Tidal Equations on a sphere in order to simulate tidal flow and thermal energy dissipation in these oceans, neglecting the presence of an icy lid. The model is applied to Titan and Enceladus, where we explore how Rayleigh (linear) and bottom (quadratic) drag terms affect dissipation. The latter drag regime can only be applied numerically. We find excellent agreement between our results and recent analytical work. Obliquity tide Rossby-wave resonant features become independent of ocean thickness under the bottom drag regime for thick oceans. We show that for Titan, dissipation from this Rossby-wave resonance can act to dampen the rate of outward orbital migration by up to 40% for Earth-like values of bottom drag coefficient. Gravity-wave resonances can act to cause inward migration, although this is unlikely due to the thin oceans required to form such resonances. The same is true of all eccentricity tide resonances on Enceladus, such that dissipation becomes negligible for thick oceans under the bottom drag regime.

Since its discovery, the Enceladus plume has been subjected to intense study due to the major effects that it has on the Saturnian system and the window that it provides into the interior of Enceladus. However, several questions remain and we attempt to answer some of them in this work. In particular, we aim to constrain the H2O production rate from the plume, evaluate the relative importance of the jets and the distributed sources along the Tiger Stripes, and make inferences about the source of the plume by accurately modeling the plume and constraining the model using the Cassini INMS and UVIS data. This is an extension of a previous work (Yeoh, S.K., et al. [2015] Icarus, 253, 205–222) in which we only modeled the collisional part of the Enceladus plume and studied its important physical processes. In this work, we propagate the plume farther into space where the flow has become free-molecular and the Cassini INMS and UVIS data were sampled. Then, we fit this part of the plume to the INMS H2O density distributions sampled along the E3, E5 and E7 trajectories and also compare some of the fit results with the UVIS measurements of the plume optical depth collected during the solar occultation observation on 18 May 2010. We consider several vent conditions and source configurations for the plume. By constraining our model using the INMS and UVIS data, we estimate H2O production rates of several hundred kgs-1: 400–500 kg/s during the E3 and E7 flybys and ∼900 kg/s during the E5 flyby. These values agree with other estimates and are consistent with the observed temporal variability of the plume over the orbital period of Enceladus (Hedman, M.M., et al. [2013] Nature, 500, 182–184). In addition, we determine that one of the Tiger Stripes, Cairo, exhibits a local temporal variability consistent with the observed overall temporal variability of the plume. We also find that the distributed sources along the Tiger Stripes are likely dominant while the jets provide a lesser contribution. Moreover, our best-fit solutions for the plume are sensitive to the vent conditions chosen. The spreading angle of the jet produced is the main difference among the vent conditions and thus it appears to be an important parameter in fitting to these INMS data sets. In general, we find that narrow jets produce better fits, suggesting high Mach numbers (> 5) at the vents. This is supported by certain narrow features believed to be jets in both the INMS and UVIS data sets. This tends to rule out sublimation from the surface but points to a deep underground source for the plume. However, the underground source can be either sublimation from an icy reservoir or evaporation from a liquid reservoir. A high Mach number at the vent also suggests subsurface channels with large variations in width and not fairly straight channels so that the gas can undergo sufficient expansion. Additionally, the broad spreading angles inferred for the µm-sized grains (Ingersoll, A.P. and Ewald, S.P. [2011] Icarus, 216, 492–506; Postberg, F., et al. [2011] Nature, 474, 620–622) cannot be due to spreading by the gas above the surface alone. Some other mechanism(s) must also be responsible, perhaps occurring below the surface, which further points to an underground source for the plume.

The plasma conductivity is an important issue for understanding the magnetic field structure registered by Cassini in the Enceladus proximity. We have revise the conductivity mechanism to incorporate the plume nanograins as a new plasma species and take into account the relevant collisional processes including those accounting for the momentum exchange between the charged dust and co-rotating ions. It is concluded that in the Enceladus plume the dust dynamics affects the Pedersen and Hall conductivity more efficiently than the electron depletion associated with the presence of the negatively charged dust as has been suggested by Simon et al. (Simon, S., Saur, J., Kriegel, H., Neubauer, F. M., Motschmann, U., and Dougherty, U. [2011] J. Geophys. Res., 116, A04221, doi:10.1029/2010JA016338). The electron depletion remains a decisive factor for only the parallel conductivity. In the parameter regime relevant for the Enceladus plume, one finds increase of the Pedersen and decrease of the parallel components, whereas for the Hall conductivity the charged dust changes both - its value and the sign. The associated reversed Hall effect depends significantly upon the local dust-to-plasma density ratio. An onset of the reversed Hall effect appears to be restricted to outer parts of the Enceladus plume. The results obtained can significantly modify Enceladus’ Alfven wing structure and thus be useful for interpretations of the magnetic field perturbations registered by the Cassini Magnetometer during the close Enceladus flybys.

The Cassini Radio and Plasma Wave Science (RPWS) instrument can detect dust particles when voltage pulses induced by the dust impacts are observed in the wideband receiver. The size of the voltage pulse is proportional to the mass of the impacting dust particle. In this paper, we show RPWS measurements of dust particles in Saturn's dusty rings. The differential size distribution of the dust particles can be characterized as a power law dn/dr∝rμ, where μ ∼ − 4 and r is the particle radius. The observed particle radius ranged from 0.2 to 10 μm. The dust density profiles of the dusty rings are derived from the impact rates measured by the RPWS wideband receiver. The radial density profiles show peaks near Enceladus’ orbit and the G ring. The region around the G ring is found to be a very thin layer of dust particles with no observable vertical offset of the peak density from the ring plane. The vertical scale height of the E ring varies with the radial distance from Saturn with a local minimum at Enceladus’ orbit. The vertical density profiles of the E ring show dips at the equatorial plane near Enceladus’ orbit and vertical offsets of the peak locations away from Enceladus’ orbit. These observations are consistent with previous modeling studies and measurements by other instruments onboard Cassini.

Enceladus has a dynamic plume that is emitting gas, including water vapor, and dust. The gas is ionized by solar EUV radiation, charge exchange, and electron impact and extends throughout the inner magnetosphere of Saturn. The charge exchange collisions alter the plasma composition. Ice grains (dust) escape from the vicinity of Enceladus and form the E ring, including a portion that is negatively charged by the local plasma. The inner magnetosphere within 10 R S (Saturn radii) contains a complex mixture of plasma, neutral gas, and dust that links back to Enceladus. In this paper we investigate the energy distributions, ion species and densities of water group ions in the plume of Enceladus using test particle and Monte Carlo methods that include collisional processes such as charge exchange and ion-neutral chemical reactions. Ion observations from the Cassini Ion and Neutral Mass Spectrometer (INMS) for E07 are presented for the first time. We use the modeling results to interpret observations made by the Cassini Plasma Spectrometer (CAPS) and the INMS. The low energy ions, as observed by CAPS, appear to be affected by a vertical electric field ( E Z =−10 µV/m) in the plume. The E Z field may be associated with the charged dust and/or the pressure gradient of plasma. The model results, along with the results of earlier models, show that H 3 O + ions created by chemistry are predominant in the plume, which agrees with INMS and CAPS data, but the INMS count rate in the plume for the model is several times greater than the data, which we do not fully understand. This composition and the total ion count found in the plume agree with INMS and CAPS data. On the other hand, the Cassini Langmuir Probe measured a maximum plume ion density more than 30,000 cm −3 , which is far larger than the maximum ion density from our model, 900 cm −3 . The model results also demonstrate that most of the ions in the plume are from the external magnetospheric flow and are not generated by local ionization. The origin of the ions in the plume was investigated using two different velocity models. Most ions were created by the interaction with background magnetospheric plasma and by photoionization. INMS and CAPS also detected water cluster ions. We will interpret these observations as a result of ion collisions with neutral water clusters, (H 2 O) n , originating in the tiger stripe vents as suggested by Tokar et al. (2009). We also estimated the process of generating cluster ions based on the INMS observations. We suggest that the most likely source is reaction of H 3 O + with neutral water clusters or dimers such as (H 2 O) 2 formed in the plume vents.